pablocruise

Hi guys, I installed poly sway bar bushings and endlinks front and back. Then I had an auto-x last Saturday. This time out, I had much more oversteer than the previous auto-x with stock rubber sb bushings. Previous tire pressures were at 35 and this time I went to 40 psi all around. I noticed I wasn't plowing through turns anymore and body roll was dramatically reduced and was pleased about that. But I kept fighting oversteer all day. I dropped rears to approx 38 on the last run but still suffered os. Gas in tank was approx same as previous.
No other changes from previous auto-x.
Is the pronounced os a result of the polys or tire pressure? Should I have dropped rear psi more?
I'm kinda new to suspension tuning and am beginning to "feel" these kinds of adjustments. What do you make of these conditions?

urtoslo

So the rear of the car was spinning out (oversteering) instead of the front plowing and not going into the turn (understeering)?

If you were mildly oversteering, try putting fresh rubber bushings in the rear sway bar, leaving the poly stuff up front. What size sway bars do you have? Try increasing the size of the front bar to reduce a more pronouced oversteer. If it's really bad, do both.

You do have to adjust how you drive when the car does not understeer and tends to lose the rear first oversteering. Personnally I can autox my car faster when it is not understeering, a point and shoot style instead of slowing down till it finally turns..



[Modified by urtoslo, 7:55 AM 7/17/2003]

jackmott

its kinda hard to diagnose suspension issues through print. Any corvette can have serious oversteer issues if your right foot is just a hair too enthusiastic.

the car should react to everything quicker with the stiffer bushings so maybe that is what is getting at you.

anyway, its always good to change one thing at a time. go back to 35psi and see how that feels. then work from there.

ZR1 MK

What tires are you using? Why go from 35 to 40 psi? Tire pressure seems very high unless you have specific reason. Try 32-34 front and 28-30 psi in the rear. Many people seem to like high pressure. I never did, except when I used BFG G Force R1. Put rubber bushings back in rear bar.
Mike

pablocruise

I dunno where I picked that up. No specific reason. I wanted to put white shoe polish on the edges do a run and come back to see how it wore off, but I didn't have any. I still don't have a pyrometer either. Sway bar up front is 30mm so don't know if I can go any bigger? Rear is 26mm. Tires are my daily driver BFG KDW's. 275/40/17. I will go back to 35psi then. Maybe less. I did fairly good at that psi last time out.
I threw away the front rubber, but I still have the rears. Judging from my timeslips and a video of the runs, I was still accelerating slower compared to other C4's. Second slowest of the group. In conversing with other drivers, most were using 2nd gear and occasional guy in 3rd as was I. I just couldn't figure it out and got frustrated/called it a day.

ZR1 MK

I use 30 mm/26 mm solid bars with urethane bushings at bars and end links. My car has basic nuetral steer. Slight understeer to 40 mph, nuetral to 70 mph and slight oversteer to 125 mph. This has been with 1990 ZR1 spring rates and coilover with 500/300 springs. I mainly do road racing, but for autocross, I never ran higher than 30 psi in tires.
Mike

rbeckham

(Edit re sway bars)

My experience is that increasing tire pressures UP TO A POINT helps, but once you go over that point, I tend to skate around like on ice. Depending on your setup, this could show up as either under- or over-steer.

Other factors of course are track temps and conditions. As someone said, it's best to change only one thing at a time to try to see how well changes work. I'm sure your poly bushings will help, but also for sure you will need to experiment with pressures to balance the car. (FYI - I know extremely fast C4 autocrossers who run much lower pressures than you).

Also FYI - if you have oversteer, you do not want a larger front bar. This stiffens the rear roll resistance and will increase oversteer for sure, particularly on uneven track surfaces. Once you have everything sorted out, you could consider a larger rear bar, but I doubt you will need or want it. Good luck.


[Modified by rbeckham, 6:28 PM 7/17/2003]

jackmott

you seem to have your front and rear rollbars confused.

a stiffer front bar will increase the front roll resistance, and move towards understeer

also, regarding tire pressure, for a given tire and load, there is a pressure where the tire gets maximum traction. Above OR below that, and it will lose traction. Normally youll want the front and rear at their ideal pressures. Usually that will be higher for the front since it has a higher load on it.

rbeckham

jackmott - you are absolutely correct re the swaybars. On a rear drive car, the bigger front bar increases front roll stiffness (duh) and tends to increase understeer (or decreases oversteer, depending on your setup and point of view). I misspoke off the top of my head. But I don't think this is the problem. These are reasonable bars, and should work.

RE tire pressures, I don't think the results are linear up and down from "optimal," whatever that may turn out to be. I'm not recommending any particular pressures, but too high gets you skating, and too low also is bad, but still provides grip. Regards, Robert


[Modified by rbeckham, 7:14 PM 7/17/2003]

urtoslo

I've got a 32mm front bar for sale. That will squelch any oversteer!

You could also put 11" rims with 315's on the rear to reduce oversteer.