kimmer

Ok guys, have a 92 that has an understeer issue while auto-xing. Wondering if I am running tire pressure too low, 29 front and 28 rear. I have chalked them and thats how I came up with these pressures. Also have 1.5 neg. camber all around and about a 1 1/2 body drop. Any thoughts? Thanks.

VetteDrmr

Corner entry or exit?

Entry is, at least in my case, coming into the corner too fast.

Exit is, again for me in my C5, mainly due to tire size. 275s all around oversteer quite a bit, 275/315 understeers on power down, 275/305 is nicely balanced.

I'm not good enough to notice small changes in tire pressure. 5 psi, maybe.

HTH, and have a good one,
Mike

jaa1992

I had some oversteer issues at CMP and it was reccomended to me to play with the tire pressures.
Rear - both tires the same, Hoosier R6 30-32 psi
passenger front - 1 psi less
Driver front - 2 psi less
It seems to work, I didn't break the rear end loose the rest of the weekend.
Even ran it that way at VIR and Charlotte with no issues

I have 0 camber on the front (can't get any ) and right at -1 on the rear

kimmer

Is the difference on the fronts making up for no passenger. BTY I run 275's front and 285's rear. Don't know why you can't get neg. Iv got the same year and it's lowered.

kimmer

Yeh, corner entry, if
im going too fast I'm just pushing.

racerbob4

To decrease understeer: lower front tire pressures, raise rear pressures, more negative front camber, more positive rear camber, more front toe out, more rear toe in, more front wheel caster, softer front springs, harder rear springs, soften front sway bar, harden rear sway bar, use larger fron tire section, use smaller rear tire section. If you have balast in the car move it rearward.

Solofast

C4's tend to like a square setup. You haven't mentioned if your car is stock otherwise, or have you changed springs or sway bars. Without knowing what else you have done it's pretty hard to tell what is wrong or how to fix it.

Fill us in on how the car is set up and we can probably get in the ballpark.

kimmer

Sorry, It's a stock setup except for being lowered 1-1 1/2 inches and 1.5 neg. camber on all 4 .

RX7 KLR

Well there you go, you found the problem, now you just have to fix it. Slow in, fast out.

But there are things you can try... A square set of tires can help. Also try some rake, lift the rear a 1/2".

shifter77

Lower rear tire pressures by 2-3 lbs from what you have up front. Check tires only when COLD.

jaa1992

Yep, for time trials I only run a square set up 275 or 315 on all 4 corners. I don't know why the different pressures on the fronts work, but it does. I only see understeer when I enter a corner too fast, I try to follow the slow in, fast out school of thought which is why the oversteer I had threw me off. I had never had it on that turn before.

kimmer

I'll bring up the rear a tad for this weekend and see what happens. As for slow in fast out ,I try but you know how that goes.

Solofast

Does the car have a Z07 suspension or is it a base car? The base springs allow a lot of roll, and if you are going to get serious about autocross you should get a set of Z07 springs and bars. If it's a base spring car you could go to the Z07 rear bar, but that might be overkill.

Most of my experience is with Z07 setups, and they were very tunable, but they liked being square. If it is a base suspension car you can put some additional preload under the rear sway bar rubber bushing on the frame, (like a large 1/16 inch washer) that will help give it a slight increase in rear bar stiffness and that might help.

Here is the spring charts for C4's... http://www.corvetteactioncenter.com/...usp_chart.html

0Vector Vette

Quick and cheapest solution is to change you toe setting. Your front should be 3-4mm toe out. You will not like this for highway travel though, so learn how to change it.
Robert

kimmer

I'll have to find the sheet with the specs of when I had it set up at the begining of the year to see what the toe is. I do know that the camber was -2 all around.