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#1071221 - 06/29/21 05:43 AM Front sway bar project  
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mmc427ss Offline
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As another part of the evolving suspension changes to the 86 to make it handle a little better on the street and navigate an autox course a little quicker requires gaining some knowledge about the 'big bar theory".

The suspension/frame projects have made huge improvements in the cars handling. Only the original frame and front LCAs remain and both of those have been modified. Modified front LCA relocation, rear LCA relocation, adj rear bar, springs, shocks, braces, body bushings, suspension bushings, steering linkage, the list goes on and on on all the parts changed or modified.

The one thing lacking was bigger wheels to mount a fat sticky 275/40x17 200 tire, that piece of the puzzle was recently done and made 14 runs at an autox event. Surprisingly the car did pretty good but there is still room for improvement.

Most of the CAM T cars that are fast run huge front bar rates. This is the area to concentrate my next research and project. Do some bar rate measurements, see if it's possible to install a higher rate bar using easily obtainable parts, and see if it's possible to easily swap bar rates for the 98% of the time the car spends on the streets.

For years with the basically stock suspension ran the stock 1 /14" solid SS bar. with good links.
When front suspension upgrades were done in 1999 installed an Addco 1 1/8" solid bars. Advised by Global West that was the bar i needed with their front suspension package.
After pulling a 36 mm from an 3rd gen F donor, swapped that into the car and have been running it since 09, over a decade now. I think it's a good match for the car on the street.

In my sway bar rack there is a stock 1 1/4", the Addco 1 1/8", a 34 mm F bar and a spare 36 mm F bar. One thing I have never known as fact is what their rates truely are. Very few venders of G-body sway bars advertise their bar rate, just mine is bigger and badder. So you find virtually no info anywhere on stock replacement bars rates.

First piece of this sway bar project is to build a rater and test those four bars to see what the actual rates are. About a year ago a rater was made to test several rear coil springs to use in the 86. That rater can be repurposed with a few dozen mods to check sway bar rate. I believe all those bars will test out significantly less than 500 lbs per inch, even the big 36 mm I suspect 400ish. But the rater should give me good numbers.
The stock configuration of these bars with all the twists, turns and bends make it a little difficult to get accurate measurements to plug into sway bar rate calculators. This one I found may be the most accurate calculator, it's from Fred Puhn.
http://www.gtsparkplugs.com/Sway-Bar-Calculator.html
With that calculator and rates i get from my bar rater I can end the what is the rate of that bar question.

Once I know what a 36 mm bar rate is and knowing how it behaves on the street I will be more able to apply that info to what happens if you double the rate just for a fun day at the autox.

My shop has been cooking hot, my time is kinda limited right now. The spare 36 mm bar has already been on the bench at the shop eyeballing what a rater needs to look like. The MIG is buried over in the corner, there's enough steel laying around,, incentive in short supply.

Going to schedule the Aug 7 autox event to run the car 16 more runs. Another very hot day i would suspect, this time will be prepared with copious amounts of cold water, not for me.

A new sway bar will not be installed for that event, maybe the Oct event I also plan to run. Trying to catch up on the autox time we didn't have last year. That will total about 40 runs on the car this seasons. Should have a better understanding of what the car is capable of and what to improve upon.

One thing that will be done for the Aug 7 event is a little more camber. Now have 1 1/4 degree set for my street setup. Will mike the shim stacks again and make solid shims for that street setup. Then add a shim to every location to add another 1/2 to 3/4 degree of camber. This is not a new trick, been around for decades. You can actually make a one piece shim unit for each UCA and just change them out quickly, a 1 1/4 and 2 degree drop in. Something I was going to do when doing the 9 1/2 degree caster mods to the front suspension, just never got around to it.

So begins the sway bar project.
Bob

#1071224 - 06/29/21 02:02 PM Re: Front sway bar project [Re: mmc427ss]  
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I'd be really interested to know the bar rates myself, same with the rear. As you said, manufacturers often don't cover the specs just bigger is better, shiney-new, etc. I did talk to Ridetech last week, their rear stock style bar is rated at 200 ft lbs, he had to look it up, now I forget the size but I think it's 1/8" larger than the stock bar I believe. Their adjustable I have the rates or can find the range online. In spring I called and asked them the rate for their new one piece front bar and their older 3 piece bar, I recall the newer one piece is rated higher but didn't write down the rates anywhere. I never could find anything on the stock bars.

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#1071226 - 06/29/21 03:15 PM Re: Front sway bar project [Re: mmc427ss]  
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One thing to keep in mind, and Bob, I'm sure you already have a good handle on it but just wanting to state it early on for all of us to keep in mind... There's a difference between forces, moments, torques, etc on these bars. If you take the same bar and use longer or shorter arms, the "bar rate" stays the same (since it is the same cross section), but the force applied to to other wheel will be different. Longer arms should make the bar act "softer" and vice versa. That also holds for moving the point on the suspension arms that the bar connects to ((just like changing the coil spring position on the A-arm changes the overall car rate).
That is probably a good chunk of why there isn't much listed information out there. The bar cross section will dictate the strength IF the arms are the same length and mounted the same way, but the farther you get from stock applications, the less you can count on that similarity and the more likely a force measurement is needed to get apples to apples information (i.e., Bob's rig he'll be building). So unless all the measurements are done the same way and readings listed the same way, one manufacturer's ft lbs vs another's pound-feet vs another's pounds per degree are all pretty much irrelevant to each other.


Shawn

'85 MC with budget 5.3L swap, TH350 with stock 2.14 rear end
It ain't much off the line, but it's nice on the highway
#1071230 - 06/29/21 05:48 PM Re: Front sway bar project [Re: mmc427ss]  
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A lot of what we attribute to a "weak" bar is a too high of roll center height and vehicle that is under sprung.

an example of a very capable car with a stock style bar is the UMI Green Machine with a UMI 35MM? bar and 850lb front springs. The motion ratio of the g-body front suspension turns what appears to be high rate springs into soft wheel spring rates. I just got my full UMI Cornermax kit, in a week or so I'll have it installed and tell you how it rides. going from a 550 front spring to a 750 was an improvement, so I'm imagining going to 850 will be even better.

The other areal to look at it the amount of deflection in the poorly designed sway bar end links on these cars. I am POSITIVE we're losing spring rate through deflection with the factory style double bushing ends. I was going to modify some things and try a double ball style endlink on my existing suspension, then i decided to just buy what was is proven to work from the UMI catalog.


86 SS 6.2l LS3, Ilmor intake, Summit Stage 4 Cam, Stainless long tube headers, Stainless 3in exhaust, Tremec T-56 Magnum 6 speed, Eaton Truetrac 8.8 LSD, UMI Cornermax Front Suspension, 3-link Rear suspension w/ UMI Control Arms, UMI Front & Rear Braces, Brembo Brakes
#1071233 - 06/29/21 06:13 PM Re: Front sway bar project [Re: mmc427ss]  
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Big front bars are better...WAY better... Wish I would have done mine years ago, would have saved a lot of hassles, time and expense.

I'm pretty sure I can still get all the pieces like mine from Allstar, you could build a clone. Then for daily use all you'd have to do is swap out the center bar.

Like said above though, the swing arm length is key. If you want to still go over the tie rods, you'll need a stock style 12" long arm whereas I use a 8.25" short arm that drastically stiffens my rate. You just make up for that difference with the center bar rate.


Lance
1985 Monte Carlo SS Street Car
#1071243 - 06/30/21 04:46 AM Re: Front sway bar project [Re: mmc427ss]  
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If, when, I build a front bar is will be a fixed tube to the frame and the bar inside the tube. But as Lance said the 12" arms would need to be used. Have sourced all the parts already, torsion bars are pretty much produced from just two companies. Arm spline count is one factor limiting whose arm to used with what bar spline/length.

UMI has some links that may fit my install, rod end on top, poly stock type on the bottom.

At Carlisle this past weekend the hood was up on the Green Machine. Looking at the front bar at first i thought it was a 36 mm factory configuration, shape, bar. Looking at UMIs site they only show a 1 1/4" solid front bar for the G, 32 mm give or take. The bar on the Green Machine is their 3rd Gen F 35mm solid bar. UMI says 22% stiffer than a 36 mm hollow 1LE bar. interesting.
UMI's 71 Camaro uses their own three piece setup, 1 1/4" solid with 5 holes for rate adjustment.
Might call UMI tech and see if I can get some numbers on their bars.

It's getting the correct shape arm to fit my install which is the biggest issue.
I see the bar's arms length and shape as the big problem. On pass side the idle arm and the swaybar are side by side fighting for the same real estate. To run a short arm the sway bar would need to move rearward 4", or as Lance did mount the bar using standoffs to lower the bar, move it rearward to run a 9" arm. The arm length of stock appearing G sway bar is 13". The sway bar links and how they stay close to perpendicular as the suspension moves through full travel is important. So arm length is important to not stressing the links. When you start to get very high bar rates the load is on the links, to much misalignment can't be good.

As with pretty much all my projects they end up a little over built. When you start to look at some of the cool NASCAR sway bar setups you can see the time and effort and quality bar bearings and trick parts they use. Super strong, over built and lite weight. The bar tubes are used as part of the front frame strength. Intention is to make something similar that also adds some rigidity to the front suspension. And to simply, easily change to a lower rate bar is also required.
Bob

#1071246 - 06/30/21 01:19 PM Re: Front sway bar project [Re: mmc427ss]  
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SSLance Offline
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Something to watch with mounting the tube up on the frame...can you still get the center bar out without removing a fender?


Lance
1985 Monte Carlo SS Street Car
#1071267 - 07/01/21 02:01 AM Re: Front sway bar project [Re: mmc427ss]  
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Yep, that was something very necessary, Once you have everything installed it's no big deal to swap out those bars, couple bolts on the arms.

The shop was near 100 degrees today but found time to dig out some steel. A nice 7' length of 2x2 square tube, 1/4" wall. Should do nicely for the test frame. Just need to mount a pump jack/inch scale on one side and my analog weight gauge on the other.

Today took the 86 to drop off, donate, some 15x8 rally wheels my slicks were mounted on at a friend's garage. Temps in the mid 90s that black car just sucked up the heat. In the next day or so will stick the gauges on the A/C to insure it's up to snuff. Been 15 years since the R&R of the R12 system. With possible 800 miles of 90 degree driving in the very near future it's nice to have ice cubes coming out the vents.

Bob

#1071315 - 07/10/21 08:07 PM Re: Front sway bar project [Re: mmc427ss]  
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MAP Offline
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Hi Folks,

Just to transfer the comment here - sway bar stiffness, measured at the ends of the swing arms, goes as the inverse square of the side-view swing-arm length. That means if we double the length, the bar stiffness goes down by a factor of four, so the sensitivity is very high.

Yes, please measure all stiffnesses with the actual bushings that would be used in the actual installation.

About the front RCH, it's when it's too low, rather than too high, that makes the car roll too much. For reference, the stock front MCSS RCH is very roughly -3" to -4". The steady-state roll angle goes as the product of the car's roll compliance with the normal distance from its roll axis to its COM. Don't forget that the car's roll axis location includes the effect of tire compliance.

Also, since the discussion seems to be headed in the direction of a very stiff front roll bar, please remember that the car is very loosey-goosey and hardly reacts as a rigid body. Some years back I took a measurement of the frame's torsional rigidity between the front and rear axle CLs (attached with OE rubber bushings to the body) and got roughly 2,100 ft-lb/degree. IOW, terrible. Many modern cars, even economy models, are five times higher. Some, even thirty times. Here's an eye-popping link: http://youwheel.com/home/2016/06/20/car-body-torsional-rigidity-a-comprehensive-list/ (N.B.:1N.m = 0.738 ft.lb.)

You could make the front sway bar infinitely stiff on our cars and the abysmal lack of front-end rigidity alone would allow the car to roll.

HTH,
MAP

Last edited by MAP; 07/10/21 09:39 PM.
#1071321 - 07/11/21 07:11 PM Re: Front sway bar project [Re: mmc427ss]  
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Just to continue a bit about chassis rigidity - the FOM I hear most often is that the chassis should have at least 10x the torsional rigidity of the suspension. We can actually take a stab at what this number should be for a 3,800lb MCSS (this includes the driver) that rolls 2.5 deg at 1g cornering acceleration. I'll assume 1/4 of this roll angle comes from the tires, so the suspension alone rolls about 1.9 deg/g. If the car's roll axis, including the effect of tire compliance, is about 12" high at the rear axle and about 0" high at the front (I'm assuming the front of the car has been lowered from stock, which raises the front RCH,) then the car's COM height of about 21" gives us a roll moment arm of roughly 16". The total moment in roll with respect to this axis is therefore roughly 5,000 ft-lb/g. If the car's suspension is rolling 1.9 deg/g, then the car's torsional suspension stiffness is about 2,700 ft-lb/deg. Using our 10x FOM, the chassis' torsional stiffness should therefore be higher than about 27,000 ft-lb/deg. Once again, I measured a measly 2,100 ft-lb/deg, so we're lacking by a factor of about 13. This explains why stiff tires and stiff everything else basically don't play well at all with a floppy chassis like ours. Yes, the numbers might still look great on a track, but on the street, the car will ride like a Caterpillar tractor, with all due apologies to Caterpillar. Thanks, GM...

IMO we consistently attack handling and ride problems with our MCSSs from the wrong end. Fix the chassis first, and everything else will fall into line with little fuss. But do it backwards, and the car's basic design will fight us every step of the way.

The purpose of this isn't to deflect from the topic of the front sway bar stiffness, but to highlight that front sway bar stiffness alone doesn't give us the entire picture of what the car is doing in roll. We had the same problem with Project Sound at my former employer: in a meeting with MB engineers, I highlighted that active suspensions don't work wheel with a floppy chassis, and they agreed. Unfortunately, fixing that problem is hardly trivial, even at the OEM level.

HTH





Last edited by MAP; 07/11/21 08:08 PM.
#1071322 - 07/11/21 08:00 PM Re: Front sway bar project [Re: mmc427ss]  
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Hi Bob - back on center to your sway-bar quest - that Puhn formula does indeed take a stab at bending stiffness of the side swing arms with that c^3 term in the denominator. Because of the units, it's hard to tell whether the swing arms are assumed to be cylindrical or a rectangular solid. While the number of significant figures in the denominator term constants are unjustified, the basic form of the equation is spot-on and is as detailed as you can get without doing some actual FEA modeling of the part.

Last edited by MAP; 07/11/21 08:04 PM.
#1071325 - 07/11/21 10:24 PM Re: Front sway bar project [Re: MAP]  
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Originally Posted by MAP

You could make the front sway bar infinitely stiff on our cars and the abysmal lack of front-end rigidity alone would allow the car to roll.

HTH,
MAP


Coming from someone that has tested this theory extensively...and has the track time and video evidence along with the seat of the pants feel to back it up...these cars with their 57-58% front weight bias love a 1500ish pound front sway bar.

My car (which regularly sees 1.3 lateral Gs) stopped lifting the inside rear tire on corner entry when I put my big front bar on. This alone made the car tons faster on course and easier to drive.

Bob's car is like mine, the front grip is great. More or less roll is not going to change it's front grip much, but less front roll is most certainly a great way to fix the rear grip loss that Bob's car is experiencing now. Undisputedly... And with basically zero side effects as well.


Lance
1985 Monte Carlo SS Street Car
#1071332 - 07/12/21 06:29 AM Re: Front sway bar project [Re: mmc427ss]  
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I said that infinite bar stiffness would still allow the car to roll; I didn't say how much - I'm guessing 0.5-1.0 deg, not counting the rear's roll stiffness contribution.

My point about the car's poor torsional stiffness supplements the discussion about the front sway bar; it doesn't negate its inherent attributes.

Last edited by MAP; 07/12/21 07:33 AM.
#1071463 - 08/01/21 01:27 AM Re: Front sway bar project [Re: mmc427ss]  
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Have been a little busy with getting a working A/C system back in the car, so no progress on deciding what to do about a front bar upgrade. In the near future will get back to building a bar rater to test 4 commonly used front bars. Have shopped all the suspension parts venders who sell a front bar for a G to see what they offer. Typically their big bars are around 400 lb per inch and a couple bars are high 5s and UMI comes out the winner with their 3rd gen F bar at 618 rated. The steel type used (materiel), bar OD and hollow verses solid are the choices. Pretty much all will have the same shape so just bar OD and hollow or solid determine the winner. UMI bar is 35 mm solid and isn't sold for a G, it's sold as an F part. But we all know front G and F bars are interchangeable. The brackets that hold the bar to the frame are different though, gotta use a G on a G and an F on a F.

My guess is the 36 mm hollow F-body bar which I have run for years is in that 400ish+ range. So looking for a lot higher rate bar would need to be a three piece.

Building a three piece bar requires sourcing several parts which may be a little more difficult these days. A three piece has it's arms splined to the torsion bar. There is no bolt-in 3 pieces bar available today for a G, you need to buy a bunch of parts and make them work on your car. Just makes the project a little more entailed.

I get the torsional chassis stiffness thing and how much it impacts handling. But for years haves made numerous subtle changes to improve the noodle. In the end it's still about polishing the turd and it's the turd I chose to drive. Pretty happy with the marriage of the body and chassis after all the mods.

Last time the car was scaled was years ago, 58.75 front, 41.25 rear, nothing to be proud of. Every time I turn around seems another lb or two gets added to the front end. And no plans for any car diet to lose 100 pounds. With a slightly higher CG run on the car due to needing a little higher ride height, and ALL that weight over the front tires, and knowing front roll angle is more than wanted, it makes the monster front bar install look like something I need to do just find out for myself what it's all about. Already can see, fell the drawbacks out on the crappie intersections.

The biggest reason to try one is see if a huge up front bar can really help the rear tires. Less roll in the front should mean less roll in the rear. It's a follow the leader game.

The prime goal of my suspension mods lately is more rear stick, it has always been the problem child. The newly installed Falken RT660 were a major improvement front and rear, just looking for more yet without cutting the car to do rear roll center changes.
Bob

#1071464 - 08/02/21 01:09 AM Re: Front sway bar project [Re: mmc427ss]  
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Hi Bob,

To minimize weight, definitely go with a hollow bar if you can get enough stiffness out of it. To say that differently, for the same stiffness, a hollow bar will always weigh less than the equivalent solid bar.

Yes, with a front-heavy car, and even with a lot of floppiness in the chassis, I can see where a stiffer front bar in the front is generally going to help. I say that not only theoretically but from direct experience as well.

#1071466 - 08/02/21 02:03 AM Re: Front sway bar project [Re: mmc427ss]  
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You and Cory should work together on this Bob as he's now convinced one is in his future as well. Can't wait to see you and your car in a couple weeks...


Lance
1985 Monte Carlo SS Street Car
#1071471 - 08/03/21 06:12 AM Re: Front sway bar project [Re: mmc427ss]  
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When I saw Cory at KOTM he was busy, a very short conversation.

Bill, me and the car will be there. But i will tell you the old 16" Comp tires and old wheels will be on the car, not the new setup. That's a 550 mile trip for the car and can't see a reason not to wear the old tires and save the new soft Falkens for autox.
Bob

#1071575 - 08/13/21 07:49 PM Re: Front sway bar project [Re: mmc427ss]  
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Hi Bob,

OK, taking this back to essentials, I'm getting that you want more roll stiffness in the front in order to keep both wheels in the rear in better contact with the pavement.

You could do this by lowering rear vertical stiffness at each wheel, through some combination of reducing rear roll stiffness via the sway bar and reducing rear spring stiffness.

But you're doing it instead by increasing front roll stiffness.

I know you spent a ton of digital ink talking about how you optimized your rear suspension, but for the sake of clarity going forward, could you summarize in a couple of sentences why you are where you are?

This isn't necessarily to question the configuration, but a clear answer could better inform where you're going with the front suspension. For example, you could just as well increase front roll stiffness by using stiffer springs rather than a stiffer sway bar.

Last edited by MAP; 08/13/21 08:13 PM.
#1071591 - 08/14/21 02:00 AM Re: Front sway bar project [Re: mmc427ss]  
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Funny you mention front springs.

We have a Dingo 5 hp yard cart at the shop, knobby tire thing. My 12 year grandson tore up the back acre earlier this week. While me not paying attention my 8 year grandson videoed the 12 year old cornering on two wheel around a big Chestnut tree. A projecting bump in the soil under tree he hits while turning and it's both inside wheels 12" of the ground. Of course my 8 think Pop needs to see it right away not knowing Pop would put an end to that foolishness instantly.

The cart back in the shop and note the front coil springs are broken, collapsed and doing nothing. I've hooked that same tree a dozen times and never had an issue. Those coils are only 15/16" OD, 2.25" long and .125 wire, not a lot of rate. Thinking when the 12 was hitting that bump with no suspension the front just got shocked light real quick and the rear tire followed suit.
Picked up a new spring coil to cut and fit to the cart today. Will mostly be a fit and rate check thing. The car is over 20 years old now, didn't shop for an OEM spring, maybe later.

For my car being near 700 lb up front now adding more rate is easy, even GM had a 1000 pounder in the part catalog for the 3rd Gen F way back then.
But as we know something will suffer, my butt ride quality meter will be able to tell.

I feel anything you do to improve cornering is a trade off in these cars, ride, or handling.
Bob

#1071596 - 08/14/21 04:03 AM Re: Front sway bar project [Re: mmc427ss]  
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If we look at an axle's bump time input, we could call the right side R(t) and the left side L(t). We could decompose these into common-mode CM(t) and difference-mode DM(t) signals as CM(t) = (R(t)+L(t))/2, and DM(t) = (R(t)-L(t))/2. Then, going backwards, we could say that R(t) = CM(t) + DM(t), and L(t) = CM(t) - DM(t). This is exactly how electrical signals are composed/decomposed for differential amplifiers in the electrical domain.

OK, the purpose of this is only to show that springs affect both the CM(t) and DM(t) responses, while sway bars only affect DM(t). So, springs make the ride stiffer for all kinds of bumps, whether seen as common-mode or difference-mode, while sway bars make the ride stiffer only for the difference-mode. So, for a given desired roll stiffness, a stiffer sway bar will make the car ride moderately stiffer, but stiffer springs will make the car ride stiffer yet. To repeat, this is for the same target roll stiffness.

I agree it's all a trade-off, as is true about 98% of the time in engineering virtually anything. But in the case of the A/G body design, we're unusually poorly handicapped. That's why I'm always talking about removing the handicap. I know that sounds near-impossible to do, but when you look at all the mountain-moving effort we exert to try to "polish the turd" as you say, it's not as infeasible as it seems. Too bad you wouldn't be open to an EV conversion - it solves an impressive array of problems in one fell swoop.

Last edited by MAP; 08/14/21 05:22 AM.
#1071610 - 08/14/21 09:24 PM Re: Front sway bar project [Re: mmc427ss]  
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Hi Bob,

Just to carry the conversation a bit further...

1. About being open to an EV conversion, I already know how you'll respond, so I'll say pre-emptively that I fully understand and appreciate the reasons. Too bad we can't turn back the clock 10 years and have $100k magically fall down from the sky. Even so, if you could add up all the time and money dealing with this and that A/G body issue over the years, and assign a reasonable value to your time, I don't think $100k would sound crazy anymore.

2. Again, what I'm hearing is a desire to keep both rear tires in contact with the ground for a greater fraction of your driving time, or at least your auto-xing time. I return to the question, why not reduce rear roll stiffness? I think you answered that somewhere in the multitudinous multifarious manifold multiplicity of multiplied millions of words you've written in your "Rear spring options for handling" thread, but for the benefit of those with less-than-perfect memories, like me, could you summarize the reason(s) here? (That's a friendly stab, not sarcasm!)

Just trying to help.

Best,
MAP

Last edited by MAP; 08/14/21 09:33 PM.
#1071638 - 08/16/21 05:31 PM Re: Front sway bar project [Re: MAP]  
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SSLance Offline
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SSLance  Offline
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Peoria, AZ
Originally Posted by MAP
Hi Bob,

2. Again, what I'm hearing is a desire to keep both rear tires in contact with the ground for a greater fraction of your driving time, or at least your auto-xing time. I return to the question, why not reduce rear roll stiffness? I think you answered that somewhere in the multitudinous multifarious manifold multiplicity of multiplied millions of words you've written in your "Rear spring options for handling" thread, but for the benefit of those with less-than-perfect memories, like me, could you summarize the reason(s) here? (That's a friendly stab, not sarcasm!)

Just trying to help.

Best,
MAP


I'll take this one as I've tried just about every method available to do just this. There are two ways to accomplish this using existing roll center heights and several others using altered RCHs.

First off using existing RCHs a stiffer rear sway bar or stiffer rear springs are your choices, the stiffer rear sway bar just lifts the inside tire and makes everything worse. The stiffer rear springs work pretty well actually and I had great success with high speed cornering Lateral grip with 600# rear springs in the car. The downside to that setup that I eventually decided I just couldn't live with any longer is reduced weight transfer to rear tires under acceleration which resulted in excessive wheel spin. Switching back to a 250# rear spring greatly helped in weight transfer and added rear forward bite, enough so to more than make up for the loss of a little lateral grip. Adding an adjustable soft rear sway bar back in the mix fine tuned with some shock adjustments tailored the rear lateral grip to "acceptable" form for me. The huge front sway bar actually made the whole setup have better rear lateral grip than the 600# spring setup I ran before.

Going down the "adjustable rear roll center height" rabbit hole is an option and I know several friends that are working on this setup now in their G Bodys. I choose to sit back and watch and see how they do first as it can be a daunting task to do correctly and I want to see if the results are as magically as some claim before making these changes to my car. I am going to need to see a 3 link rear perform better than my current setup on track before deciding for sure to try this. The few that I've ridden in, driven, and watched race haven't convinced me yet that it's worth the trouble but I remain hopefully someone will hit on something.


Lance
1985 Monte Carlo SS Street Car
#1071645 - 08/16/21 10:05 PM Re: Front sway bar project [Re: mmc427ss]  
Joined: May 2002
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MAP Offline
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MAP  Offline
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Yuma, AZ
Hi Lance,

Let me try to parse your answer:

1. "First off using existing RCHs a stiffer rear sway bar or stiffer rear springs are your choices, the stiffer rear sway bar just lifts the inside tire and makes everything worse. The stiffer rear springs work pretty well actually and I had great success with high speed cornering Lateral grip with 600# rear springs in the car."

So stiffer rear springs help but a stiffer rear sway bar doesn't. Since both methods raise rear roll stiffness, then rear roll stiffness tuning alone isn't the whole answer to the problem.

2. If springs are affecting rear bite under longitudinal acceleration, then that's a consequence of anti-squat or anti-lift, in addition to the springs. So adjusting the side-view instant roll center location can help or hurt that significantly. The stock rear design is 25% anti-squat as I recall, but with a live axle, it's easy to get that above 100% so the rear of the car lifts instead of squats under acceleration. And if the rear lifts, then the upward force required to generate that lift is seen as a downward-force reaction at the tire patches which increases traction.

So, combining lateral and longitudinal aspects of rear traction with a front-heavy car, there are now at least three tools for the rear instead of just two: springs, sway bar, and anti-squat. Shocks have a transient effect and so that's somewhat a separate discussion.

About the rear RCH, on a flat surface, there is little consequence. High RCH makes the rear "bite" laterally faster than a low RCH, so that steering response feels more instantaneous and synched between front and rear axles. With a low rear RCH, the rear needs time for car roll to cause lateral weight transfer to be communicated to the rear tires so that appropriate slip angles are generated and the rear develops its full lateral grip. In other words, with a low RCH in the rear, the car has a distinct "1-2" steering response: front first, then rear roughly 1/4-1/2 sec later. With a high rear RCH, it feels like one immediate hit, so the steering response is sharper.

On the street is where it gets a lot more fun and interesting, and where the low RCH really shines.

Back to Bob's quest: increasing front roll stiffness should accomplish nearly the same thing as reducing rear roll stiffness. Bob, it's certainly your prerogative to keep the discussion restricted to measuring and then selecting the correct front bar, but that's likely to cost money and even more weight on the front axle. Beyond this, the car will roll a bit more with the second option, but we're probably only talking a fraction of a degree, and even then, through playing around with caster and camber in the front, you can more than compensate without undue ill effects. Given the excessive front caster you've got plus what you've done with camber, you're probably fine as you are - HTH.

Last edited by MAP; 08/16/21 11:06 PM.
#1071648 - 08/16/21 11:02 PM Re: Front sway bar project [Re: mmc427ss]  
Joined: Dec 2007
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SSLance Offline
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SSLance  Offline
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He just needs more front sway bar... laugh Like a LOT more...


Lance
1985 Monte Carlo SS Street Car
#1071649 - 08/16/21 11:07 PM Re: Front sway bar project [Re: mmc427ss]  
Joined: May 2002
Posts: 4,301
MAP Offline
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MAP  Offline
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So, how do you know?

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