Posted By: mmc427ss Rear roll center improvement. - 09/13/22 08:04 PM
Needing to improve the rear suspension grip in this car and knowing the Winter doldrums are approaching it's now time to take another look at options for better stick out back.
It wasn't until recently with Bernie's bolt in three link that a better option appeared. It's to late in the game now to junk the 8 1/2" rear so a weld in third leg can't replace the two UCA there now, needs to be a bolt in 3 link like Bernie's. That's what makes Bernie's three link viable for me, essentially a bolt in. But the one downside is how to deal with the tailpipes, or the lack of room for tailpipes. That's for me is a huge disadvantage, no tailpipes, couldn't tolerate the noise level.

One of my projects for the car this Fall, reupholstering the driver's seat with added bolstering to fit my torso, is now also a Winter project, the upholstery shop's slow time. A visit there this past week started the ball rolling for the seat mods and possible harness install. That in process it's time to spend time on the rear roll.

First I think i need to get under the car and do a full workup on what i actually have for all the pick points needed to find the roll center I have now. Don't expect it's much different that a 100 other G similar to mine, but I don't trust answers when someone says "it about 18". I like number and like exact numbers, not almosts. it's not a big deal to get precise pickup points and angles for all the moving parts, just got be persistent.
And because I'm a modeler, making a full scale rear suspension to mimic what's under the car isn't difficult either once you nave the numbers.
Yes much easier on software available to play with design, but I'm just an uneducated type who like to hold stuff in their hand and watch it break.

So measuring, thinking, reading, understanding, looking at all the possible avenues to approach rear roll center is the new Winter project.

This Thurs at my local tire/repair/alignment/racers shop i need to get my PA annual inspection stickers on the windshield. If the computer alignment rack is available when I go there we may throw the pickups on the wheels and see if his computer rack gets the same numbers as my alignment rack in my shop using old school tools. Have done that comparison before and we deviate very little and it's not anything to be concerned about.
One check that is important is I can see bump from ride height to full droop on his screen while raising the front of the car on the rack and compare it to the last bump number I got with my bump gauge. Bump on this car has been an ongoing correction for 20 years. Currently it's actually very good, the computer should verify that.
A good thing about a 4 wheel align computer rack is you get all the number for the rear suspension, camber and toe per wheel and also thrust angle. These are useful number to know for the next project.

I have four corner scaled this car before using that shop's Intercomp scales. After asking about using them he said we can throw them in the trunk and take them home with me for a few days. That I will do. Scale the car with driver as i did before. I doubt the number changed much since done 10 years ago.
But there is a method to find the center of gravity using scales that i will look into. Everyone always say, "oh, it about the centerline of the camshaft". Sure would be nice to really know how high that number is. When we talk about roll center the guesstimated center of gravity usually is just a guess.
Being one of those anal people i'd like to know just exactly where that is on my car.
There is a process to do that accurately, just need to find the procedure, acting it out ain't that hard.

So finding, modifying, improving the rear roll center seems to be this Winter food for thought and project. Last year the biggie was the huge front sway bar project, that took nearly a year of thinking. Hoping i don't run out of brain cells by next Spring.
Posted By: MAP Re: Rear roll center improvement. - 09/13/22 10:49 PM
Hi Bob!

About RCH, I know I've said forever that it's about 18", but that depends on ride height, among the other usual suspects.

Speaking of a software model versus a physical model, I'll take the physical model every time. Having to convey everything through a computer screen has certain insurmountable drawbacks.

Any chance you'd tolerate the exhaust pipes exiting the car ahead of the rear axle? To minimize the impact on ground clearance, you could create an exhaust tip that's short in height but very wide to conserve exit area; you know, something like an inch tall but ten inches wide.

Exiting ahead of the rear axle normally makes it a few dB louder for the driver relative to a full rear exit, but there are ways to reduce noise to make up for the difference. Doing this, however, would add to the weight and complexity of the exhaust system. Or, you could attack the noise problem by using passive methods inside the cabin with heavy, lossy IIR rubber sheets that you'd apply to the cabin sheet metal. Expect to add about 30-50lb to the car (as usual, the eternal weight-gain issue.) Continuing on the weight angle, you would at least reduce weight by eliminating roughly four feet of tubing for each side of the exhaust system relative to the full rear exit which is, what, 15 lb?

But the upside is that you may be able to avoid that packaging nightmare with the rear axle...

About determining the car's COM location: I never tried it but always wanted to. I haven't looked at the stated procedure in many years, but it involves four-cornering the car with it horizontal and then tilted in roll a considerable amount (the more the better, to a certain point.) BUT, when you do this, the tires and the suspension move around in ways that are difficult to control. Even the exhaust system tends to shift a bit. Plus, the gas in the tank moves. All of this slop tends to give a falsely high estimate for COM height, maybe something like 5-10 percent (??) In other words, if you wanted to get the true COM position within an inch or so, you'd be hosed.

Me, I'd remove everything that can move from the car prior to this measurement. Then, knowing the total weight of the car in this state and the determined location of its COM, I'd measure the weights of all the removed components, along with each component's COM location relative to the car, and then calculate a final revised COM location for the entire car (mathematically, it's trivial.)

As a compromise, you might be able to chock/clamp the car under the frame in a way that immobilizes it, yet without changing the scale weights (harder to do than it sounds.) Empty the car of gas. Calculate your COM position. Calculate an adjustment for about 55lb of gas in the tank. Or, do the opposite: fill the tank completely, then adjust for about a 55lb weight reduction. Done this way, you might be able to hit about 1% of accuracy for the COM height.

This is a lot of work but I'd do it!
Posted By: Buick Runner Re: Rear roll center improvement. - 09/14/22 01:47 AM
Some states do allow the exhaust to exit in front of the rear axle due to safety reasons with carbon monoxide entering the cabin. Not sure if PA is one of them.
Posted By: MAP Re: Rear roll center improvement. - 09/14/22 05:16 AM
Didn't think of that - thanks for pointing that out.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/14/22 05:23 AM
This RC and CG thing has been something smoldering on a back burner for many years for me, 100s of hours surfing, reading peoples thoughts and ideas looking for that light bulb to turn on and have that revelation. Unfortunately I've never found that and as a result you move onto something else that bugs you, and back on the back burner that rear RC problem, thinking goes.
As the candle in the wind gets dimmer, time to make another go around at that damn lousy rear RC on these cars.

Being I'm retired, surfing, as in the web type, I'm pretty good at. The patience needed I learned back in the last millennium dial-up days. Didn't take long to find the technique for locating CG using the four corner scales. Also found several plugin calculators to quickly spit out a CG height. Simple you need wheelbase length in inches, weight at each corner at ride height entered in the calculator. Next raise the rear suspension as high as conveniently possible with an equal height stand and scale under each tire and check the corner weights at all four corner again, enter info into calculator. Enter the height the rear was raised and press "enter". A roll center height is produced. Sounds easy.

But as Mark said there are couple things that can effect the CG accuracy. Fuel level in the tank. Will insure that it is near 100% full so slosh is not an issue. About the only advantage I could ever give having the fuel tank filler being behind the license plate is this time. With the xxxx end of the car raised 12" or more no fuel will be at the cap. All the air in the tank will end up there.
A requirement of all my weight measuring is full tank of fuel. All my racing starts with a full tank. The car benefits running a full tank,

Had to laugh a couple years ago at an autox safety inspection they asked why I didn't take the spare tire out for the event. All i could think of as an answer was, WHY? Everything the car does is with a spare tire in the truck, it get weighed with a spare, it had a 4 lb difference rear side to side with the tire, WHY would I remove it.

The one PITA of this scaling the car method is as you shift the weight, load, to the front because of raising the rear the ride height is changed on both ends, this skews the results. So to negate ride height change you remove all four shocks and replace them with a solid shock of fixed lengths. The length is set to mimic ride height when the car is set back on the ground and sitting just on the scales. Now when you do the jack the rear of the car up and set on the 12" stands both the front and rear heights don't move and you get a more accurate CG height.
If you spend time in a chassis shop notice a fixed length of steel rod duplicating suspension geometry ride height will be substituted for springs and shocks during chassis construction.
Tonight dug out some 3/8 x 1 1/4" flat stock, a couple lengths of 1/2" threaded rod and think I have what I need to make the two front solid "shock". Using threaded rod I will be able to tweak the ride height at the top using nuts. Being the Viking front shocks have tie wraps on the shock shaft to show max compression traveled I can use the tiewrap to indicate the position of the shaft at ride height. Just jack the front suspension into droop, slid the tiewrap down all the way. Drop the car back to ride height and the tie wrap will slide on the shaft, now indication the ride height. Removing the shock, compress so the tie wrap touches the lower tube and measure the length needed for a solid shock. Sounds pretty easy.
For the rear I have a set of Comp Eng shocks that I may weld the shaft to the bottom tube so it doesn't compress or rebound. Same method using a tie wrap as the indicator on the shaft to find ride height length of solid shock. Those Comp shocks are near brand new, hate to destroy them with a welder, but tried to give them away and no one bit on them. They may be donated to the cause and end up in the scrap pile afterwards.

As far as the exhaust, tailpipe thing goes. On the car is an ATR 2 1/2" stainless exhaust system made for the GN, installed in 2000, and still very nice condition, it doesn't rust, wear, corrode, it's a near forever exhaust. Cutting them up may require and act of god. Tailpipe exits are GN, behind the rear tires . Noisier that the stock SS bumper exit but tolerable. All my custom exhaust vehicles had behind the tire exits. It is very important for me to continue dumping the exhaust, behind the tires.
I believe here in PA exhaust legally can not dump under the car and needs to exit behind the cabin.
The law reads thus:
"An exhaust system shall extend and discharge completely to the outside edge of the vehicle body, including a truck bed, or as originally designed."
But along with window tint, tires outside the body lines, cambering, illegal lights, exhaust is just another violator of the law often done.

Tomorrow have a few hours to spare in the morning. If this week I want to borrow the four corner scales for a few days I need to produce four fixed length shock substitutes quickly. Shouldn't be a big deal, just, more thinking.
Posted By: Travis Jones Re: Rear roll center improvement. - 09/14/22 12:43 PM
Your best bet for a 3-link and exhaust is probably to emulate as much of the S197 mustang 05-13 as possible. That's what I run, with the right amount of work I'm sure I could make tailpipes work and behind the tire would be easier than out the back due to the PHB.
Posted By: Hunter79764 Re: Rear roll center improvement. - 09/14/22 04:22 PM
Locking out the suspension seems like a good idea, I'd be curious how much it affects it either way? Also, do you have to weight the rear in the lifted position? I'm seeing that you need front and rear weights when level, but only front weight (and height) when lifted. That should make the process a little quicker.
Do you plan on doing the measurements at multiple heights? The higher the lift, the less sensitive your measurements will be, but if there is any shifting happening, the lower height would be more realistic for real world use. Maybe a matching pair of jack stands on each axle tube, then take a reading at each notch? I'm sure you'll have something nailed down for that.

Anyway, looks like another fun, practical science experiment. Hope it goes well!
Posted By: SickSpeedMonte Re: Rear roll center improvement. - 09/14/22 08:10 PM
We measure CG height just about every day in our lab. For passenger vehicles, we have a cradle that you drive an axle onto and lift the cradle with an overhead crane.

We put a load cell in the rigging and zero it out with the cradle suspended. Then pull the front of the vehicle on to it and lift as high as we can before the rear bumper hits the ground. Then we take a laser line level on a rolling cart that rides on a rail and get it shooting a vertical plane perpendicular to the vehicle centerline. Then roll it along the track to shoot the center of the crane hook and the shackles on the cradle, mark the floor, and measure the distance from the axle that's on the ground to the laser. That, combined with the load on the load cell tells you how much moment there is about the reaction point on the ground. That load is reacting the weight of the vehicle acting at the CG. Knowing the weight of the vehicle, we can calculate how far the CG is from the reaction point on the ground and draw a vertical line on the vehicle (using a poster board usually to mark on). Then when you let the vehicle down, that line is no longer vertical. Repeat the process with a rear lift and you have an X at the CG. Some of my colleagues did a similar exercise using wheel scales (you only need one pair if you know the total weight of the vehicle) on their nitrous pro-mod. The trouble is getting enough inclination on a low vehicle to get an accurate number. Using the wooden block wheel stands would improve the lift, but you'd have to be really careful.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/15/22 05:39 AM
Glad to see some input on this CG project, and much appreciated .

Bernie did you ever have you car CG done at work?

Today a trip to the hardware store and 6' of 1/2" threaded and a 10 nuts and flat washer procured. Decided 3/8 x 1 1/4" steel was overkill for the project and went with 1/4 x 1". This material to make the solid shock replacements rods.
For the rear rod welded one end of an 18" piece of 1/2" threaded rod to a 1/2"-13 bolt at a 90 degree. That end will bolt into the rear lower shock mount just like a shock would. At the top a 1/4 x 1 x 3 1/2" flat stock got drilled 1/2" in the center and 3/8" at each end. The piece will get bolted into the top shock mount location and nuts and flats on each side. I will we able to install that rod setup, one each side and adjust the nuts at the top to lock the rear suspension at ride height.
At the front similar setup except made a 1/4 x 1 x 3 1/4" plate to mimic the lower end of the shock mounting. A 1/2" rod 14" long is welded to a 1/2" hole in that plate's center. At the top nuts and flats to lock that threaded rod at ride height.
With this threaded rod setup I will be able to adjust all four corner to my known ride height before the scaling procedure.
So that is done.

The car will be scaled at ride height twice before removing the shocks and installing the threaded rod at four corners. Once with me in the driver's seat and the second time no driver. The no driver weight will be used in the calculation to find CG.

The threaded rod as shock replacements will now not allow the suspension to move, compression and rebound negated, which should allow more accurate info.

The plan is to raise the rear suspension as high as I can safely. My minimum is 12" under the tires and will try to get higher if possible. My understanding is the higher the more accurate the numbers are.
Found these calculators:
Posted By: MAP Re: Rear roll center improvement. - 09/15/22 05:57 AM
Hi Bob,

(SSM - thanks.)

As much fun as it's going to be to know the exact location of the COM, what, may I ask, are you going to do with that information? (With limited resources, I always skip getting information that serves no practical purpose, no matter how fascinating.) Anyway...

So to summarize, COM in x,y can be gotten in a single measurement set with the car horizontal. To get z, you have to create two different directions of the car's weight vector relative to the car, which is where tilting enters the picture. So you wind up taking two sets of weight measurements: one horizontal and the other tilted. I always heard of the tilting happening in roll, but I generally like pitch better provided the car's suspension and tires have higher pitch stiffness than roll stiffness (Bob, with your massive sway bar, I wouldn't immediately assume that's the case!). If we immobilize the suspension, then tire stiffness still counts, but that strongly favors the pitch method. And yes, the greater the tilt, the more accurate the z calculation.

Either way, you're faced with the challenge of completely immobilizing the car during tilting and preventing stuff from shifting around. If you don't do that, you'll always get a falsely high estimate for the COM height.

Posted By: mmc427ss Re: Rear roll center improvement. - 09/15/22 02:57 PM
Mark have been busy reading my new book and how back in the 60s before we had all this software based knowledge it was modeling that won the day. Amazingly for a large part of the handling research done back in the day was driven by the Corvair, and Chaparral. More later as I wonder through the 800 pages of that book.

Yes I know that finding the CG of my car is just an experiment in wasting time as it may be. But unless someone does the actual foot work and spits out the CG from their foot work I will pursue my own testing. I know the variables on the testing procedure can influence the results and will try minimizing them with the goal of getting a good number. Would think anything + or - 1/4" would be good info.

Something i need to be aware of during testing is the Longacre 4 corner scales I'm using are borrowed, not mine and damaging them is not going to happen, i hope.

Posted By: MAP Re: Rear roll center improvement. - 09/15/22 09:11 PM
Hi Bob - well, +/- 1/4" is about +/- 1%, so yes, you need to be careful about the immobilization aspect. And I can also appreciate pursuing information just for the fun of it. Sometimes apparently useless information I got later became useful in ways I didn't foresee.

Also, before I forget, to get an accurate rear RCH, you'll need to know the front RCH too. The coupling between the two is weak but not negligible.
Posted By: MAP Re: Rear roll center improvement. - 09/16/22 06:23 AM
About ^^^ sorry, I was thinking ahead to the whole roll axis deal - for the rear RCH, you only need data from the rear suspension.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/17/22 12:21 AM
Yes, mapping all the rear suspension pickup points and angles is just down the road in this project. Actually easier than mapping all the front points when i did them.

Yesterday my safety inspection was canceled by my shop guy, he was swamped and no time for setting up the computer rack for the 86's rack time. Also didn't pickup the Intercomp scales yesterday. But, made a trip there today and hauled his scales back to the shop. We scaled the car a couple years ago, maybe 5, and I thought they were Longacre, nope Intercomp.
New 3000 bucks when he bought them.

One of his young workers loaded the scale box in the van, I wonder why. When I unloaded it in the shop i found out why, it weighs 100 lbs, each scale weighs 23lb.

When reading the instruction book on the scales they show how to set up the program to measure side to side CG and also vertical CG. You just do the setup, program in the needed numbers and bingo the digital display tells you the answer. Cool.

Never had a set of wood cribbing to put under the wheels of the car. In the shop was several old wood custom sized 2 x 4 x 8' which were dimensional 1 3/4" x 4" x 98". My brother gave them to me and they were laying around for several year in the shop. Made two 16 x 16 x 10 1/2" high cribs. Have several large pieces of 1" thick 17 ply plywood, again a donation from my brother. It's the material they use in railroad pass car seats, awesome stuff. I cut two 17 1/2 x 17 1/2 " pieces to top the cribs so the 16x16" wheels scales sit nicely on the cribs. So the rear tires can now be safely, comfortably sit on 11 1/2" high cribs. DONE.
If needed and can be done could add another 1 3/4" height to the cribs easily.

The Intercomp is battery powered, it's charging now, tomorrow looks like a play day with scales.
I don't mind borrowing expensive tools, always return them in cleaner condition and don't keep them long. Goal is to get them back by Tues.

Have a good idea how to go through the whole CG process, not difficult but will do it a couple times to insure accuracy.
Finished making the four adjustable 1/2" threaded rod shock replacement the other day, Once the car is scaled with me in the car, with no driver in the car, both at ride height I will pull the shocks and install the threaded rod "shocks" and then adjust then to my non driver ride heights at both ends. The threaded rod allows that easily.
Then it on to the CG testing phase. Is 11 1/2" high enough to get an accurate number, time will tell.

Years ago the car was scales with me in it on these same scales. It showed a 58.75% weight on the front end, OUCH. Yesterday I measured the wheelbase again and verified it at 108 7/8", yep, not factory 108". Moving the front balljoints forward 3/4" to obtain 9 3/4 degree caster along with moving the rear axle rearward 1/8" to add endply to the driveshaft and bingo, 108 7/8" wheelbase.

On the concrete floor you can do the layout of the horizontal CG of the car as scaled at 58 3/4%. You can also measure, layout what a 50%-50% horizontal CG would be. There is 9 1/2" difference, again OUCH. Transferring those measurements to the driver's door and it gives you the prospective relative to the driver's seat. My knee caps are at the 58 3/4 and my navel is near the 50. So most of my mass is located rearward of that 58 3/4. When scaling the car with no driver I expect that 58 3/45 will be even higher, oh boy.

Moving right along.
Posted By: MAP Re: Rear roll center improvement. - 09/17/22 09:16 AM
With that amount of lift, I'd guess about 82- ish lb increase at the front axle and the same amount decrease at the rear axle (assuming you lift the rear.) You can't increase the lift? The accuracy of the scales will be the big determinant of your COM height calculation. To hit 1% COM height tolerance, your weight tolerance will need to be better than about +/- 0.6 lb per scale - that might not be possible with your scales. Do the usual corner jouncing prior to each measurement to minimize any stiction effects.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/17/22 03:03 PM
As I mentioned the shocks will be replaced by sold constraints that will allow no suspension compression or rebound, so no need to account for springs, or shocks, while measuring. And there is no rubber, all delrin, at any point in the suspension, zero stickion.
Yes much higher elevation of the rear axle would be good, starting at 11 1/2" to see how that works out and if i can safely go higher. I have the necessary stuff to go higher. Lifting the rear of that car, done under the centersection is a little tedious. The sway bar on the car limits where the floor jack can be placed to raise the rear, safely.

If you ever had a vehicle move, slide, while jacking the safely thing become the primary goal. Having put jackstands under the car numerous times to get the rocker panels 24" off the ground the safely thing is always the concern. Even to the point of now owning 10 ton stands used for HD trucks.
Posted By: MAP Re: Rear roll center improvement. - 09/17/22 10:37 PM
Hi Bob - Forgot at the time I posted that you were locking the suspension - good deal. So no jouncing. Delrin - you can still get some stiction, but again, thankfully, you're bypassing the issue. Safety - of course.

Also, you're locking the front and rear axles from rolling during this test, right? And, the surfaces that the tires are resting on will tilt along with the car as you raise it? It will be very important to lock the car from moving longitudinally during the test. A shift of more than about 0.026" will introduce more than 1% of error in your height calculation, and that's on top of scale accuracy issues.

If you just rest the tires on horizontal surfaces throughout the test and don't lock the axles nor the chassis, so the car is free to roll forward and backward, then your COM height measurement will be relative to the heights of the rolling centers of the front and rear axles. Of course, that's quite doable and legitimate as well, and might actually be preferable because it's easier.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/18/22 12:53 AM
Today had a few hours to play with the car and the scales. It was a day to work on CG technique and just get a feel of doing the whole weigh in thing.

Once you set the four scales on the floor, attach the leads and then zero the scales. The first thing you find is the rechargeable battery in the digital box is dead, won't take a charge anymore despite having it plug in for 24 hours. To operate the scales you need to leave the charger plugged in and fly that way. Battery is one of those antique 9 vdc NiCad from a decade ago. I'll deal with a replacement before the scales are returned.

Once zeroed jack up the car, place the scales under each tire, physically settle the car back to near ride height and look at the results. Yesterday a trip to the Sunoco pumps and 50 bucks squeezing in as much gas as possible, everything out of the trunk except the spare and jack, now time to turn on the scales.
OH MY!!!! Just sitting, no driver this fat turd put on almost a 100 lbs from the scaling several years ago. 3710 lbs today, I just can't believe that.
Last time was 3610. I know i have maybe 4 more gallons of fuel in the tank from last time, 35 lb, a catch can installed, four 17" wheels and tires that are a little heavier than the old 16", a cup holder, an A/F gauge, a couple other things that added a few pounds but, 3710!!!!!

Next is jump in the seat and scale, 3880, when we two driver the car it then becomes 4055, OH MY!!!

Next is jack up the rear and put 11.5" under the rear scales. That transferred 29 lbs from the rear to the front. This was just a see what happens test, shocks are still on the car. And I didn't follow the procedure correctly when allowing the computer to do the math and it was skewed. Couldn't get the vertical CG answer from the scales readout.
Will pull it all down again soon and go through the whole procedure correctly again beginning at the zeroing. As mentioned it's a procedure.
Will then remove and replace the shocks locking the suspension travel at ride height and test again.
Yes it's all fun and games with probably no real useful info except I know the car is a fat pig.

I did bring the numbers from the testing today and plugged them into one of the calculators on line and it said 20.4" vertical CG from my inputs. I really do hope that is not true, that's terrible. Doing the weight transfer math 1.9% moved forward with the 11.5" cribs. Some say 10" or more is good, some say a lot more height is more accurate. My Lincoln floor jack is near it full extension. Next would be putting the jack on a 6" solid crib and also adding 6" to my wheel cribbing, 17 1/2" would then be possible.
We'll see how all that plays out over the next couple days.

I forget what that saying is about insanity? Something like this, "Insanity is doing the same thing, over and over again, but expecting different results.”
I'll keep playing that the scales until I get less than 20" height.

Posted By: MAP Re: Rear roll center improvement. - 09/18/22 01:21 AM
4 gallons of gas: 25.0lb.

Bob, some of those numbers just plain don't make sense. How exactly were the tires and car supported during the test?
Posted By: mmc427ss Re: Rear roll center improvement. - 09/18/22 02:33 AM
Weights I feel are 100 % correct from the scales. Fixed measurements accurate. Yes i was generous on the 35 lb gas weight, brain fart was thinking 8.4. That 3710 number threw me for a loop.

The car is sitting with the front tires on the front scales, the scales are sitting on my concrete floor, where my alignment rack is setup when i need it. The turnplates for the alignment rack and the front scales are set up in the same location on the floor. I neglected to set the front scales up in the same manner as when the turnplates are set up for alignment. Each turnplates is leveled in two planes and then both turnplates into the same height plane. hope that makes sense. I learned that from shimming those turnplates dozens of times. Now each plate has specific shims and location on floor, Just makes all that easy.

When I do the second go around with the scales they will be leveled and in plane, doesn't take long to do once you know the drill. I think front corner weights will change a hair, but no big deal.

At the rear tires after zeroing and the front tires have scales under them next is the rear tires just set on the scales. Now all four corners are pushed down to measured ride height using the ride height gauge, the drywall T square, great toy with lots of uses. Gotta be 2 decades now with 24" to 30" inscribed markings on it.

Next press a couple buttons and corner weights, F-R % bias and total weight, stares you in the face. Ouch.

There's a process to input measurements for wheelbase, axle height and how high the stand is and I inputed that after the fact, not before, my bad, but was anxious to get a number and knew i was also running out of play time. But also knew today was just a run through.

As I mentioned using the Robineete calculator it said 20.4" CG. I'd love to hear ten other people say they know the CG on their car and they were surprised how high it was.

Ya know all this CG is just to know what the moment arm really looks like. I said to Bill in the shop the other day, it can be like a short breaker bar or a short breaker bar with a 12" pipe on it. It can get pretty serious torque wise quickly.

Posted By: MAP Re: Rear roll center improvement. - 09/18/22 04:12 AM
OK. The fact that you have to enter axle height (front and rear; they should be slightly different,) tells me a lot.

1. So you start with the four scales in a common plane and exactly horizontal. Right?
2. You park the car on top of the scales. The car is free to roll, but because the scales are horizontal, the car stays put. Right?
3. You get in the car and press a button to get weight measurements at all four corners. Right?
4. After this, or maybe during this, you press another button and get the scales to read zero as a reference. Right?
5. You get out of the car and jack-up the rear. Right?
6. You set the two rear scales on a pair of 11.5" box-like stands with horizontal top faces, and set the scales down on those, directly beneath the rear tires. Right?
7. You lower the rear jack so the rear tires now rest on the scales. Right?
8. You get back in the car and press a button to get the weight changes at all four corners. Right?

Just curious what all the numbers were, and what is the accuracy of the scales? Thx.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/18/22 06:10 AM
Mark once I've done the second go around and know I got good numbers from the second, more refined setup I'll post some numbers.
As said first go around was just to get familiar and see what would make the second, third or fourth number crunching the most accurate.

Intercomp claims .1% accuracy.

Andy won the 3rd UMI KOTM.
Posted By: MAP Re: Rear roll center improvement. - 09/18/22 07:57 AM
0.1% of 2,200lb is +/- 2.2lb, which is 7.6% of 29lb. By the way, is that 29lb for the axle, or for each scale? I'd contact the manufacturer to get a more precise understanding of the kind of uncertainty involved. It may work differently when you're zeroing the scale to get a difference.

But that aside, I'd still strive for a higher inclination.
Posted By: 1 Slow SS Re: Rear roll center improvement. - 09/18/22 10:21 PM
Have you targeted a goal from where you are now?
Posted By: MAP Re: Rear roll center improvement. - 09/18/22 10:59 PM
Bob, just jumping ahead a bit here, if you say you're actually going to do something about your COM position, I'd say two things: One - fantastic and good for you! The car needs it, at least in x. All you have to do is move the engine to the trunk. Two - I'd be dumbfounded. If back in the day at somewhere around post 1,432,986 of the sway bar thread where the idea of doing a chassis-stiffening experiment was proposed but ignored, if such low-hanging fruit was left on the tree there, then why strive to reach such high-hanging fruit here? (All said in good-natured humor - promise.)

One more thing about scale accuracy: you can duplicate the horizontal phase of the test and add a 20-ish lb weight known with high accuracy to each scale in succession while the car is parked on top of them, and see how accurately each scale reports the weight difference. This will tell far more than any commentary from the factory. If there isn't enough room on the scale for the weight, then divide the weight into parts and place the parts around the tire for a tighter grouping - you get the idea.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/19/22 12:58 AM

In the shop is a bathroom scale, 169, at my cardiologist office last week I was 170 on his fancy scale, second gross car weight to check was me in the car, weight went up 170 lbs I know all those scales can't be dead on and would give then the + or - a couple pound errors.

When these fat tires are on the scales there is barely enough space left to put a small wheel chock in front and behind the tire.

I'm not going to worry about the scales being off by 10 lbs on a 3700 pound reading, or even a little more. Has no real impact on finding CG.

Here's a short video, Intercomp hyping their wireless scales. If you go to their website you will see they are just not some fly by night scale manufacture. They make scales for everything. And i have their digital angle gauge, one of the most useful tools I every bought.

Today droped the car back on the floor, pulled all four scales, positioned and leveled the front scales, zeroed, set the car back on the scales at ride height. Exactly the same 3710 gross saw yesterday. Corner weights were slightly different by a few pounds but went through the correct process to find CG on their processor.
Checked it two times, same number 20.9". When I ran the numbers through the Robinette calculator yesterrday it said 20.4"

Started the next stage of testing which is replace shocks with 1/2" threaded rod. The rear shocks are out and the threaded rod is fitted, bolted in, and looks like an easy way to just tweak the static ride height of the car now. Tomorrow will install the front rods, that allows the suspension to be locked at ride height and removes the springs and shocks as a variable.

Yes Mark I know I'm anal about stuff. I dislike when asking and can't get a straight answer. I've been doing this car thing long enough to know that most often if you want to know the answer you need to find it yourself.
You would think with millions of 4th Gen G's built someone could say yep, vertical CG is here on my car. Love it when the assumption is the car rotates around the cam centerline. How do they know that if they didn't check it?

About the diet necessary to lose a hundred pound or maybe 300, ain't going to happen at my age, or the car's age. The car had been 3500 lb for years when the new engine went in. A lakewood scattershield, block plate, 204 lbs of a Little M block, 4 core copper rad, A/C, cruise control, power windows and locks, line lock, a catch can system, heavy gauge stainless exhaust, the list is long of the pounds added, didn't even list the numerous extra pieces of steel used for structural improvements, or new wheels and tires, all this drives MY curb weight to 3710 with a fuel tank maxed with fuel.
To move some weight rearward mean losing some of the frills and of course a striped down alum LS. So horizontal CG is what it is, with a heavy truck engine up front.

Yes the goal of all this is to see by what means can the rear suspension be improved farther. This is just another exercise.

Posted By: mmc427ss Re: Rear roll center improvement. - 09/19/22 02:35 AM
Found this, interesting.
Quote from:

"In addition, C7’s low-mounted torque tube is gone and C8’s exhaust system sweeps upward as it flows rearward, raising the height of the catalytic converters, Juechter noted. The net result: the C8’s center-of-gravity height is 470 mm (18.5 in), 15-mm (0.59-in) higher than C7’s."

Posted By: SickSpeedMonte Re: Rear roll center improvement. - 09/19/22 04:16 PM
Originally Posted by mmc427ss

OH MY!!!! Just sitting, no driver this fat turd put on almost a 100 lbs from the scaling several years ago. 3710 lbs today, I just can't believe that.
Last time was 3610. I know i have maybe 4 more gallons of fuel in the tank from last time, 35 lb, a catch can installed, four 17" wheels and tires that are a little heavier than the old 16", a cup holder, an A/F gauge, a couple other things that added a few pounds but, 3710!!!!!

Didn't you add a big sway bar?

More tilt (pitch) absolutely equals greater accuracy. The most accurate way, theoretically, would be to maximize pitch to 90 deg by hanging the car vertically from some height (Z-location on the vehicle coordinates) until the front and rear wheels (with locked suspension at ride height) are plumb. Imagine if you had some sort of (magically massless) fixture on the rear of the car with vertical slots, and you could grab the rear of the car from anywhere along that slot and hang it. Whatever height in that slot that puts your tires plumb is your CG height.
Posted By: MAP Re: Rear roll center improvement. - 09/20/22 04:02 AM
Hi Bob - Interesting article about the C8. Yes, glass is a poor noise barrier, especially at higher frequencies. Plexiglass is much better, ironically. COM height of 18.5": Nice! Couple that with the C8's considerably wider track width relative to an A/G body, and you get less lateral load transfer per unit transverse acceleration for sure.

About upward weight creep and front portliness: Like death and taxes. Stubborn in the extreme. I'm just pulling your chain with the engine in the trunk comment, of course. You're at a point like Lance, where any major gain is going to mean doing something truly radical. At this stage, you're thinking more of taking the exterior MCSS body and draping it over entirely new underpinnings rather than modifying the original car piecemeal as most do. The original design is just too limited...

About the COM height calculation and scale accuracy: if you're trying to detect a 14-ish pound weight difference at each wheel and your uncertainty is 10 pounds, you're hosed, Bob. As I said, you really need to check the scale's ability to detect small changes with accuracy rather than getting good relative accuracy on a gross reading. Different scales behave differently and you simply can't afford to make assumptions. For example, if you detect a difference in weight upon tilting of 14lb at each wheel, and your uncertainty is 10lb, then if you calculate a hypothetical COM height of 20-ish inches, your uncertainty will be +/-14 inches. Repeatability alone doesn't ensure accuracy; it indicates precision.

Even if your tires fit the scales to the full, I'm sure you can still do something clever to add a known weight. Just don't put the weight on any part of the car, however.
Posted By: Hunter79764 Re: Rear roll center improvement. - 09/20/22 05:00 PM
Full scale accuracy is one thing, I agree, see if you can grab a 2 lb sack of sugar or a few known pounds worth of metal and see if setting them on top of the tire gives you the right reading. If so, you're in good shape.

I'd also propose doing the measurement at your 11 1/2" (or whatever your existing cradle is), then again at something higher. And maybe for grins, another one lower as well. If they all align, you can feel pretty good about it. If they don't, it doesn't necessarily mean the measurement isn't valid, but the higher lift should be the more accurate. And just checking, you are accounting for the weight of the cradles themselves, right?
Posted By: MAP Re: Rear roll center improvement. - 09/20/22 09:51 PM
^^^ Good point about the weight of the cradles. Or, maybe you're just sitting the scales atop the cradles/boxes/whatever is lifting the rear 11.5"?

Bob - on edit, I ran a check on your numbers. 4,055lb total weight, with 58.75% on the front axle. COM calculated height: 20.7". Assumed axle height: 12.5".

With the car horizontal, I get 2,382.2 lb (theo.) on the front axle. With the rear lifted 11.5", I get 2,414.8 lb (theo.) on the front axle. So each scale is seeing a 16.23 lb (theo.) weight difference.

If you want your COM height calculation to be accurate to +/- 1/4", then the detection of weight difference at each scale needs to be more accurate than +/- 0.50 lb, and even that is quite generous. +/- 0.25 lb is closer to reality (Root-sum-square of 4 tolerances of +/- 0.25 lb each = +/- 0.50lb in the aggregate.)

If your scales are that good, they are worth their weight in gold.

Again, I - we - urge you to run a sanity check on all this. And yes, increase inclination if you can. I like SSM's method if you have the room and the courage to do it.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/22/22 03:13 AM
Finished up with the sanity check today, scales are all cleaned up, everything back in their cases and will return them to my friend tomorrow.

Have a dozen or more tests done and numbers on paper and know within a 1/4" where the horizontal CG is.
Also know from numerous tests of the scales they are very accurate and very consistent. I'm 170, Bill is 225, weights from the barbells, bathroom scales, all show less than 1 lb at 3700 deviations between all four scales. Intercomp says .1% and that would be 3.7 lb at 3700. For 20 year old scales they are awesome. The scale owner said they were 3K buck back then. I'm just one of dozens of cars that have used them from drag cars to dirt track guys. I believe they are as good as portable scales can get. Intercomp scales are now wireless, 1/2 the price too.

Scaled the car several times, actually several several times trying all kinds of way to see how they weighed out.
Did every way of scaling possible. Initially was not happy with the corner weights and wondered how they could be so much different that several years ago when I scaled the car at the owners shop. But realized i had done a lot of work to the suspension since that time. Cut the front spring to drop front height a little, relocated the front LCA to allow 9 1/2 caster. and have 9 1/2 caster instead of the 4 1/2 run previous, added an adj rear sway bar in the rear, double adj front and triple rear shocks, 3rd gen F rear springs cut and measured, and recently installed a 1200 lb front sway bar. ALL these things can and will effect corner weights and most of all crossweights. And as far as the car being a 80 lb heavier than last scaling way to many things changes and sum of the parts added up.
Don't really care the car is fatter, it is what it is.

For many years have known what the measured ride height has been to the 1/16" on all four corners, have measure and logged it dozens of times. A few years ago i did the rear spring project, made a spring rater so i could check rates on the four different rear springs I was testing. Ended up using a 3rd gen F progressive spring which I cut down to get the rear ride height I wanted. 3rd Gen F rear springs are tangential at the top and can be cut to fit. Both spring on my rater where cut to achieve the exact same rate at that the same compressed height. Effectively were equal springs in length and rate at the time. They were installed and ride ht was higher on the right side, as it has been forever, That right rear ride height is always 3/8" higher than the left rear.
Several years ago when the body bushings were done i tried to correct that and found it was most likely due to body irregularities, and possible frame. Just another reason they put Chevy badging on the car. You live with that.

Fast forward to this week and I was seeing my left rear scaled weight 75-79 higher than the right rear side, and this also carried into the front corner, making the right front higher by a lot also. Effectively whether I weighted the car as static or with driver the cross weights were way off. Yesterday as a test i pulled the rear springs and reversed them side to side and instantly the crossweights got MUCH better, both in static and with my butt in the seat. After that swap all the testing showed much better crossweights and rear weights with a driver was only 24 lb.

On my car when a 170 driver is in the seat 75% of his weight will apply to the left side and 25% to the right side of the car. This is good to know info when a pass of 170 is in the pass seat. I guesstimate the side to side balance of the car is very slightly effect but horizontal and vertical is minimal.

So the big question after multiple testing looking for that magic number. On the scales I used which calculate the CG for you when you install a scale ON TOP of a 11.5" crib is 20.5", same number was achieved several times. When I go to two different online calculators and run the numbers they come up with 20.3 and 20.4. On the CG calculators you can make small change in dimensions and weight to see the effects on the final number.
As i mentioned early on a + or - 1/4" variance i was expecting so 20.3 and 20.5" is well withing my expectation.

What did I learn:
Removing the shocks and installing solid 1/2" threaded rod and locking it to my ride height specs showed a marginal difference in CG. This due to the 700 lb front springs on the car. Lower rate front springs should show more of a weight shift difference and should show a different number for CG. How much, minimal i think.
But the solid shock 1/2" rod take all the spring rate choice issues out of the equation. Installing them and locking them down to ride height just made things a little more reliable.

Would going higher produce more accurate CG number, I would say yes, how much i think is insignificant. My good 30 year old Lincoln floor jack was max in height to allow the crib and scales to slide under the rear tires. I would not be afraid to go higher but that would require at least 6" to make a difference and safely go 6" higher is a chore.

Was able to see my static corner weights, my butt in the car corner weights and cross weights and in the end correct with a rear spring side to side swap with made corner and cross weights MUCH better.

Being the huge front sway bar project was finished a couple months ago I was concerned about how adjusting the solid 1/2" rod end sway bar links could effect the front corner weights. I knew when I did the final install of that bar and was installing the right link that there was preload on that link. I had to raise a front corner just a hair to install the last link bolt. If that bar's rate is 1100 lb per inch, and the 12-20 threads of the link equals .050" per one turn. 1100 divided by one thread of .050" would be 55 lb of preload on that link, Rounded off say 50 lb per .050".
When my butt sits in the driver's seat the left front corner weight is increased 71 lbs.
The point is even if i set the bar up at zero preload when i get in the seat it will preload the driver;s side of the bar 71. So at this point I will not concern myself about how much preload the bar has now. Because my front L to R only has 33 more on the L when I'm in the car.

Today after days of playing I removed the 3/4" threaded rod from all four corners. Pulled the scales aside and pulled the car out of the shop, then down the street, then back into the shop in the opposite direction. Jacked the car up and after relocating each scale to the proper tire dropped it back on the scales. Each scale is dedicated to a tire, FR, FL, and the wire is color coded to that scale.
Turning the car around was to see how much of a factor the level of the shop floor is.
The reading on the scale showed the total weight went down 7 lbs to 3694, a 7 lb difference. The now rear tire scales, closely positioned where the front tire scales were earlier, showed an 8 lb total loss collectively and the front tire scales showed a 1 lb gain.
What i learned was my floor is not perfect, I didn't laser set the four scales to exactly the same height. And ideally the place to do a four corner weighing is on an alignment rack that is dead on level in every plane. And a rack with the ability to raise the front or rear via built in lifts would allow much higher lifting of the front or rear to put cribs under the tires.

Did any of that make a difference in anything in the world today, NOT. But I'm a need to know guy, and i think I know now.
Tomorrow is rain, may have time for one last thing on the car concerning CG, May have time to return the scales,. May be able to get state inspection stickers and maybe even some time on the alignment rack in his shop. If it isn't raining.

In my quest for info i ran across this from 20 years ago. I was a member here in 2002 but still busy over on the Monte Mailing List. Mark, Marcus and even Norm contributed to this discussion. This might have been Marks introduction to lie in the forum.
In that post is talk of CG, with a 20" number thrown out there, "center of gravity height (guess only) of 20"" hmmm.
Posted By: MAP Re: Rear roll center improvement. - 09/22/22 08:16 AM
Bob, Bob, Bob. You're making this much harder than it needs to be. Get a weight that's in the vicinity of 15lb, but known with very high accuracy. (I presume you have other scales that you trust for this range of weight?)

Get the car on the scales, level or not level. Suspension locked or not locked - it doesn't matter. But don't get in the car, because if you shift just a bit during the test, you'll invalidate it. Now, add that 15-ish lb weight to each scale in succession, and watch the change in weight that each scale tells you. 1. Is the increase within 1/4lb of what you know the added weight is when you measured it by itself? 2. Remove the added weight. Does the scale return to within 1/4lb of its original reading?

If you get a total of 8 "yeses" (i.e., 2 at each wheel,) you're golden and can trust your COM height calculation.

If not, you're more-or-less hosed.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/22/22 09:32 PM
Apology Mark. LIE was not the intended word, it was live. Not sure how i missed that correction as I usually catch most of my errors, misspelled.
"This might have been Marks introduction to lie in the forum".

Mark, did all that testing on the scale's accuracy and satisfied they are accurate.

Today scales went back to their home.

I'll be back in my shop tonight to wrap up my paperwork on the insanity. Today with time to play I made a plastic plumb bob and have it hanging from my rearview mirror. It is located near exactly the CG of the car. Interesting picture. May have it posted later.

This Sat three of us are making the trip to Cecil County Dragway. Me it's just my annual see how it pulls in 3rd and 4th and hope it hasn't lost a step. Been 15 years since i built this truck engine. When i put the McLeod twin with organic discs that ended the use of slicks. Now you just run what ya brung.
One friend is trailering his 65 Olds wagon with the new Rocket Racing 460 something inch. He's sorting it out and hoping to get into the 10s before it gets to cold here. Four new tires went on it last week, Mickey and Moroso got real expensive now.
The other friend will trailer his 69 Camaro with a 327, he's run 11.80s. I may take my 13 year old with me if he's interested. The 9 year old has a baseball game so he's out. He would love Cecil, starting line spectating he would love, it very up close, and personal.
Posted By: 1 Slow SS Re: Rear roll center improvement. - 09/22/22 09:41 PM
Hey Bob,

Why didn't you put the car on your alignment rack? It would have made things much easier for you.

Secondly, I would definitely invest in true coil over shocks at all corners with remote canisters.

Posted By: SSLance Re: Rear roll center improvement. - 09/22/22 11:43 PM
Just now had the chase to read and catch up on this thread. I'm impressed with Bob's can do attitude!! I had a hard enough time just swapping out the ignition components on my car before last Saturday's race... laugh And that change actually made my car faster!!

I'm actually a little scared to scale my car again now. I can't remember what it was the last time I scaled it but I know it's gained weight as well. I lost 25#s myself though do it may be about a push.
Posted By: MAP Re: Rear roll center improvement. - 09/22/22 11:57 PM
Hi Bob,

Lie - not even the slightest problem - I hardly noticed it, and knew one way or another you meant nothing bad. But too bad about the missed opportunity to really test the accuracy of your COM height determination. Based on everything you've said here, it looks like you can trust a calculation of 20.5" to within a couple of inches.
Posted By: 88ssBrent Re: Rear roll center improvement. - 09/23/22 01:21 PM
Couple of Bob's pictures.

[Linked Image from]
[Linked Image from]
Posted By: 1 Slow SS Re: Rear roll center improvement. - 09/24/22 12:26 AM
Car looks pretty clean still.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/24/22 01:47 AM
Just had to make that little red nylon plumb bob, couldn't resist showing where the horizontal, lateral and vertical CG of my car fell. The cool thing every time I'm in the car now I can look down and imagine that CG and how it is moving around. Is it exactly correct, maybe not if you want to measure down to the 1/8", but damn close for my car. And is that where all Gs fall, absolutely no. Most every car is different, some may be the same. It's just the CG of my 86 with all it's particulars.

Never owned a set of cribs before, now need to find a spot to store two.
You can see those sweet ATR stainless tailpipes and why i would hate to throw them away in favor of a panhard.

From all the info obtained from dozens of scalings at ride height and at 11.5" rear raised no mater whether I use the processor in the Intercomp scales or one of those online calculators they all spit out the same number, 20.5" + or - 1/4" when you input the same weights and measurements from the scales. "20.5" within a couple inches", I'm a lot more anal than that.

In process of doing the paper work that shows the testing and results in a simplified manner. Every time I study the results i see more data that is used to get other results. For anyone that has been around oval of road course racing or even drag racing scaling a car is done often and with purpose.
Also now can see the weight changes that happen with just a driver and then a pass added and it's net result on corner and crossweights.
This scaling insanity has put the car back in what I think is good balance with one or two 170 pounders in the car.
Now ready to do some looking at ways to effect roll center and with a static CG height can get better idea of how all this stuff will interact.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/24/22 03:14 AM
Ron that is the advantage of owning , servicing and preserving a G-body for 30 years of ownership. Only 10 years as a daily driver for the wife. Car now has 174k miles on it and would say you would be hard pressed to find another 50 footer G with original paint and interior that has the body integrity and lack of corrosion this old turd has. One of the reason i don't mind working under it, no rust.

As far as sticking coilovers and reservoir shocks on the car that's not in the cards. Us old guys remember the progression of shock development for the past 50 years. My couple year old 72 Chevelle SS had good, as in good for the day, factory shocks. Gabriel or Monroe were the available replacement/ungrade over the counter shocks 50 years ago. Our MCSS came with OK factory shocks for an F41 suspension. When in 2000 it was time to upgrade the shocks you could now get Bilstein, Koni, KYB and some specialty shock companies then now offered a single adj via a knob on the shock shock. I opted for the Koni Red in 1999 for the 86 because it was a rebound adj shock.
About the same time I bought a used 95 AWD Astro and tore the entire front suspension out for freshening, installed Bilstein on all four corners. Between the Koni on the 86 and and the Bilstein on the 95 both had the best bang for the buck shocks of the day. And you knew it when driving them, very good over the counter shocks of the day.

Fast forward to about 2010 and the 86 was going through another phase of development. Street manners and drag racing was the priority. That's when the Viking DA shocks went on F and R. They were the least expensive, great quality, way to get into a DA shocks that could be tuned for all types of uses. Have since swap the rear DA for the Viking TA and find for the most part the rear DA would have been satisfactory for my needs.
I knew the price of shocks got out of hand about that time as I had friend's running drag cars with 1000 a shock SA Penske that were dyno tuned for their cars. Soon after the reservoir shocks appears in the marketplace for anyone with money and the need. My thinking is it was the stadium and rally trucks that drove the tech and the need for way more suspension control than any off the shelf shock provide for them. The net result was the trickle down soon found Fox (Ridetech) and a host of others now filling the market with very expensive shock few needed and few could ever justify the need for. The Ridetech reservoir/coilover alone for my car is 2 grand an axle.
And if you would install the fronts, new coilover LCAs, throw on a set of AFX spindles, 13" brakes, and there is a pocket full of cash outlay necessary. And as we know the collection of parts we can throw at a G has progressed a piece at a time. And as we know the market is now saturated with parts that may or may not play well together.

For me just can't justify that outlay after all the trials and tribulations of suspension upgrades to the 86 in the past 23 years. Few have all the mods done to their car not on the cheap but on the necessity when nothing was available for these cars.

Ron I think you now have a better grasp of the marketing and availability of product for our cars. Being able to drive an old SS over to the Rod Shop and bring home a slick, one off chassis with all the right parts to impress even our car show gurus to drool over. Where do we draw the line on our need to impress others, or us.
Pretty much every Winter another suspension upgrade, or attempted upgrade, or insanely silly some would say upgrade on my car is done, not on the cheap, but trying to improve on a turd.
Posted By: MAP Re: Rear roll center improvement. - 09/24/22 07:53 PM
Hi Bob,

One of the reasons why your COM is as low as (it seems) to be is because virtually all of the weight you've added to the car is at or below this height. IOW, I suspect a more stock MCSS would have a higher COM. I'm also guessing your MCSS has a lower ride height than most MCSSs, which is another factor that would portend a lower COM height. Btw, can't resist asking, but how high is the center of your camshaft? (I'm not asking to suggest this means much, but I'm still curious.)

I'm glad you have a lot of faith in your calculation of COM height, but I don't share all of your confidence.

Btw, I think you've done a terrific job of getting the most performance for your money with your car - congratulations.
Posted By: 1 Slow SS Re: Rear roll center improvement. - 09/24/22 10:51 PM

I would love to buy the Roaster shop chassis for street drivability and ease of installation. Unfortunately I can't justify giving up my new, last Camaro SS for the same money. I have a complete Speed tech set up with AFX spindles and an extra frame. I'll buckle down this winter and build a fresh chassis for the old SS while enjoying the time.

As fare as remote shocks go, I suggested that as I thought you were taking the three link dive and whacking the rear tail pipes. Your exhaust is one of the cars strong points IMO. I like the fact it still retains the factory cross over position with large matting Y pipes that are nearly impossible to find these days.

These cars aren't turds, they're very cool! Back in the day I drooled over all these G bodies, SS, 442, Hurst Olds, GP, and the GN's.

As fare as your 50 footer goes, I wouldn't be too hard on a car that old that retains it's factory paint. It's a miracle the car is still alive after being a daily driver for ten years, looking as good as it does. Mine had a 114K on it and 24 years of being a spring, summer, fall driver and it was in worse shape than yours.

Posted By: mmc427ss Re: Rear roll center improvement. - 09/25/22 09:22 PM
TURD, is a term of endearment when referred to my 86 SS. Since the early 60s when the A, B. G suspension platform was created by some guys with the mandate to improve upon the 50s design we were stuck with it for nearly 35 years, up to the 90s B-body. We all know economics drove the production of cars for decades, not how they would compete on a road course, or autox or even when pushed on the street. It was do what was need to sell cars, and make big profits. The result is today we try to compare today's computer engineered mass produced by robots cars and compare to the turds they sold us 30,40, 50 years ago. I think most of us that have these old cars know the battle to catch up to the new tech is almost insurmountable. The new tech's benchmark is always moving to a higher standard. The parts expense for us to try to fix these cars is, was, will always be, expensive to dream off.

So that said for me with a limited budget for car parts, no 20K chassis in the budget, and obviously a limit education on design or engineering I've stumbled along making changes that hopefully will improve on the turd's original design. Today we have the internet and it's many avenues to look at what others have done with turds. You will find everything from the 200K 1st Gen F body and A-body builds to the budget builds, all though only budget in their mind, of odd ball cars. All with the same intent, polishing a turd and trying to make it competitive with today's cars. It has been done by some, at a high cost in one form or another.
Enough said about turds.

My grandson and I made the trip to Cecil yesterday to educate him on drag racing, make a few runs on my car and watch two friends trying to make fast runs. Previous 6 years or so Cecil hosted the Buicks event, old A's and 4th Gens. Not sure what happened but some kind of in fighting, maybe, and only the old Buicks were there, saw no GN and only one sweet T-Type. But can say it's cool watching 70s Buick running 11s, 10s and some even as quick as 9s on all motor, carbs. Also there was a nostalgia Super Stock event, again 60s and early 70s race cars. Those cars bring back the memories from my time at the tracks back in the 70's, cool.
Only a small hand full of new stuff, a Hellcat and SRT Scat Pack, a C7 ZO6 and the 3.
At Cecil you are 10' to the side of the start beams, I placed my grandson and I to watch launches there. The FAST Chevelle on skinny redlines and the Tesla 3 that made zero noise and zero tires spin at launch he found interesting. Loads of wheel stand leaves.

That 3 ran a 12.75 @ 114 which i thought was slow for them. I made three passes of which only one was notable, 12.77 @113.1 on a 1.89 60'. Respect for my 20 year old T56 demands granny shifting through the gears. No idea how the Falken RT660 tires would behave causing me to blow them off the first run, and a missed 3rd gear, made that run a waste of the first pass. But I come to expect that on your first pass on something you do once a year. I don't get embarrassed anymore.
My third and final run just showed near the same mph, 112.6 and verified i was on the rev limiter at 6200 at the top end, I knew that was possible with these short tires. Spun through 1st and 2nd, hit the limiter in 2nd and my grandson said he saw flames out the tailpipes, go figure. Ran a 13.3 on a 2.09 sixty. Not a good run but over with.
The autox and now 1/4 miles runs need the 3.73s back in the car.

Both friends cars made 3 passes.
The 65 Cutlass station wagon ran a best of 11.85 @113 on a 1.69 sixty. The ATI 3500 stall converter is a problem, it get pulled and sent for a fix, way to loose.
The 69 Camaro with the little 327 ran an 11.8 at 114, on par for the car. I had to jump his battery to trailer the car.

This engine is now 15 years old since I put it together, over a hundred passes at the 1/4. For the life of this engine every year several 1/4 passes made to insure it's still healthy enough to run down the road with only a flashlight and wheel lock key for tools. Sorry, forgot the folding chair in the trunk.
Next weekend, weather permitting is 16 more autox runs on the car, near 80 in a little more than a year.

Had goals for this year with this car:
32 autox runs, check, should have 48 after next weekend
drag race runs, check, made three, one OK
2000 miles, nearly a check, 1850 as of today
suspension improvement, check, front sway bar
minimum downtime, check, maybe two weeks
car shows, check, a couple is enough anymore
gas costs, check, spent a lot
Fun, check, no tow trucks yet

Posted By: MAP Re: Rear roll center improvement. - 09/26/22 01:15 AM
Well, all of your viable options for the rear will - guess what - add weight to the car. At least it will be on the side of the car that needs it wink. Added weight would probably be in the vicinity of 30lb, maybe more (50llb?) if you tie the two frame rails together horizontally for the frame-side of the PHR mount, which you really should. Too bad that roughly 20lb of that will be added to the rear's unsprung mass. That's the last thing it needs.

It looks like you've packaged the rear so precisely that anything that would upset that more than minutely, would probably require chopping into your exhaust system. If the rear pipes can be separated from the mufflers, it looks like you could rotate them (~ 10 deg?) to splay them apart for more clearance around the top of the differential, and possibly enough to clear a RHS axle mount for the UCA. Of course, then you'd have to add about 10 deg of further bending prior to each straight-pipe exit to restore the ground height at each exit.

Btw, don't forget to tilt the axle PHR mount rearwardly to fit a longer UCA for better suspension linearity.

Clearance for the PHR itself is hard to judge from the photo.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/26/22 03:28 AM
A few posts ago in one of my ramblings I noted a post from way back in the early days of the forum.
I had wrote this with this link at the end. Sorry Mark, should have triple checked that.

"In my quest for info i ran across this from 20 years ago. I was a member here in 2002 but still busy over on the Monte Mailing List. Mark, Marcus and even Norm contributed to this discussion. This might have been Marks introduction to life in the forum.
In that post is talk of CG, with a 20" number thrown out there, "center of gravity height (guess only) of 20"" hmmm."

It should have been this link:
In my quest for info i ran across this from 20 years ago. I was a member here in 2002 but still busy over on the Monte Mailing List. Mark, Marcus and even Norm contributed to this discussion. This might have been Marks introduction to life in the forum.
In that post is talk of CG, with a 20" number thrown out there, "center of gravity height (guess only) of 20"" hmmm.

Have now read over that 11 pages of discussions several times trying to see where that 20 years old topic has progressed to after all that time. Aside from a hand full of people who currently are running a three link on a G, inclusive of a few here now, I can still not see any clear path to the fixing of the wows of the G 4 link.

Mark i may be wrong but if i lower my ride height two inches the CG will drop two inches, but will be exactly in the same place as it is now, but 18 1/2" instead of 20 1/2". The end result is the location of the CG in relation to the car is still the same. That height in relation to the roll created will change. Lowering the car 2" will change the RC produced by the 4 link. My enter effort to locate the CG has to do with how to effect the RC via that moment arm. If you don't know where the CG is you can't know it's effect on the RC.

Next up is located the RC in the rear, plumb bobs, angle gauges, measuring sticks and more modeling coming soon. It's a Winter project.
Posted By: MAP Re: Rear roll center improvement. - 09/26/22 06:29 AM
Thanks, Bob. Aaaahhhhh - the good old days, when discussions were more patient and more in-depth. We actually wrote in complete sentences back then: "I went to the store," instead of "Went to the store." I miss Norm a lot, because he added lots of technical knowledge, and kept the discussion at a high level.

Anyway, little has changed. Blue Hog's comments about being able to push a G-body rear much harder than a 3rd-gen F-body rear remain somewhat a mystery. Marcus, in reference to the 2nd-gen F-body, told me just the opposite.

Then, as now, the A/G body's dismal 3:1 sprung/unsprung rear mass ratio was plain awful if you care at all about handling on bumpy surfaces. As I've mentioned in other threads, the lightest way to suspend the rear axle remains the OE C4L and its flipped upside-down counterpart, the Satchell link. Everything else, including the 3L-PHR, is heavier. But nothing will substantially beat 3:1 at the rear until and unless you switch to an IRS. I don't want to belabor that further here since I'd just be re-hashing what I've written in other threads, but IRSs on flat road courses with a front-heavy car aren't going to win against a live axle with generous anti-squat. IRSs shine on the street with rear-heavy cars where, once again, bumps actually matter.

About COM height: if you lower the suspended car by X, the COM height will probably be reduced by roughly 85% (+/-) of X, because once again, you're dropping the sprung mass and not the unsprung mass, and elements of connection between the two will divide the height difference. So to say that the COM location relative to the car doesn't change isn't really correct. And, if you lower the car, the rear RCH will increase, so the moment arm one would define between the roll axis and the COM would decrease in a compounded way. (You'd need to look at what happens to the front RCH too in order to define the whole roll axis, so this aspect is open for the time being.)

But I'm still confused about how you're going to leverage (no pun intended) the knowledge of the COM's location in relation to the car's roll axis? A shorter moment arm is good because it means less roll in cornering for a given transverse acceleration and roll stiffness, but that's far from the whole story: if it were, you'd just set the front and rear RCHs to 20.4" as you've found for your COM height, and voila, a roll-less car! Of course, its handling would be downright evil, however...
Posted By: SickSpeedMonte Re: Rear roll center improvement. - 09/26/22 03:27 PM
The COM of the sprung mass is what you use to calculate the moment arm between the roll center and the COM, so a 2" drop would reduce that moment arm by 2", assuming you could maintain the same roll center. So if you really want to make use of your COM number, you'd need to use it to calculate the sprung mass' COM. You do that by weighing the axle and anything attaching it to the frame (separately), as well as the front upright/brake/wheel/tire assembly and anything connecting it to the frame, estimate each component's COM as-installed, and then do some maths. The sprung COM will be higher than the total COM that you measured, as most, if not all, of the unsprung mass is below 20.4".

Even still, you see that with such a high roll center in the rear compared to the COM height, your rear roll stiffness is much more influenced by link forces than the front.
Posted By: MAP Re: Rear roll center improvement. - 09/26/22 09:39 PM
I mostly agree but tire compliance influences the roll axis as well, causing it to migrate toward the ground. To that extent, unsprung mass still participates in roll. As Greg Locock says, "Cars don't roll about their roll centers." But from simple geometry, however, we imagine that tires are rigid, so I agree that it's the COM of the sprung mass side that counts - thanks, SSM.

But I repeat the question, Bob, how are you going to act on the knowledge of the location of the roll axis in relation to the car's COM? (Let me clear up a potential ambiguity here: in a gravitational field with zero gradient, i.e., gravity is constant, the center of gravity and the center of mass are the same thing. So "COM" and "CG" are practically the same.) The goal of minimizing the moment arm will lead you toward increasing, rather than decreasing, the front and rear RCHs, and that, in turn, will lead toward a car that handles horrifically over bumps.

Or from a different perspective, what are you going to do differently about your rear suspension to influence (geometric) roll axis now that you know where your COM is? I would argue that within the typical confines of how far we can move the COM and the (geometric) roll axis, the two can be thought of entirely independently.
Posted By: mmc427ss Re: Rear roll center improvement. - 09/27/22 02:47 PM
Not sure if I had mentioned I've been following Craig Coburn's video channel on the building of his 1966 Volvo. When I was getting started on the front sway bar project I found his build and quickly became a follower. Craig is, was, a college prof in Canada involved in world geography. A very bright guy who isn't afraid to fail and continues to evolve the car. Unfortunately in Canada the pandemic's strangle hold on sourcing material really put a bind on his build and slowed it immensely. That caused delays in his build and deviating from goals. I find him very entertaining and very much like me in the "need to know why" aspects of cars.
The interesting thing about Craig is the areas of car building he gets into, it's every piece of the build.

There are 80+ videos on his channel involving his build, from making carbon fiber body parts, hammer forming stainless, making his own tools, but most interesting his Volvo's suspension build. Whether it's the rear IRS he installed using the Supra rear parts or his adapting a C4 into the front of his build it's all worth the time to watch how someone in a little garage goes about finding the problems, and finding cures.

Here's on video which pertains to this post about RC. Very much watching his take on RC.

If you have nothing to do and like watching someone do things you will never find videos of it's Craig, slightly insane but very direct with approach. Here's his Throttle Stop Garage site.

Posted By: MAP Re: Rear roll center improvement. - 09/27/22 09:25 PM
Hi Bob,

That was fun. I have a predilection for Volvos - my first car was a '71 164. Car was a joy to drive - 3L-PHR Dana rear, SLA-IFS, But the car was tall and narrow by today's norms, and the skinny stock tires resembled bicycle tires more than car tires. Unibody. Yeah! Around 2,800 pounds as I recall with a straight six, with an interior cabin volume that was close to that of an A/G body. The Volvo in this video is an older model (122?), but still, quite tall and narrow by today's standards. I haven't watched any of his other videos but I'm sure packaging wide-body C4 things into a narrow-body Volvo was a MAJOR headache. (An A/G body is somewhat intermediate in width compared to a C4 and a Volvo.)

So he describes two ways of getting the RCH: one by getting an instant center as defined by the links as they move in response to vertical suspension motion, and another by plotting the lateral migration of the center of the contact patch as the suspension moves up and down. It's a shame he didn't plot this in the same way as he did for the link-centric definition, but his method amounts to constructing a normal to the path of this CPC migration, and seeing where that normal intersects the vehicle centerline, where the height of that intersection point would be the CPC RCH. So, two different methods, and two different RCHs.

Note that the first method is independent of the lengths of the links. But the latter is highly influenced by the length of the links. In fact, the latter is a function of the former, but with the modification of link length.

So I think (and I hope SSM comments) that the real question is, if the two methods are related but different, which one is more relevant? Which "counts" more? Or more fundamentally, why do we even care about RCHs?

First, let me use acronyms to reduce typing. I'll call the first method, which is the one we see in videos and in texts, the "LIC" (Link Instant Center) method, and the latter the "CPC" method (his acronym - Contact Patch Center.) Let me use another acronym as well: LLT (Lateral Load Transfer.) I'll further take this to be considered per unit transverse acceleration, which amounts to the car's lateral acceleration at a given constant speed in a turn of constant radius. We could take that acceleration to be 1g, for example. LLT is more-or-less directly proportional to lateral acceleration, so I'm talking about the ratio of the former to the latter.

If we ultimately care about LLT in a turn (btw, SSM, your previous reference to roll stiffness is really a reference to LLT) then the LIC method is what counts. The CPC method is a measure of lateral tire scrub and amounts to a forced tire slip as the tire rolls over bumps or the car rolls. It only has a transient effect on forces imparted to the car as the suspension moves up and down. That is to say, it has no steady-state consequence. LLT, on the other hand, has very important consequences in a steady-state turn. This is because it determines the difference in vertical tire loading between the two tires on a given axle (that, plus the spring/sway-bar contributions in roll.) And this difference determines things like ultimate lateral grip (going back to how tries don't obey the laws of Coulombic friction,) and induced tire slip angles, which brings us to the matter of understeer/oversteer when we also consider what the other axle is doing. LLT is gotten from the LIC method plus the spring/sway-bar contributions in roll.

Again, CPC is a transient-only effect and I think is relatively unimportant. LIC has transient and steady-state effects. CPC does matter for transient bumps, because it imparts a lateral force to the car which, in turn, is reflected back to the wheel as a modulated coupling mass. I wrote about this a lot with respect to live axles with a high (LIC) RCH, in that a high RCH couples a surprisingly high fraction of the car's sprung mass to the unsprung mass of the bump-affected wheel.

Anyway, that's my top-of-the-head reaction - I'm probably forgetting something...

Posted By: MAP Re: Rear roll center improvement. - 09/27/22 09:57 PM
Incidental remark, just to muddy the waters even more, as if they weren't already sufficiently muddy:

Bob, two posts ago, I said that one effect of tire compliance is that it causes the car's true roll axis to migrate toward the ground. Generally speaking, but not always, this migration is a good thing. In the case of an A/G body and its stock geometric RCHs of about -2" in the front and about +18" in the rear (or about +6" for the roll axis height at your COM location, disregarding tire compliance,) this is certainly a good thing, at least with respect to handling over bumps at the car's rear.

So have you thought of testing unusually soft tires in the rear, such as what you get with a tall sidewall? Soft tires make the car's steering response more sluggish, but they will improve handling over bumps and reduce your car's understeer. It might ultimately be a faster way around turns, and even better, you may not have to change your rear at all. To test this, you could swap a set of stock MCSS tires for the rear. Of course, ultimate grip in a turn will be reduced (to restore that, go with wider tires and softer compounding,) but it will tell you a lot about handling characteristics over bumps and the car's understeer/oversteer balance.
Posted By: 1 Slow SS Re: Rear roll center improvement. - 09/27/22 11:04 PM
Cool video, and methodology thanks for sharing. I hope all this produces a serious recipe for you Bob.
Posted By: MAP Re: Rear roll center improvement. - 09/28/22 01:21 AM
Heads up: the fun starts when trying to apply the CPC method to the stock rear. Depending on how the rear is constrained, you either get a very low or a very high RCH (!)
Posted By: mmc427ss Re: Rear roll center improvement. - 09/28/22 01:47 PM
I take everything as food for thought. Craig is one of the most interesting builders you will ever meet, talented, smart, persnickety and PPPppppaaaaaatttttiiiieeeennnnttt, patient.
Episode 47 on his sway bar build I watched a dozen times for inspiration.

Had a little time today and being a beautiful day put an easy 30 miles around the block. A stop at a used car lot looking for my quest for a cherry Trailblazer, some cheap Sunoco 94 octane, a blow through at the car wash to clean the rubber off the tailpipes and a trip to see my friend with the 65 Olds wagon. Looks like we will be swapping the converter soon in it.
Back in the shop sitting full tank, ride height, with a bullseye on the drivers door to mark CG height, fore-aft. That on a black door, with ok old gloss, and a white China marker + to stare at for the next couple months. I then lean back in the comfy shop chair, stare at the car and wait for that epiphany. There will be a lot of that going on before something changes in the rear susp.

This Sat morning coming is autox in Harrisburg, Hurricane Ian, may have something to say about that. To early to tell but the weather forecast didn't look good for Sat. Haven't driven the Falken RT660 in the rain, it's said they are ok in the wet, don't need to find out. Autox is a 160 mile round trip.

If I go it will be a cold morning, only 70 for a high, won't need to spray tires. All four previous 16 run days were 90 or higher, we sprayed the last event.
Front shocks cranked up, 14 Comp - 14 Rebound.
Rear shocks 0 L Comp, 0 H Comp, 8 rebound to start. Will back down if, when hop shows up.
Spohn adj rear sway bar is currently and at last event on it's middle setting. As a S&G experiment with nothing to lose will set it to full stiff. I need to see if I can tell how much effect a lot of rear bar has on a car that had oversteer. Set at a full stiff rear bar I have driven the car on many times, it was my drag race setup. For some time after drag racing i would leave it full stiff. Along with that bar setting the LCAs in the rear were dropped to improve the drag racing IC. I never liked the way the car drove on the street with that drag racing combo. Better at the track, yes, also ran a right side bag, and bias slicks. But I had never driven the car with the bar at full stiff and my normal street LCA angle, middle hole.

So nothing to lose, crank up the rear bar maybe tomorrow and be able to burn some more of that 94 before the weekend.

Oh, forgot. In the shop, just staring at the car from all corners imaging how the attitude of the car is as it navigates high speed corners. Sitting back in the chair, staring at the left fender from the front, close the eyes and watch that front left corner as it turns into you, you visualize all that jacking, wow that's a lots, the result of all that caster, how much weight is that pulling off the left rear as that body lifts on that left side, man, that's the inside rear tire.

Good thing there is a couple months to just sit back and think about if there is any hope.

If you take the time to watch a few of Craigs suspension follys, you get a good grasp of how difficult it is to make changes to benefit all aspects needed. Compromise. An effort was made by him and suspension software modeling to try and find his way. He does point out something I've know for a long time and that is life is 3D, suspensions are 3D, to much concept is 2D. In his own way Mark has said that.

Cool thing is I'm doing my homeroom reading, trying to find a pertinent chapter in the Milliken RCVD to read 10 times to understand those boys. it's seem every time those boys did something they built it and beat the snot out of it, then formed opinion and finally some truths from the results.
Posted By: SickSpeedMonte Re: Rear roll center improvement. - 09/28/22 04:32 PM
I watched the roll center video in bits and pieces last night. Getting 25 minutes uninterrupted to myself during the week is rare these days! This is some very interesting content. I think I understand what he is doing to calculate RCH via the CPC method... he's essentially getting a gradient of dY/dZ of the contact patch and then multiplying that by half the tread width to get the height of the pivot point above the tire contact patch, at the vehicle centerline. That calculation simplifies nicely.

So as MAP said, the LIC method is not affected by the link lengths, and it also does not require you to move the suspension. It is a snapshot, so to speak, of the point about which the chassis kinematically rolls at a given instant. As Craig notes, the RCH moves around as you exercise the suspension. So by using measurements at different heights, he is approximating the average RCH location over a range of vertical suspension height. I suspect that if you differentiated the curve to get the gradient at each point instead of taking a gradient from two points at +1" and -1", you would end up with the same RCH at each vertical suspension position as the LIC method. The trouble is there is some noise in his measurements which gets amplified during differentiation, so a simulation would prove that better than real data. If I'm right, the CPC method gives you an average of the RCH over the range [-1", +1"].

I disagree with Craig that an analysis of the pivot points is inferior to his method. By the looks of it, he would have been time ahead to have measured the link positions and simulate it. He seems pretty sharp, and I bet he could develop a spreadsheet to determine the instant center locations using 3D coordinates if a 2D simplification doesn't satisfy him. A 2D simplification to find the Y/Z coordinates of the inner arm pivot would be to take the intersection of a vertical-transverse plane passing through the arm's ball joint with the axis of rotation of that arm. If you want it more precise, you have to define the plane of each arm using the 3 measured points (your two arm bushing points can be anywhere on the axis of rotation, so no need to estimate the center of a bushing or rod end, just use the end of the shaft), then find the intersection of the upper and lower arms which defines an instant axis, and then find the intersection of that axis with a vertical-transverse plane through the wheel center to find the front-view instant center.

See the 6th post down, by Jack Action, with some links to how to determine this mathematically. The last link to use the instant axis to determine the front-view instant center is dead, but extrapolating the axis to a given X location is the easy part. You could make a spreadsheet using these equations and have it spit out the IC and RC locations.

Having a high RCH not only makes the car handle awful over bumps, but it also makes the car act like it has a bind in lateral transients. When you turn in, the rear tires develop lateral force before the body starts to roll. The lateral force, with a high RCH, causes the tires to transfer load to the outside instantly. With a low RCH, you don't get this and the tires don't transfer load until roll rate creates damper forces and roll angle creates springs and bar forces. That can cause imbalances in tire lateral load transfer distribution during transients (turn-in, slalom, etc) and upset the balance of the car.
Posted By: SickSpeedMonte Re: Rear roll center improvement. - 09/28/22 04:38 PM
Bob, I think you'll find that more rear bar will spin the inside tire on corner exit. The Trutrac acts like an open diff at some point if you don't have load on the inside tire. I think I commented to Jason that I felt it doing that when I drove it.
Posted By: SSLance Re: Rear roll center improvement. - 09/28/22 06:41 PM
I would disconnect the rear bar completely before I'd put it at full stiff.

Speaking from a LOT of experience, these cars do NOT like a lot of rear sway bar entering, mid turn, or exiting a corner.

I like your shocks settings...leave them there. But soften the rear bar way up, trust me on this...
Posted By: MAP Re: Rear roll center improvement. - 09/28/22 07:55 PM
Bob, try the soft rear tire experiment. This is incredibly low-hanging fruit.

To be very clear: a high rear RCH means the CPCs of the two tires do a lot of lateral scrubbing as the rear works over bumps. (Yes, at higher, mass-controlled frequencies where the body is essentially grounded.)

With short, stiff sidewalls, that scrubbing "spends" a lot of the tire's traction circle to stay attached to the pavement. If you hit bumps in that condition, you could break loose, and that's not a good outcome in a turn or with throttle or brakes, or some combination thereof.

A tire with lots of lateral compliance "spends" less of its traction circle fighting that lateral scrubbing, yielding better traction over bumps. It also yields a softer ride, but I know you're not much concerned with NVH.

A soft tire does yield a slower, duller steering response, but I'm guessing not of a magnitude that's likely to be significant in a road race scenario. In fact, the reduced understeer and improved traction over bumps (back to the street in this case) could actually make the car faster.

The trick is to maintain the rear traction you have now, so the recipe would be a tire with a tall, soft sidewall, but with the widest, stickiest tread you can fit.

A stock MCSS tire will likely give you the first two out of four, so you'll get a good sense of handling over bumps and some sense of a new understeer/oversteer balance.

Based on past experience, I know you like to stay inside the box, but breaking out of the box can sometimes bring surprising dividends...

Further about the CPC RCH: the center of the contact patch traces a smooth curve as the suspension translates up and down. The curvature of this path is likely to be so small that it could be well approximated by a straight line, at least for modest translations like +/- an inch. The inward-projecting bisecting normal to this line creates an RCH based on its intersection with the car's centerline. We could certainly get mathematically precise about this by invoking derivatives (of first and higher orders,) but this simple geometric method could be done with a pencil and a ruler.

On edit: Yet another occasion where I wish we could all be standing in front of whiteboard! This is very cumbersome to convey with words, but a picture or two would make this easy for even a third-grader to understand. The CPC RCH method is literally child's play, it's so simple. But anyway...


Also, to amplify a bit of what SSM said: the vertical forces imparted to the wheels in a turn could be classified into two broad camps: one of horizontal link-transmitted forces that act instantaneously as the car turns, and another where (ignoring damper action for now,) spring/bar forces develop in response to the relatively slow accumulation of roll as the car starts a turn. Tuning the rear involves finding a comfortable balance for the driver between the instant "kick" of link-transmitted forces, and the slower "thud" of spring/bar-transmitted forces.

If good handling over bumps were the whole object, the rear RCH should be zero. But steering response would likely be judged to be slow, so that upon initiating a sudden turn, we would get a distinct transient response from the front of the car, followed by the rear "falling in" a noticeable fraction of a second later. A one-step two-step response, if you will. Or a zig followed by a zag, if you also will. Raising the rear RCH amplifies the link-transmitted "kick", but it comes at the price of good handling over bumps. So the driver needs to find a subjective balance that works best for him. That's why I recommended starting with a rear RCH of around 8" - 9" to explore this balance.

Btw, needless to say, as the rear RCH is dropped, you need to increase roll stiffness in the rear suspension to conserve LLT.
Posted By: MAP Re: Rear roll center improvement. - 09/28/22 10:09 PM

So yesterday's video with its CPC RCH was new to me. I've been thinking about this a bit more, and just to share:

The LIC method, which is what we're all used to for determining RCH, is the one I believe rightly deserves the lion's share of attention. From it we get link forces and can deduce LLT in a steady-state turn.
The CPC method matters only in a transient state when a wheel is going over a bump. The lower this RCH is, the lower the tire's lateral scrub, and the lower the fraction of sprung mass that's reflected back to the wheel, which is a good thing. I believe the goal with the CPC RCH is always to minimize it.

Tuning the LIC RCH is more important and more nuanced. Making this RCH zero is not necessarily the best solution, as partially evidenced by the kick vs. thud balancing exercise.
Posted By: SickSpeedMonte Re: Rear roll center improvement. - 09/28/22 10:37 PM
I think the RCH derived by the CPC and the LIC methods arrive at the same answers (as a function of suspension position), as the distance between the two measurement points for the CPC method approaches zero. You can't minimize one RCH without minimizing both, as they are really two ways of measuring the same geometric roll center.

Multiplying by half the track width in the CPC method interpolates the line from the tire patch to the IC that the LIC method defines.
Posted By: 1 Slow SS Re: Rear roll center improvement. - 09/28/22 10:45 PM
Originally Posted by SSLance
I would disconnect the rear bar completely before I'd put it at full stiff.

Speaking from a LOT of experience, these cars do NOT like a lot of rear sway bar entering, mid turn, or exiting a corner.

I remember seeing your rear wheel off the ground I believe from too much rear bar.
Posted By: MAP Re: Rear roll center improvement. - 09/29/22 03:38 AM
SSM, on thinking further about this, I think I agree with that. In the y-z plane, as the wheel moves up and down, it is always virtually revolving about the intersecting construction lines running through the UCA and LCA pivot points. Since the wheel is a rigid body, the path of translation of any point in that wheel would have to yield the same effective center. Where things get squirrely from a motion standpoint is that especially for short, unequal-length UCAs and LCAs, suspension travel can cause that IC and thus its RCH to drift in space a lot. For example, for a restricted portion of suspension travel, the instant RCH might draw close to the ground, even though we might consider the nominal RCH to be quite different. So the CPC method immediately reveals the squirreliness of the IC as the suspension moves up and down, while the LIC method tends to be thought of mostly with the suspension near its rest position, where the IC location is relatively stationary. (Btw to get the full IC locus, one needs the first and second derivatives of the y,z path. So we can get quite sensitive to measurement noise.)

Anyway, point is, these different perspectives yield different impressions of what's happening to RCH (as it did to me,) but the same information in a global sense. Another important distinction: we have to remember that the location of the IC derived from the CPC method will be relative to the location of the CPC, and not relative to a frame of reference ["FOR"] fixed to the ground. OTOH, the LIC method customarily uses a FOR fixed to the ground.

So where this gets really interesting is for a live axle rear. With an IFS or IRS, plotting the IC and thus the RCH is simple because it can be seen as a one-wheel system.

Not so with a live axle.

We have to choose how to constrain the motion of the entire rear. We can either anchor one wheel's CPC, in which case the CPC defines the IC, so we get zero RCH as the other wheel moves up and down, or, we can laterally anchor the car's sprung mass, in which case the RCH will be our famous 18-ish inches above the ground at nominal ride height. The former creates a lot of lateral body motion with no lateral tire scrub, while the latter does the reverse. The former amounts to the compliance-controlled low-frequency regime, and the latter to the mass-controlled high-frequency regime, and comes from a Hamiltonian constraint of minimized energy. Bumps usually fall in the high-frequency regime, so this is where soft tires to accommodate lots of lateral tire scrub can really help us. As well, it can restore a more neutral understeer/oversteer balance with a front-heavy car. Or, we can instead drop the rear RCH and increase rear roll stiffness for conserved LLT, reducing the "kick" part, but making the "thud" part sharper. (Softer tires also reduce the "kick" by dropping the rear of the effective roll axis.)
Posted By: MAP Re: Rear roll center improvement. - 09/29/22 11:17 AM
Ron, Lance - that's my recollection too. With limited rear traction, start with the least possible rear roll stiffness and work up carefully from there. Reducing the rear RCH helps too if that's an option.
Posted By: 88ssBrent Re: Rear roll center improvement. - 09/30/22 12:22 PM
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Posted By: MAP Re: Rear roll center improvement. - 09/30/22 10:13 PM
Hi Brent - could you explain that further? Thx. Also about two different RCHs, that's true for any suspension. We normally jump immediately to the fixed-body scenario, so that's where the suspension's effective point of rotation (i.e., that IC) with respect to the body comes into play. The IC at one CPC on an un-bumped wheel thing (i.e., the trivial case of the suspension and body rolling as a unit over a one-sided bump) prominently comes up in the case of a live rear because we're immediately confronted with how to apply motion constraints with two coupled wheels. And must say it's quite useful to think about this in the frequency domain...

But anyway! Hope I'm not making this too sticky and boring.
Posted By: mmc427ss Re: Rear roll center improvement. - 10/01/22 02:20 AM
I need to apologize to Brent, sorry.

That pic is a chart of my scaling of my 86. Brent just helps me out at times posting a pic. Of the dozens of times the car was scale i could break it down to three areas of interest.
First is the three top weights. This were done with four sold shocks, 4SS.
No driver, ride height.
Second is driver, ride height.
Third scaling is no driver, rear raised 11.5" and shows a 20.5" CG on the scale.

Second set of four scalings are rear springs swapped side to side and no shocks at all.
Notable is how the crossweights got a whole lot better and how that 20.5" CG number appeared again.

The bottom set of three scaling was to see if the concrete floor level effected the scaling. Both the car and the scales were turned around in the shop bay. The individual scales are specific, LF, RF, LR, RR. i found a little deviation in gross weight of 8-9 lb, crossweights not effected much and still got the same 20.5" CG.

As someone said the ideal place to scale a car is on an alignment rack that is dead level. If i had more time with the scales i could have got my water level out and did the dead level shimming necessary to get all four dead level to each other. As this is termed silliness doing all this weighing I felt the water leveling would just be pushing the weighting into the funny farm crazy realm. I may be off a lb or two here or there but still think there was very useful info obtained.

Got state inspection stickers on the car yesterday. Unfortunately the shop latest and greatest Hunter alignment rack is waiting for a service call because of a dead head. So no rack time for the 86. I need to know that rack is spitting out good numbers, and I'm especially interested in what the rear suspension numbers are. As we know the solid axle rear still has camber, toe and thrust to measure and deal with.

After stickers put 40 miles on the car, needed to see an old guy about a ZL1 67 Vette he put together. Today another 35 miles on the car. Tomorrow was supposed to be an autox event, it's already raining, Ian is coming this way.
Being i had the rear sway bar at full stiff and the rear TA shocks set at 8 rebound I wanted to put miles on the car to get a feel of that setup. So far haven't found a corner that the evil rear bar shows up but I'm still pushing on the car. I can say that 8 rebound setting causes even me to be totally bothered by the ride quality. Every irregularity in the highway is transmit to the car, busy is not the word for it. Finally late this afternoon i pulled into a parking lot and cranked the 8 down to 2 in the rear and presto, tolerable, NOTICEABLY better. Comps the entire time was set at 0 - 0.
I'll drive, play with the car in that setup for a couple weeks coming.
Posted By: MAP Re: Rear roll center improvement. - 10/01/22 05:22 AM
Hi Bob,

Thanks! Those videos are a lot of fun, btw. He does a great job with them.

Back to the numbers: if you can just share your front and rear axle heights, we can confirm the COM height. Your garage floor doesn't need to be exactly level to calculate this number with good accuracy, as long as you're initially level to within a degree or two (most garage floors are canted about one degree for fluid drainage.) But, front/rear weight balance will show some sensitivity to this inclination.

Need to say it once again: if you've got some soft, skinnier tires to try in the rear, you should test them. Put it this way: you're probably thinking about the 3L-PHR in order to drop the rear RCH from the stock towering 18" (+/-) to something lower, like 8" to 10" as a first guess.

But I've repeated what Greg Locock said: a car "...doesn't roll about its roll centers." That's because we have compliance elements in the force transmission chain between the ground and the sprung mass, chief of which is tires. One effect of tire compliance is to reduce the true roll axis height. And the amount of that dropping could be quite significant depending on how soft the tires are, possibly even comparable to what a 3L-PHR might do for you. Now softer tires wouldn't yield exactly the same changes as a 3L-PHR, but the overlap is significant enough to be well worth exploring. Plus, you'd get a softer ride. Plus, you'd probably need less dampening at the rear. Plus, the inner tire would be less likely to part with the ground in tight turns. Plus, you'd get some free reduction of understeer, which you probably have in spades.

Again, this is just for experimental purposes. If you really like it, just go a lot wider and stickier.

What do you think?
Posted By: 88ssBrent Re: Rear roll center improvement. - 10/01/22 11:10 AM
No apologies needed Bob. When I posted the picture I meant to put a description with it, something came up and I completely forgot to come back and edit.
Posted By: SSLance Re: Rear roll center improvement. - 10/01/22 01:17 PM
The evil rear bar shows up on a heavy braking corner with a quick turn in. I used to recreate it in a church parking lot.
Posted By: mmc427ss Re: Rear roll center improvement. - 10/02/22 05:48 AM
If you liked Craig's other video of the RC his follow up is a necessity. So many thing he puts in prospective as he simplifies the front suspension pickup points for his car.

Ya know it took me a long time to find his name and who he was, is. You won't find that anywhere on his Throttle Stop Garage channel. I can read some of his facebook page, you won't find personnel on him there either.
I drink coffee, have said a dozen times I should order one of his coffee cups, time is now.

Mark as far as tires go only have two different tires for the rear. The autox tire, Falken RT660 is a 275/40x17, 200 rated which I've run at 32 predominantly, street and race. It's a super sticky tread and a pretty stiff sidewall. My other tire is a 7 year old BFG Comp 2 Summer in a 255/50x16, when new a 315 rating and now with old age not as good. Old, tired, and not good at the autox but OK for the street for every day use for another year. I ran 36 psi rear pressure with the Comp 2 tire. I had Hoosier A6 which I gave away, had Hoosier QTP slicks which i threw away, has Mickey drag radials which i gave away. Forgot about the M60x15 a friend gave me just to see if i could burn them off. They were so old and hard you couldn't burn them out, just could constantly haze them.
So yep had a few different tires in the back and got an idea how a soft tire feels as the rear rotates around.

I don't see any tire available that could just be thrown on the rear to try. When the Rt660 are gone the replacement will possibly be these Continentals.

I'm still looking for a place that has lots of flat,good looking blacktop that doesn't mind a guy doing a couple spin outs at high speed.

Posted By: MAP Re: Rear roll center improvement. - 10/02/22 07:31 AM
Too bad you don't have something you could throw on. I'm not sure what to say - maybe try running a very low tire pressure? You can actually get an idea of tire vertical stiffness by measuring axle height as you lower that end of the car to the point of barely touching the floor, then releasing the jack entirely and measuring the axle height in that fully-loaded condition. The difference in height will be kind of proportional to vertical tire compliance.

It's a shame you couldn't the original A/G body tires, which were 195-75R14s - soft as a pillow.
Posted By: MAP Re: Rear roll center improvement. - 10/02/22 10:01 PM
Hi Bob,

Just a bit further to say that the easiest way to soften your tires is to let the air out of them, to a point. (10psi? Wild guess only.) I'm sure you'll find that point sanely and safely.

I really like Craig's videos. On that last video, though, couldn't say I agree with all that he said (!) but still, a great video.

Have fun,
Posted By: mmc427ss Re: Rear roll center improvement. - 10/03/22 12:22 AM
I find Craig's approach to most everything he does entertaining, enlightening and it's his quest to figure stuff out and produce from scratch that I like. Although his garage is really tiny he has a wealth of knowledge and tools stuffed in it. It a shame that restrictions from Rona, and the difficulty and expense to get things across the border slows everything he's done down. I've watched more than 1/2 of his videos.
Ordered a coffee cup from his today to help support just a little.

Having rained now for most of Sat and Sun spent time surfing and reading pieces of my new book, RCVH. One of the interesting points made in several sections is about CG and how you need to know where it is is you want to play with improving handling. Even pages of how exactly to arrive at a CG, and how my procedures mimic their process. Exception is I didn't need to struggle with trig, the scale processor and the calculators on line did the hard part.

I email Susq SCCA to see what their last event of the season is, Oct 29. I may be able to make a few more runs on the car then. We'll see.
My friend with the 65 Olds station wagon is in process of swapping in his old torque converter, a 9" 3500 stall, to see if the 8" 3500 stall converter in the car now is WAY to loose. Intent is a mid 10 second boat in the 1/4. If he make a trip back to Cecil County I may go with him and also make a couple more passes on my car. Just to see if I can get my truck engined, single exhaust, 3700 lb turd to run low 12 on the Falkens. In another month or so the Summer only Falkens need to be removed and my old Comp 2 Summer go on for the Winter driving. That may sound contradicting, it is, but it's not.The Falkens need to be stored about 50 degrees for the Winter, the Comps are soon retiring.

Oh, only about 50 miles needed to drive to hit my 2000 miles annual goal on the 86. It's why i still have the car, because i like to drive it, and do drive it, sometimes pointlessly, just to, drive it. To many of my car friends have cars they put 100 miles on a year, just don't see the point to owning a garage queen.
My car will soon hit 175K, bought almost new with 9200 in 1991. It's been a toy car since 12/01, the wife says money pit, but being a toy I can play with it any way i want. Sure would be fun to run a road course someday.

Happy motoring.
Posted By: mmc427ss Re: Rear roll center improvement. - 10/05/22 02:05 AM
Being it has been raining now for 4 days that's both a bad thing and a good thing. The car has been high and dry since last Thurs, that's the bad thing, the good thing is here in PA I heard they lifted the drought restrictions for most of the state. That's a huge good thing, more important than the car.

So in the shop staring at what the next part of this new Winter project is, will be, the first order of business was finding the static CG, which I believe is done and accurately done. But we know just like every suspension mod we look at in the static mode and drive the car in anything but the static. But ya gotta know what ya got before ya start.
So the process begins with a doing more measuring. And being someone that needs to know asking for what other have for RC is a waste of time. And my car is unlike most others so my numbers only represent my car. Disclaimer.

Was laying under the front of the car and just eyeballing with the help of a straight edge, a digital angle gauge looking for an idea of getting actual, accurate points of interest located. Locating that IC is the key, and think will need to have wheels removed to locate it. That is the difficult part, IC location. I think once you have IC the RC is easy to find.
When I had the entire front suspension out of the car a few years ago I played with RC measurements/location and a 2" number kinda showed up. Looking now to do the exact location.
The front suspension went through major surgery to improve it a few years ago. No need to make any changes up front, just need to know what it's RC is so the rear's RC can play nicely with the front, like more nicerly, ain't a word.

The rear's RC should be easier to find now because I have a pair of those 11.5" cribs built. Put the rear tires up on the cribs, jackstands under the front at #2 body bushing and shim the stands to bring the car to ride height + 11.5" up front. Plumb bobs, tape measures, and angle gauge should make rear RC location not a week long project to get done. May even be able to use the creeper to roll around under the car at that height.
For S&G the 18" rear RC number for stock suspension I've heard, should be interesting to see how that guesstimate fits my car.

Wet weather, colder days and COLD nights is here now. Before that concrete floor in the shop gets to cold to work on gotta get the simple RC numbers on paper. Then I can think about how, if at all, I go drastic change to the rear suspension.
My last chassis project took a year from the first thoughts of the why and what needed to be done to actually done and testing. Oh yes, and a million words written about a simple subject, sway bar.

Yesterday made another mod to my quick stick camber gauge. It's simply a straight 1/4" x 1" X 18.75" alum alloy flat stock, it's used to touch the wheel's alum bead across the center of the wheel, at plumb. The 1/4" thick edge is the straight edge used to measure. The digital angle gauge first is used to plumb the alum measuring off the alum's 1" face. Plumb on any camber gauge is necessary to insure accuracy. Then the gauge is flipped to the 1/4" edge and camber read on the gauge. Worked great for the past couple years. But needed a little more done to make it a 30 second per wheel tester. And it was raining.

Yes i do have a good caster/camber gauge for doing a full alignment. It requires pulling the wheel center caps, and the hub bearing caps to use it. When I found that process as a PITA method the quick stick camber setup was made.

From my sway bar arm's scrap steel ( leftovers ) a 3/4" x 1 1/2" x 1 1/8" block of that steel was whittled with polished parallel faces to .001". This screwed to the center of that alum and dialed in parallel. Now the magnetic digital gauge has three steel faces it can hang on. It was a PITA to lay on the floor while trying to hold the alum plumb while you flipped the gauge 90 degrees to find camber angle. That steel block sure made that easier, but not happy yet.
Holding the alum's 1/4" edge against the wheels bead needed improvement. On one end a Tee has been made. This will allow that T end to have 1 1/4" of surface touching the wheel' s lip, not the 1/4" wanting to rock on a polished surface. The side of the T is curved to match the wheels curvature. I dropped that mod off at the welder's shop this morning for him to glue it together.

That tool when just a piece of alum and a digital gauge worked wells but a couple rainy days and some "nothing to do moments" that tool should be super now, may not even need to lay on the floor to use it. It along with the dozen other suspension tools I've made makes this whole suspension thing a little easier.
Posted By: MAP Re: Rear roll center improvement. - 10/05/22 05:53 AM
Zooming out a bit to the big picture, what don't you like about the existing rear's handling?

After getting the rear to articulate freely and precisely (I.e., no squishy rubber bushings to let things flail about,) it's all about RCH, anti-squat, and unsprung mass.
Posted By: MAP Re: Rear roll center improvement. - 10/05/22 09:18 PM
Hi Bob,

About this: "The rear's RC should be easier to find now because I have a pair of those 11.5" cribs built. Put the rear tires up on the cribs, jackstands under the front at #2 body bushing and shim the stands to bring the car to ride height + 11.5" up front." Doing this doesn't load the suspension the same way as having the front wheels sitting on a flat, rigid surface would. If you support the front at #2, the rear will sit considerably higher than it would under normal conditions with four wheels on the road.

Also, back to the central question, let me amplify a bit:

1. What specifically about the rear's handling are you expecting would improve by changing the rear RCH?
2 My statement about "no squishy rubber bushings..." may sound contradictory when I was previously talking about deliberately adding "squish" with soft rear tires. But having lateral compliance in the suspension and in the tires doesn't create the same results.

I've said a number of times that a problem well-stated is half-solved...
Posted By: SickSpeedMonte Re: Rear roll center improvement. - 10/06/22 11:39 AM
I can't necessarily speak for Bob, but I can tell you why I did it. I wanted to make the transient cornering characteristics of the rear, whether the transients are induced by changes in steer angles or road irregularities, more consistent with the front. I feel that I have absolutely accomplished this. With the C4L, I hated slaloms. I really had to take it easy or I would lose the rear. Driving my car and Bob's car back-to-back really cemented the difference for me. I can be much more aggressive through a slalom and trust the rear is 1) going to stick, and 2) let loose more slowly if it does, providing an opportunity to recover. Another benefit was the reduction of roll oversteer which quickened the lateral acceleration response of the car, but that wasn't my primary objective.
Posted By: MAP Re: Rear roll center improvement. - 10/07/22 03:08 AM
That aligns well with what other 3L-PHR converters have reported, like Lance and Travis. Bob, your comments on this? Do you have anything to add?

Most of that post was about steering transient response, where the rear's "kick" and "thud" come into play. With Bob's car's front-heaviness and preponderance of front roll stiffness, it's hard to see oversteer as an outcome unless significant throttle is applied.

Also, when you reduced the rear's RCH, did you increase rear roll stiffness to restore LLT?
Posted By: SSLance Re: Rear roll center improvement. - 10/07/22 09:42 PM
Originally Posted by MAP
Also, when you reduced the rear's RCH, did you increase rear roll stiffness to restore LLT?

I'll speak to the other adjustments I made to compensate for the lower Rear RCH.

I softened the front sway bar (1485 to 1100#), Softened the rear springs (250 to 185 I "think") and stiffened the rear sway bar one hole from full soft to middle.

The rest of the adjustments were made by twisting the knobs on the shocks.

Mainly what I found was the car now has MUCH better rear grip in a large sweeper, nearly the same but maybe a touch better rear grip in a slalom and slightly too much rear grip in a brake\dive hard turn in corner (pushy). I could get the hard braking turn better, but it would screw up the sweeper and I REALLY like how much more speed I can carry in a sweeper now. Really what I need to learn to do is to back the hard braking corners up, go in a bit softer and come out faster. That's what Bernie did back in April to get by me on his final run I think.
Posted By: MAP Re: Rear roll center improvement. - 10/08/22 01:46 AM
Hi Lance,

That was quite helpful, as well as SSM's comments. So far I'm getting that the real secret sauce in all this is reducing RCH, as we suspected all along, truth be told. Everything else seems to be secondary or less...

I must ask since I've never driven on an autocross surface - are these surfaces really dead flat, as in no bumps and no ripples? Or are we still dealing with a small amount of irregularity? As I recall waaaaaaaaaaaaay back to my C4L rear in my '78 Malibu, with cylindrical TPU bushings and boxed arms (a terrible recipe in retrospect,) the rear didn't like any bumps at all. The ride was quite "nervous," in a manner that instantly reminds me of what SSM is saying, but probably far worse. I'm trying to reconcile the comment about confidence in the rear sticking to the ground in a slalom, and what might be signaling that something bad could happen - is it bump-related or not?
Posted By: SSLance Re: Rear roll center improvement. - 10/08/22 03:27 AM
The skidpads we race on are mainly pretty smooth but they are mostly crested in the middle for water run off. We'll have cracks and seam sealer to deal with at times and the crest can be an issue but for the most part the surfaces are pretty smooth.

The gyrations we put the cars and their suspensions through during a run are violent though, there is nothing smooth about them at all. There is never any moment when a load isn't being transferred in some direction pretty harshly.
Posted By: mmc427ss Re: Rear roll center improvement. - 10/08/22 03:47 AM
Mark not sure what that says. "I must ask since I've never driven on an autocross surface." I assumed you know that these autox venues are usually just old parking lots. Currently the Elmerton lot in Harrisburg is ok, old blacktop, two near 360s at each end to make it long enough, off camber in lots of places, slick blacktop in some, I've seen most every types of car spinout due to it ain't flat and can be slippery. The Tesla 3 was cool seeing it loop on a fast 90. It's predecessor from Back To The Future spun out 25 times one afternoon.
Warmister Park was an old runway, camber to the left both running out and then back, concrete, blacktop, concrete, blacktop, sectioned.
Maple Grove pit area was far from ideal, Chicago boxes only a Miata could love. Many old and newer blacktop patched areas.
Hershey Park lot, the most enjoyable due to size, high speed, wet, my co driver looped the car, a fun day, fun off camber sections.
Just a couple autox surfaces I've been to or on. Most all are not someplace you would normally want to beat the car and find it's handling problems. You go there because it's the only choice. For me a novice and not much good seat time adapting to the course is always a challenge and slow learning.

Had the car out today for anther jaunt around the country side. Stopped at Bill's house to see how he like his ROUTE 66 drive last week with his wife. He has a sweet C4 Vette with a T56, averaged 32 mpg he said. They only got to Oklahoma, turned around because they ran out of time, and the dust on the old highway was in places unbearable. Another Dust Bowl out that way.

So out on a few really tight corners today the tail got a little happy at speed, more so than normal. Mid corner after entry and was easy to catch. One note here is prior to the big front bar when the car would start the oversteer you could feel the inside rear tire break loose and then bite again, sometime a couple times depending on the corner. Now with the much higher 1150 front bar you don't feel that, the rear tires loosing traction is much more subtle and smoother.

One thing to note here and what I consider important is the car went straight to the gas station after that corner and 9.1 gallons went in the the tank. It's not normal for that tank to be run close or less than 1/2 tank. 55 lbs of fuel in the tank helps tame the rear oversteer.

Came to my senses and will put the car on cribs on all four corners to more accurately assimilate ride height before doing RC measurements. As
mentioned I think the rear is just a PITA but easily done because you can mechanically measure and plot all the points.
The front RC LCA is easy, the UCA is the PITA, but I'll figure it out.

Something i may have mentioned is jacking from the front corners effecting the rear corners. More jacking is a product of changes made in the front suspension, mainly running high caster to gain a better camber gain, curve. When I have the car on the turnplates and you crank it left or right you can see the amount jacking. Even when the car is just sitting on the floor my imagination can visualize all the jacking and it's effect.

More to think about.
Posted By: MAP Re: Rear roll center improvement. - 10/08/22 04:49 AM
Hi Bob - "I must ask since I've never driven on an autocross surface." I just never had the luck of living in a place anywhere close to auto-x stuff. Back in Massachusetts days, the problem was that I didn't have a car worthy of auto-xing. Back in upstate NY, I was in the sticks - only course not very far away was Limerock, CT, but I just never got on the grounds there nor knew anybody that was connected. I don't even know if they did home-brew-style racing anyway. Waaaay back in Long Island days, nothing. Here in Yuma, all we have is sand, lettuce, and the occasional tumbling tumbleweed. What can I say...

OK, so it sounds like you have bumps and irregularities too in auto-x, for the most part. For you there in PA, Bob, it sounds like they are nearly as pronounced as what you encounter in ordinary street driving. As I vividly recall from MA and NY days, some of those bumps could be real whoppers. I've said before that just one transit of the Cross-Bronx expressway could reduce most suspensions to a loose pile of nuts and bolts. Here in AZ, a lot depends on the age of the roads. Up in Peoria where Lance is, most of the developments are recent so roads are pretty smooth. But the brutal AZ summers will eventually turn them into tectonic plates with upwardly-curled edges separated by cracks that will definitely cause some higher-frequency VH reactions from the suspension. Yuma is older so our streets have pretty bad temp-induced cracking, but none of the big suspension-bottoming stuff you get in the northeast.

Anyway! So the lesson for me is you get bumps in auto-x too. OK, I'm recalibrated - thx.

So, as we know, a high RCH doesn't play nicely at all with bumps. With a one-wheel bump at the rear, instead of the wheel doing a simple up-down deal as it does at the front, it does a weird side-to-side jig as well, and this may break traction if you're trying to accelerate at the same time. It gets even worse with high unsprung mass. Basically, recovery from a one-wheel bump, while turning, while applying brakes, is a very unhappy combination for the stock rear. Reducing the rear RCH helps with some, but not all, of that. Reducing LLT helps too. Reducing the rear RCH helps with that too (i.e., the "kick" part), as well as reducing the rear roll stiffness (i.e., the "thud" part.) But the price to pay for this is more understeer.

Moving the COM rearwardly and increasing the ratio of sprung to unsprung mass at the rear (hopefully a lot) would complete the remedy.
Posted By: SSLance Re: Rear roll center improvement. - 10/08/22 08:38 PM
Regarding the jacking effect the high caster on the front tires has on the rear...

I've tested the weight transfer on the rear tires while turning the front tires 20 degrees each way and the effect is minimal at most. While it may look like it's really tweaking the chassis, the scales showed only about a 20# weight change per rear tire with steering input. I'll take the extra front grip from the caster all day long every day.
Posted By: MAP Re: Rear roll center improvement. - 10/08/22 09:09 PM
That sounds thoroughly credible. It irks me that my inexpensive scheme to maintain dynamic camber at zero for all wheels, even with tire loading variation and body roll, would eliminate the need to use caster to bolster camber. The system does need sensing and signal processing, but it's very simple. Simpler than ABS, for example. It might even be back-fittable to an A/G body. Drats!

Must say parenthetically that live axles aren't all bad: you don't get a jacking effect in turns; you can get lots of anti-squat from its attendant suspension, and it does a great job with camber for free. Just dial it in for about -0.5 to -1.0 deg static camber to counter the tires, and all is just about as good as it can get.

Back to the front: until we do active control, it's a messy balancing act between KPI, caster, and camber to keep the tire squarely planted and total trail to be "nice." And even then, you can get it to work for only one wheel of the two, so the inner tire gets ignored.
Posted By: mmc427ss Re: Rear roll center improvement. - 10/09/22 04:32 AM
I didn't think to check the weight transfer from jacking when I had the scales in the shop. My intention is to get the scales back for a few days as I have other things to check with the scales. So putting turn plates on top of the scales and doing the jacking transfer weight test is in order.
The dirt track guys have been using the jacking thing for a long time as another tuning aid.

It's looking like autox is all but done for this year in this area. Today spent the day outdoors in a pavilion, multiply celebrations, a 25th and a college grad, and even the 41st for my son-in-law. Only a lite sweatshirt after lunch but the Winter coat was installed when the sun got lower in the afternoon. That is pertinent because when it comes to running these 200 autox tires I don't think they like it when the ambient get to the 50s. As we know Summer tires are not supposed to be run when temps drop into the 40s. Last Winter the 200s got pulled and stored in the extra bedroom in my house. That will happen again pretty soon.

In this area of PA we have three SCCA groups, Philly, Blue Mountain and Susquehanna. Very fortunate, but not really an advantage. The Blue Mountain does primary hill climb events, has not done an autox in about 5 years, did a few rallyx events before the Rona. The Philly group does autox only and uses that old airport runway, a mall parking lot which I've never ran at but think, it's NE Philly, I avoid Philly. They also use one of the sport stadium lots in south Philly occasionally. My understanding was it's surface was pretty fresh several years ago.
Susquehanna I've ran with 6 times now. But the two good lots I, we, ran in aren't available now. The Farm Show lot they got "removed" from because the State was going to re-blacktop it and didn't want the autox guys tearing it up. So they got booted, that was several years ago. This year Mecum was back in Harrisburg, the Dodge Thrill Ride thing was going on in that parking lot where we would tear it up. Go figure.

All that is in reference to what do these autox parking lot look like. What it always boils down to is the who will allow a bunch of guys the privilege to safely, with insurance, make several hundred autox run in their parking lot, 10 times a year. Last time in harrisburg there were roughly 450 runs made in the morning session, less cars in the afternoon but 75 made 4 runs, another 300. That's 750 runs on a Sat times 10 times equals 7500 runs. It's hard to do that without indecent to body, soul or private property. That's one of the things that makes incurring a place to play most difficult.
That said thanks to Susq SCCA for providing ANYPLACE to play.
Auto racing has become more challenging just in finding a good venue. Our local Maple Grove is in process of shortening to 1/8 mile, as is another track near by,
I understand 6 drag strips are now closing. Grandview stock car track, 60 years old, 10 miles up the road is closing, may sell out to Copart.
The list is getting long and growing.
Just one of the reasons as an old guy I gotta make use of my car and available venues before it gets even more difficult to stretch the legs of my toy.

Couple weeks ago made three passes at Cecil, took my 13 year grandson that day. Today at the outdoor parties told my 9 year old we may be making another trip to Cecil yet this year. "Do ya wanna go?" "YESSSSS, ,,,,,,,,but i need ear protection". He was talking to his brother that went with me the last time. Us old guys don't need ear muffs, we know when to stick our fingers in the ears. Call it experience. At Cecil you are but 8' from the beams, and yep it can be ridiculous dbs. I will make sure that when that Tesla launches he takes the muffs off to experience that.
Posted By: SSLance Re: Rear roll center improvement. - 10/09/22 01:17 PM
I ended up in a huge online argument with Sam Strano over the whole caster jacking issue so I ran the test just to prove my side. His mistake was thinking what applies to a 50/50 strut front car also applies to a SLA front heavy stick axle car...which it clearly does not. Plus, he can also just be an A-hole at times.
Posted By: MAP Re: Rear roll center improvement. - 10/09/22 09:43 PM
About the video, all I see is a great way to turn lots of expensive rubber into smelly soot. The point being?
Posted By: mmc427ss Re: Rear roll center improvement. - 10/10/22 03:29 AM
The point being a huge corp can come in and burn up the blacktop at a Mecum event in Harrisburg but a group of car enthusiast in a not for profit can't come in a play in that same parking lot.

That video was 2019, Mecum was not in Harrisburg for a few years but returned in 2022. I was watching the Mecum Thurs very early telecast introduction and highlights they showed the Hellcats were back in the parking lot burning off tires.

Always looking for unimportant info.

Still looking to see what Tesla 3 CG is.
Posted By: MAP Re: Rear roll center improvement. - 10/10/22 05:19 AM
Ouch. Sorry to hear that..

COM height - thought I'd try to search online for Teslas - the claimed range seems to be 17.5" - 18.5" for the model 3 and model S.
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