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 :-).

WallyStiffening 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.

[Norm Peterson]

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.Yes. And this has even started to get some mention in the monthly magazines where "increased chassis stiffness has permitted the engineers to use slightly softer springs" or wording to similar effect.

Even though it may be difficult to visualize a chassis as being flexible, they all are. Even Cup cars.



. . . using solid lower a arm bushings would lower your spring rate (although minutely) Not necessarily minutely. Information I got hold of from a Mustang site (I think) put the wheel rate effects of OE rubber bushings at somewhere around 10% of the rate from the springs themselves.

Norm

[Jeremy]

My front springs should be between 420 and 460 and I am unsure of the rear, whatever stock ws6 would be. I am running rubber bushings in the front leaf spring eye and poly in the rear eye and frame mount, not rubber springs--sorry about the typo.

I have poly frame bushings on the front sway bar and it rotates freely--I checked to be sure when I swapped endlinks today.

Someday I plan to update to tubular lower arms with delalum or similar bushings. If I can't find tubular arms I like, I will most likely just do the bushings, although the rubber work nicely for the time being.

[1977nova]

your setup is pretty much what I am going with on my Nova. I will be using ws6 front springs but still unsure of rears. Are you running subframe connectors? I dont think I will need them with solid body bushings and g braces with your modification. I did have an idea however, I was thinking you could run plate steel across the top of the firewall going across from the fender bolts. Then you could bolt the entire plate across your firewall with bolts and then you could mount your g braces to that. I just thought that would be better bc I see that they are just mounted to the sheetmetal on the firewall. This may also help stop your firewall from creaking if there was a more solid mount for those arms.

[David Pozzi]

Your car may have a front weight percentage around 55% but a front roll stiffness of 75%, so the front is stiffer than it's weight percentage, but if you increased chassis stiffness I would guess the rear is now more positively connected to the front weight mass and makes the rear carry more of the roll load.

I still don't see how softening the front bar would stop the rear from sliding around.
I'd like to see you put the front bar back to where it was and loosen the rear bar.

[Jeremy]

David,

Since it does not cost anything, I'll try to put the poly endlinks back on the front and loosen the rear ones but from what it feels like in the driver's seat, I would think that things will get worse.

Right now with rubber end links just snugged to the point that they don't rattle, the car is very neutral at first, then going to a hint of oversteer the harder I push it. From the driver's seat it feels like the car is neutral during the first part of the body roll and then as the endlinks get to their limit of compression, the car starts to oversteer a little bit.

Up to that point the car feels great, as if both the front and rear are rolling at the same rate. The car feels very level when this is happening,

Before, the front was so flat that it felt like the front end was litterally sucking onto the pavement but the rear was rolling. The front end felt like the inside corner of the car was lower than the outside corner if that makes sense. It was really un nerving to me.

[WS6]

jeremy. what goodyears do you have on the car? if they are F1 GS tires which is what those rims were wrapped in from the factory then get rid of those tires. they suck. i had a set on my formula and i could shred those things going around corners. i have since used F1 GSD3s and now i have firestone wide ovals. boths these tires are so much better than the regular F1GS tires. the difference is huge. also drop your tire pressures in the back which im guessing youve done already.

your problem has me stumped as well, but i still feel that loosening up the rear will help you out. not sure how this would or could affect you but make sure your front bar is laying flat when the car is on its wheels. you dont need it twisted up or down any. the front end links only need to be tightened enough that the bushings start to buldge a little.

do keep us informed as i dont want issues liekthis with my 79 since ill be building a similar setup.

[Norm Peterson]

. . . The front end felt like the inside corner of the car was lower than the outside corner if that makes sense. It was really un nerving to me.That may be partly a matter of perception. From the driver's seat, the reduced roll that comes from the bigger bar shows up partly as less compression at the outside front corner. If your usual visual reference has been the roll angle itself, the outside front will now look 'high', and the whole front end may appear to be abnormally high as well. That's what I recall having observed for what was perhaps the first suspension mod that I ever did - the addition of a 7/8" front sta-bar to a 1964 Dodge which had not been optioned with any bars whatsoever. As you have noted, it's definitely a 'weird' sensation.

Norm

[Jeremy]

I went to my Haynes manual and checked the torque specs for end links and found that I had the poly links way too tight, causing some bind in the suspension.

When I backed the poly links off I probably had them closer to where they needed to be. The reduced chassis flex probably let me feel the bind in the front end. At least this is the most reasonable explanation I can come up with.

I have been reading up on how to tune the chassis through tire pressures, although I still haven't got that figured out. I haven't adjusted the pressures yet. If I want to loosen the rear, do I raise or lower pressures? How does varying the pressures affect wear? I was always under the assumption that tires ran their best at the recommended pressure on the sidewall.

I'll check to see what style goodyears I have as I can't recall.

If playing with tire pressure doesn't dial it in a bit, I have thought about using rubber on the rear sway bar to soften it a bit without looking for a smaller bar.

The good news is I talked with my dad and he will be able to make a finished product for me this weekend. I should have it back by Monday and be able to get it painted. I'll post pics of the finished product when it's done.

I do appreciated everyone's input as I have learned a lot.

[Norm Peterson]

The longer this thread runs, the less sure I am that we're all on the same page with respect to defining 'oversteer'. Could you elaborate a bit on this part of your topic starter


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.including what you were doing with the steering/throttle/brakes and whether the car was trying to plow straight ahead (and was responding poorly to more steering) or felt like the tail was getting a little loose.



I went to my Haynes manual and checked the torque specs for end links and found that I had the poly links way too tight, causing some bind in the suspension.Even the torque specs for OE rubber are really too high for poly, and only serve to make it more likely that you'll break an endlink. Snug enough so that you can't rotate the endlink sleeve by hand is better yet (just use a little Locktite blue to prevent endlink self-disassembly).


I have been reading up on how to tune the chassis through tire pressures, although I still haven't got that figured out. I haven't adjusted the pressures yet. If I want to loosen the rear, do I raise or lower pressures? How does varying the pressures affect wear? I was always under the assumption that tires ran their best at the recommended pressure on the sidewall.The inflation number that's molded into the tire sidewall is the maximum inflation pressure that the tire is rated for, and that has nothing to do with what any given vehicle and tire size wants for some given usage. The car mfr's recommendations are a better place to start your inflation pressure tuning from, assuming that you're running a tire with similar load capacity to the tire size that they've spec'ed the pressures for.

Tire load capacity varies with inflation pressure, by about 30 lbs per psi in the sizes that you're likely to be interested in. So as an absolute minimum, you want there to be enough load capacity at whatever inflation pressure to be greater than the corner weight (plus some margin for lateral & longitudinal load transfer).

In moderate doses from the OE recommendations, increased inflation sharpens response, reduces slip angles, and can improve the grip. Slip angles are what define understeer or oversteer, and to move in the more understeer direction you'd either reduce front pressure, increase rear pressure slightly, or do a little of both. The converse is true for moving in the oversteer direction with reduced rear tire pressures relative to the fronts and relatively greater rear slip angles, hence the recommendations to run a little less inflation pressure in the rears earlier in this discussion.

Understand that there is such a thing as too much inflation pressure, to the point where grip is lost and slip angles rise again, and this may happen before you reach the max inflation pressure on the tire sidewall. It's a fairly well-known trick within the autocross community to use this phenomenon to help get heavily understeering FWD cars to rotate. At 44 psi in the relatively lightly-loaded rear, you may be in this situation. Understand that this is separate from whatever additional compromise you might make to suit rough pavement.

Re: the new Firestone Wide Ovals, you might want to read this review first (http://www.tirerack.com/tires/tests/fs_firehawk_wide_oval.jsp), and then some other reviews at the bottom of this page (http://www.tirerack.com/tires/tires.jsp?tireMake=Firestone&tireModel=Firehawk+Wide+Oval). If they're anything like the SZ50's you'll be happy with them except in snow/ice conditions.

Norm

[yody]

wouldn't a "bar" running from one bar to the other with a heim joint and a bracket work just as good or better than the plate? I say to use heim joints and brackets so you could remove it.

[Norm Peterson]

I think to duplicate what the plate is doing with bars would require at least two bars (three being better), and at least one of the two would have to triangulate off toward the opposite-side aftermarket bar assembly. Methinks the carb and/or air cleaner might argue . . . and working a diagonal from off the middle of a tube or using a bent tube to gain a little intake clearance are not efficient solutions for adding stiffness. By the time you've supported the bent tube and not tied it off to the middle of a longitudinal tube - you're pretty much back to Jeremy's design.

Norm

[Jeremy]

Norm,

When I am getting the oversteer, I am trying to maintain a constant throttle opening and the rear of the car wants to come around and I end up tracking to the inside of the turn. I usually have to lift off of the throttle are lessen my steering input to keep the car from running off the inside of the turn.

Yody,

I looked at trying to build something out of tubular bars but could not find a way to fabricate it quickly and cheaply. The other thing that I was looking for was as low an attachment point as I could get to maximize the triangulation. With the plate, the lowest mounting point is about an inch above the valve cover. The prototype can be unbolted in about 5 to 10 minutes so that is not a problem. I use grade 8 1/4 inch bolts with body nuts with the built in washer. This nut washer combo was really common on the 2nd gens and I had plenty lying around. The washers also have some teeth on them which help keep them from loosening up.

The other problem with tubular brace is that as it got closer to the air cleaner, due to the thickness of the bar, I would have to move higher towards the firewall. With the plate, I fit it to have about 1/2 inch clearance around the air cleaner all the way around, while staying as low as possible.

I'll have the finished product Monday and will post some close ups from different angles so you can see the tight fit.

I definitely had the end links way too tight after hearing your description of how to tighten the poly links. When I loosened them up, that is how I set them up. I looked at the poly ones again and I think I had them so tight that I have deformed them. They look a lot thinner than new ones and flat. They don't have any rounded profile left to let the bar rotate.

I would say this was a lot of the problem. The ends of the bar probably had a very hard time rotating at all.

[David Pozzi]

It might be that the front bar being too tight was affecting front to rear weight transfer. You have kinda soft ft spirngs and if the bar was binding it held the front down.

[WS6]

i think thats a good evaluation of the problem right there. this is why i dont torque end links and just check for the bushings to buldge. checking the rotation of the sleeve is a great idea. ill be sure to try that as well.

Jeremy put the air pressure in the tires at 33-35 cold. this is what i ran mine at when i was running on the street and that was fine for the street. if i thought about it when i autox i bumped the front to 38-40 and dropped the rear to 30-32. i do this with the wide ovals as well. granted this is for my formula which is a little lighter, but the weight distribution of these cars is similar.

kinda off topic but when i bought my wide ovals tire rack didnt have any evaluation info on them. i didnt even know they had them. i was going to buy Kuhmo Mxs and the guy i was talking to just happen to have already tested the wide ovals himself and though id like those better. they were $10 cheaper than the MXs so i bought them and ive really liked them. if it snows or ices down here i walk. this southern boy doesnt know how to drive on snow.

[Jeremy]

I stayed with the rubber end links and tightened them to spec as well as setting the tires to 36 psi cold. The car now responds like I thought it should when I began the project to stiffen the front end structure.

The steering feels a little heavier, but turn in is crisp and predictable. The car has a little understeer at slow speeds and as speed increases, the car balances very nicely.

As for soaking up bumps the car almost feels soft. It sails over bumps without any drama or harshness.

Thanks for all those that took the time to help me figure this out as it just did not seem right to me. My best guess would be that with the end links so tight, the bar could not rotate and the body was flexing to soak up bumps. When I stiffened the front structure, I don't think there was much left to flex and things did not work right to say the least. Now that the suspension is free to rotate and the body structure is stiff, all is right with the car.

On a final note, I did not realize how much tuning could be done with tires. I have only scratched the surface with lowering all of my tires from 42-44 to 36, but the difference was very noticeable.

I talked with my dad and he is cutting the final product today. I will have it Monday and will post pics of the finished product after I paint it and get it installed.

[David Pozzi]

Jeremy,
In general the lower pressures increase the tire footprint which helps traction (bigger footprint). The limit is when the tire sidewall rolls over and the tire "gives up". The tire is a spring, (a stiff one) so lower pressures equal lower spring pressures. The car will roll a bit more and the tire will distort under load more. I'd imagine with a car as heavy as yours, you'd want 36-38 psi for large radius turns but need higher pressures if you encounter tighter turns or more severe manuvers like Autocross. Some Autocrosses have a good "flow" and some have turns where you really dive in under hard braking and turn very tight, this loads the tire very severely and rolls it over onto the sidewall unless you pump up the pressures.

[wally8]

One more note about tires:

It's been my experience that you can really only tune with air pressures by +/- 1 or 2 lbs. After that you start to compromise the contact patch too much and should switch out to another tuning method like springs or sway bars. The loading on the tire has a lot to do with it though so there could be a little more leeway in some situations.

This however is with bias ply tires. Radials could be a little different and maybe someone can chime in with numbers for them.


Wally

[David Pozzi]

Wally,
I'm not a fan of tuning with pressures for oversteer/understeer. I'd rather set pressures to prevent tire roll over and then tune the suspension to get the balance I want.

I've seen some autocrossers fussing with tire pressures and getting beat because they did too much to tune handling with pressures and ignored tuning the suspension. I allways figured if you have to change a tire pressure a lot, you are reducing that tire's traction ability. That may help balance a car but you are reducing available traction, better to tune the suspension and redistribute the traction to the end that needs it.

[Norm Peterson]

For the guys who auto-x their daily drivers, tire pressures, swapping the front sta-bar, and shock and alignment adjustments may be the only class-legal tuning options available (and not everybody can or does take advantage of all four). Some tinkering comes between runs even after a good set of hot pressures has been determined, as it is entirely possible to gain a psi or two (and 50+ lb/in of tire spring rate) during a single 45 second run.

Norm

[WS6]

i autox my car with the same setup that i drive on everyday. the only changes i make are to tire pressures. by no means is my setup optimal. all im really wanting to add to this discussion is that sitting still on a black top in between runs will get the tires very hot if your in the sun. i made better runs on cold tires simply because i could not shade my radials from the heat/sun. if its not so hot or sunny i dont have this heat issue. with heat comes increased pressure. so its worth keeping an extra eye on it on very hot days. once too much heat gets into the treads though its over unless you can cool them off. heat is what affects me most. this is my experience with my car on 9in street radials

[Jeremy]

Things are finally coming together. I got the finished brace in (still needs a little cleaning on some edges and paint) and it fits and works nice. I have attached some pics of the finished product to show how it clears the valve covers and air cleaner.

The other thing I think that was contributing to the weird handling was that the front was now drastically stiffer than the rest of the car. I fab'd an x brace that starts under the firewall and runs back to about the rear of edge of the door. The brace mounts the subframe, tranny crossmember, and subframe connectors together.

Once this was done, the car responded as we would have expected. The car now understeers slightly and feels very predictable. The ride has improved by leaps and bounds and rattles and squeaks are all but gone. I'll post pics of the x brace the next time I get the car in the air.