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.
https://www.youtube.com/watch?v=a8esG6kJuVY

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.
Bob

Found this, interesting.
Quote from:
https://www.sae.org/news/2020/03/engineering-2020-corvette-mid-engine-architecture

"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."

Bob

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.

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.

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?

^^^ 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.

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 MCSS.com forum.
In that post is talk of CG, with a 20" number thrown out there, "center of gravity height (guess only) of 20"" hmmm.
http://www.montecarloss.com/community/ubbthreads.php?ubb=showprofile&User=1629
Bob

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.

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 MCSS.com 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.
Bob

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.

Ron.

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... 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.

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.

Couple of Bob's pictures.

Car looks pretty clean still.

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.
Bob

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.
Bob

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.

Bob,

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.




Regards,
Ron

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

Bob

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 . 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.

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 MCSS.com forum.
In that post is talk of CG, with a 20" number thrown out there, "center of gravity height (guess only) of 20"" hmmm.
http://www.montecarloss.com/community/ubbthreads.php?ubb=showprofile&User=1629"

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 MCSS.com forum.
In that post is talk of CG, with a 20" number thrown out there, "center of gravity height (guess only) of 20"" hmmm.
http://www.montecarloss.com/community/ubbthreads.php?ubb=showflat&Number=329633#Post329633

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.
Bob

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...

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.

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.

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.
https://www.youtube.com/channel/UCsx-HTIYPn3CiBQFd4rMyXw

Bob