Suspension balance/correcting oversteer

[Jeremy]

I am needing some advice on how to correct some oversteer in my chassis.

The car is an 80 TA with ws6 springs front and rear, tubular upper arms with steel bushings, rubber lower a arm bushings, rubber springs in the leaf springs, edelbrock shocks, and 17 x 9 tires front and rear. On the rear, I ran the adaptors 1/2 inch wider to add some roll stiffness to the rear and in this condition it had a little understeer.

I added front structure braces from pro touring f body.com and solid body mounts and the car was balanced the way I wanted it.

I added a shear plate across the structure braces to add some torsional stiffness and to take some load off of the firewall. This worked as planned and now the front is tight and almost rattle free. The problem is now the car oversteers significantly when pushed on tighter corners. It oversteers so much that I am a little un comfortable with it.

To correct this I think I need to soften the front or stiffen the rear. I wanted some feedback before I made any changes so I get it headed in the right direction the first time.

If I need to soften the front, that would be fine with me as ride is porbably more important than max g's. There is a 6 cylinder bird in the junkyard that I can get a smaller bar and softer springs from cheap or new springs are not that pricey.

If I need to soften the front, which will have a greater effect on a softer ride, a smaller bar or softer springs?

[chicane67]

First off...... what do you have the front end aligned to ?? And what tire pressure do you run front and rear ??

[Jeremy]

The car is aligned to 1/16 toe, 3 degrees castor, and 1 degree camber. I run 44 psi front and rear to minimize tire wear.

[chicane67]

Well first of all, you can make a HUGE difference by mearly changing your rear tire pressure. Try it down about 6-8 and even down to more than 10 psi (from what you run in the front)..... also, you can run more static negative camber and change the toe to somewhere around 0 to 1/8" toe out.... before you start unecessarily throwing money and parts at it.

I set-up second gens with up to -1.75* camber, +3.5 castor and 0 to 1/8" toe out.

If I had to change anything first, I would change the rear stay-bar to a larger one first.... due to its cheap cost and simple bolt on mechanics.

[Norm Peterson]

Unless this oversteer is being caused by too much right foot, I'd look to softening the rear rather than stiffening it further. It's apparently attracting too much of the roll couple already. I'd try removing the rear sta-bar entirely as a first step.

BTW, those pressures are more than a little higher than necessary for street driving. They're closer to what you might use for auto-x or open tracking. Depending somewhat on your specific tire size(s?), you might try something like 36f/32r. Your tires' tread centers will thank you.

BTW, part II - are those tires relatively fresh? A really old set of tires tends to be "looser" than a brand-new set.

Norm

[Mean 69]

I added a shear plate across the structure braces to add some torsional stiffness and to take some load off of the firewall. This worked as planned and now the front is tight and almost rattle free. The problem is now the car oversteers significantly when pushed on tighter corners.

Very interesting. What I read here is that with no other changes, your car went from balanced to oversteer with the addition of the sheer plates, and no other changes? Do you by chance have a pic? This seems a little counter-intuitive, it feels to me that if you reduced chassis flex in the front, making the "overall" suspension less compliant (overall meaning that in this case frame flex is convolved with the actual suspension), you would tend towards understeer. It's late, I'll noodle that one in my sleep, but certainly an interesting question.

Mark

[Jeremy]

The tires are about a year old, 5 K miles or so. They are goodyear 275/40/17.

When I am in a tight corner, before the chassis would roll evenly and the car felt great. Now the front is flat and very tight feeling and the rear rolls as it did before and the rear feels like it wants to swap ends. If I add any throttle the car will quickly track to the inside of the curve.

Just from the feeling I get when I drive the car, it feels like I need to let the front suspension roll a little more now to make up for the roll/twist the body had before tightening the structure. I hope that made sense.

[Mean 69]

Yeah, that sounds like what I would expect in terms of the roll characteristics, but the reaction of the car is still puzzling. The roll resistance you add (relative) to one of the car, the more lateral load that end will carry, all things being "equal." In this case, especially a front engine car, you'd really expect the front tires to be overwhelmed at the additional loading, and the car would push. Not so as you describe things, sounds really strange.

I'd start with playing with tire pressures, as the guys suggested already. 44 PSIG is pretty darned hard, I'd guess that tire temps would show that the center of the tire is hotter than the outer edges, indicating ballooning. Getting more effective tire on the ground may be a really good first step, and it's free!

Let us know, this is an interesting problem.
M

[Jeremy]

I rechecked the tire pressures and they are at 42 after 5 miles of slow, easy town driving.

To test the theory that I needed less roll resistance on the front, I loosened the end links on the front bar until they were loose and I could move the bolt up and down. I then re tightened them until the bushings just touched the bar or a arm. This helped quite a bit as the car did not oversteer as badly. Under the same stretch of road, the car was much more predictable. It would still oversteer if I dove in hard, but the oversteer occurred at a much higher speed.

The groaning at the base of the dash was all but gone and impact harshness over rough roads is much much improved. Steering response is still crisp, just not right now as before. The car has a more refined feel to it now. It feels that both wheels can move more independantly of each other over bumps. The driveway cut that usually caused all sorts of noise, came and went without a peep.

While this setup will work and has made the car better, I may try a 1 inch bar as I should be able to get it for under $20 from the yard plus my time to pull it. The smaller bar will also save some weight off the front end if it ends up balancing things better.

I can also get a 15/16 from a gto from my dad for free if I need to go smaller still.

I would like to go with a softer spring instead of smaller bars, but the ride height is were I want it plus switching bars is easier than springs.

[David Pozzi]

If it's oversteering you need a softer rear bar OR stiffer front bar. The front bar you have can be stiffened with stiffer mounts or bushings than you have now.

I don't see how softening the front is going to reduce oversteer, but maybe I or you are missing something else that is going on.

There is quite a selection of rear bars availabe if you have the factory rear bar. You can either switch bar diameter or lengthen the arms a little to reduce it's stiffness. Stock second gen rear bars have very short arm lengths and are very sensitive to ride height changes so make sure your rear bar arms are close to level with car sitting on it's wheels. You can cut or lengthen the bar support links to level out the bar.
I have a list of second gen antiroll bar part numbers on my web page, it's on the second gen page.

You can remove just one rear link to test how the car would handle without a rear bar.

Make sure your car isn't lowered too much or it will bottom out on the bumpstops and handling will become unpredictable.

[Jeremy]

David,

That's why I posted my problem here as I was suprised that stiffening the front structure made the car want to oversteer as I thought it would have started to understeer with the mods, but it isn't.

As I posted, loosening the end links made the car more balanced so it seems that softening the front seems to be the direction to go.

For those that are interested here is a pic of the prototype I made to test the function of the pattern. It was thrown together real quick with scrap around the house. The pattern is what the finished product will look like. I like how clean the finished product will look, especially once painted.

[chicane67]

Ya know, for some reason I was looking at this as an >understeering< problem...... until I looked at the header again. So yeah, just the opposite of what I said.

Anyway. I think the smaller front bar and some more tire pressure adjustments might just be where you want to be.

[wally8]

Hey Jeremy,

Don't let yourself get confused. Softening the fronts will absolutely, positively, without any doubt, loosen your car. Reread that sentence again so you can be sure to understand.

Now, I'm not sure what's going on with your sway bars but the above sentence applies regardless. Personally, I would never use a rear sway bar for anything (well, maybe a street car).

Second: it sounds like you're serious about tuning this thing up so looking at the tires is great advice but do yourself a favor and buy an inexpensive pyrometer. You can't get good info just by looking at them. A pyro is worth its weight in gold for suspension tuning.

Last, here's my real world experience with stiffening a chassis:

We used to race GRT (brand) Late Models (on dirt of course). They were a hot car and were the favorite of some really fast guys and ours was fast as well. GRT sold a lot of cars and was looking to keep improving the chassis so they hired a race chassis engineer to help them go over the chassis completely and tweak up the design (the guy came from a successful background in IMSA, IIRC). The new car was much, much stiffer. It was called the Rigid as a marketing name. Lots of people bought them, including us. With similar setups the car was way, way looser than its predecessor. Even with drastic changes in setups, no one really ever got the car to turn well on a dry track and it never hooked well off the corners. It never really progressed much beyond one season. We sold ours off that year as well.

What it seemed to be doing was making the rear springs more effective and thus loosening the car. In other words, it was basically adding to the rear wheel rate (for newbies, this is the true spring rate that the tire actually sees). Any deflection is actually a spring, reducing it increases spring rate. Think of it in that way and you'll see how this will cause the car to become more loose. I'm sure Norm can put the math to this for everyone :-).

This is also not to say that a rigid car isn't desirable on a road course (it obviously must be since that engineer was successful on road courses), just not desirable on dirt.


Wally

[MrQuick]

Hey Tom, I figured you were just having another typing dycslikeit moment again. LOL

I think your tire pressure has alot to do with the oversteer problem.

[Norm Peterson]

What it seemed to be doing was making the rear springs more effective and thus loosening the car. In other words, it was basically adding to the rear wheel rate (for newbies, this is the true spring rate that the tire actually sees). Any deflection is actually a spring, reducing it increases spring rate. Think of it in that way and you'll see how this will cause the car to become more loose. I'm sure Norm can put the math to this for everyone :-).

Wally

Stiffening the chassis also makes it much more effective at shifting cornerweights, either intentionally where you want them to go during setup or unexpectedly in unpredictable (from the driver's seat) directions when one or more wheels randomly hit bumps.

Partly, it's a springs-in-series problem, which mathematically looks something like this:

1/[Koverall] = 1/[Kspring] + 1/[Ktire] + 1/[Kchassis]

Kchassis is not a single number, either, so that "equation" is only intended to illustrate how chassis stiffness fits into the overall picture. Chassis stiffness is a collection of stiffness terms rather than a single value. Discussion of stiffnesses in much more detail than that goes off into computerized structural analysis territory.

No cut and dried math/simple formulas exist for the kind of analysis that you're trying to do here for a car. This is one of those situations where you either need to have the base of experience to suggest when you have enough [chassis stiffness], enough without compromising something else too far, or a really good handle on interpreting what the computer output is trying to tell you with respect to your specific situation.

Jeremy's sheetmetal panel *may* be reducing front wheel compliance camber change and making that end stick enough better that the car can be made to go loose more easily . . .

Norm

[Mean 69]

Great stuff. The thing that puzzles me even further is how much relative effect the plate made things. If you look at the primary conrering forces, you can see that this plate really isn't in sheer, but rather in bending. In aggregate, being attached to the other mounts, etc, it still appears to be pretty effective. Nice bit of homegrown engineering, well done!

Mark

[1977nova]

Is what you are saying is that the more stiff your chassis is, the less need for a high rate spring? I know there are limits on this but would this mod with a lighter rate spring help? I can see your point on the fact that the chassis can affect spring rates. I kind of think now of the car chassis having its own spring rate in a sense.

[1977nova]

double post

[Jeremy]

I got a chance to drive the car more last night, this time on an asphalt country road. This road in the past I would struggle to take at 40 to 45 as it is got enough dips and rough spots that the car always felt like it was not staying glued to the pavement.

With the brace in place and the endlinks just barely snugged, the car felt great on this road. I could push as fast as line of sight would allow. The car was glued to the pavement and bumps no longer sent shudders through the car.

Just to see how far I could go with softening the front, I replaced the poly end link bushings with rubber. I have only had a chance to run the car in town, but suspension compliance is up some more. Around town turn in was still nice and it did not feel like I was giving up anything running the rubber endlinks--I have them pretty loose at that.

I'll post the results of highway driving when I get a chance. I am trying to track down a 1 inch bar to see how it works as its an easy swap. I may be able to get a little more progressive feel on the front turn in by using a smaller bar, but stiffer or tighter endlinks. I had thought about driving it in town with the end links unhooked just to see how things went.

For whatever reason bracing the two structure braces works. The braces always worked well in resisting bending between the frame and firewall, but not so good in torsion. Bracing them together has seemed to really make them work better in torsion now.

As for softer springs, it is something I would like to try, but two things are keeping me from it at this point. FIrst being my dislike of removing and replacing coil springs and second I am a little concerned that a softer spring may let me bottom out with the lowered front end. Right now I have enough spring rate that I don't constantly hit the bumps stops.

[1977nova]

what is the spring rate that you are using? I hear you on the not wanting to change the coils. I have been doing alot of researching on this so I dont have to do things twice but I am doing my entire front end so its easier for me. What type of sway bar bushings do you have for the frame? Maybe poly bushings with zerk fittings would let your sway bar move more freely? What do you mean about rubber springs in the leaf springs? Also using solid lower a arm bushings would lower your spring rate (although minutely) and also help suspension geometry. By no means am I an expert but maybe these suggestions would be benifitial to you.

BTW I love your idea for the braces. It has made me think that I can avoid subframe connectors and use your idea.