astock165

I've been struggling with the new setup on my 93 coupe a/x car this season (95k miles). Previous 2 seasons were run in AS on 255 front/275 rear Yokohoma A032R tires on a base suspension (can’t remember the code right now … FE3? … the base suspension w/o adj shocks). The car had a typical street alignment, minimal camber, touch of toe in etc. With that setup the car only had two handling characteristics: poor turn in and a ton of push.

Over the winter I replaced the old rubber suspension bushings w/ poly, replaced the original shocks with KYB non adjustable, installed a camber brace, did my own alignment (front: -1 ¼ deg w/ 0 toe, rear: -3/4 deg rear w/ 1/8 toe in) and bought another pair of 9 ½” wheels. I now run 275/40/17 V710s on all four corners. I also reduced the weight in the back a touch by permanently removing the spare tire/ jack combo and its carrier. All this and a couple of other add-ons put me in BSP.

This new setup has helped turn in and seems to have reduced the understeer. I’m happy with how the car handles in this respect but have seemed to pick up a new trait of the rear end being loose. At times under straight line acceleration the rear tires will spin, not out of control but they’ll break free. In addition, the rear end slides more in the turns.

I was slow before because the car handled poorly in that it couldn’t turn. Now it seems like I’m slow because I back off a touch as it feels like the rear won’t plant and am trying not to lose it and rotate. I’ve spun more this year than in my first 4 years combined.

I know a lot of things have changed but am trying to determine why:

Is it a function of the tires?
Is it a function of suspension?
Or a sum of everything I’ve done?

Any feedback would be greatly appreciated. I know suspension setup is a vast subject so I’d also like to also get some recommendations on reference material for vehicle dynamics and setup. Either corvette specific or just race setup in general. I’m usually a pretty good book learner so I’d like to know what to look for to read up on the subject.

Thanks in advance.
Tom

tigerdrvr

It seems like you would have more front end stick just on the basis of the front tire change, but I wonder if ride height is the same after the suspension changes and whether you have enough toe in the rear?

My first moves would be to see how much rake is in the car, maybe lower the back if the r/f rake is excessive and then dial in more rear toe in.

TravisD

Any chance this seems to start after your 1st or 2nd run? The V710's are gettin a reputation for not liking heat, at least for AutoX use. Especially with summer heat, I found mine were getting greasy after just 1 or 2 runs. I would suggest picking up a medium sized garden sprayer and bringing it into grid with you. When you get back to your spot each time, spray the tires down right away -- just a good even coverage. If it's really hot, maybe hit them again before you get back in the car to go out for your next run.

I started this at our last event and I found that it made a BIG difference in how consistent they were from the 1st run to the last.

Slalom4me

Binding in the rear suspension because of the poly bushes?

.

STEVEN13

[QUOTE]I would try one thing at a time. The first thing I would try is adding .25 more negative camber in the rear. This can help with sliding in the turns. However, if you're having a problem in straight-line acceleration this won't help.

Another solution may be to try 315s in the rear and leave all alignment settings as they are.

Good luck,

Steven

rhneff

Is it a function of the tires? Not likely - the V710 is being run extensively across the country without problems such as what you describe.

Is it a function of suspension?
Or a sum of everything I’ve done? Possibly - way too many variables to draw any conclusions from what you've said.

Before you go tinkering further with the suspension I'd first suggest you put the car on a skid pad and see what kind of temps you're getting across the tire patch. You didn't mention anything about what tire pressures you're running and that will dramatically affect how the tires work, so adjust pressures until you get consistent temperature variation across the tire patches. Then go make some runs at those pressures to see how the car responds, bleeding back down to those pressures between runs (and cooling tires, as noted in an earlier response). I've found a 1-2 pound change in tire pressure makes a significant difference on my 96.

bobmoore2

Here's another guess.

I'm guessing that you might not be getting as much weight transfer onto the real wheels under accelleration as you had before. Less weight on the rear tires under accelleration means less grip and more fish-tailing. This would also result in improved steering control (less push) under accelleration because more weight remains on the front tires. Maybe the new shocks you mentioned are too stiff to allow enough weight transfer under accelleration.

You're learning from personal experience that every suspension change improves one handling trait and hurts another one. Sometimes the trait that gets hurt is something like ride comfort, straight-line stability or tire life, which are compromises you can accept on a autocross car. Sometimes it isn't.

I could be wrong. Just a guess.

astock165

WOW, what great responses! Thank you. There have been a number of things brought up so I'll try to address individually and perhaps prompt more discussion.

I haven't checked it but I will. So, what is or where can I find a spec on rake? Rake is simply the delta between front and rear? So the car doesn't have to be perfectly level right? My concrete garage floor is fine?

It does seem to get worse as the day goes on and I attributed it to me just being more aggressive as the day goes on but I've seen a lot of guys with those sprayers. I was worried about causing damage by quickly cooling hot tires ... not an issue?

I'd eventually like to go to 315s all the way around but speed is only a matter of money right? I may try getting more aggressive with the alignment setup, will a little more toe in help with rear stability?

I usually run 32-33# cold front and 29-30# cold rear. I've come up with that by chalking the edges and watching roll. I've never used a pyrometer but think I made need to go that route. I do adjust back to my pressures between runs.

I'd agree, there does seem to be less wieght transfer and it makes sense that the rear breaks free more so I need to find a compensation for it.

I have learned a lot and for the most part it's been fun but the poor class and PAX result don't go well with my competitive nature.

Thanks again for all the feedback!

tigerdrvr

I haven't run a C4, only a C5, but...

Rake would be the delta between f/r. I'm not sure how you would accurately measure on the C4.

If you think you aren't getting enough weight transfer, one of the easiest things to try is dropping rear ride height a little. This will also give a little more camber. Benefits should include less tendency for the rear to "jack up" under braking. A little more toe in (at least on the C5, where I'm currently in the 1/4" range, depending on surface) will work wonders for off corner bite under throttle.

Of course, you do have lots of options in balancing the car. Tire pressure and shock settings are easiest, as others have said. Only other thing I would suggest is to make one small change at a time...

rhneff

Regardless of any other issues, you're definitely running too much pressure in your rear tires. Fronts are probably okay, but I'd suggest you drop the rears to 26-28 lbs. cold.

BrianCunningham

It's all a compromise. In order to get it to turn in and reduce oversteer, you make it loose on the way out.

Solofast

1.) Check the breakaway torque on the diff. If the LSD is loose it will make the rear end loose since the diff adds an aligning torque that tends to make the car understeer. If the rear end (diff) is loose, the rear of the car will be loose....

2.) You don't have enough rear camber in the car, and as noted earlier, you have too much pressure in the rear. Without enough camber, you are going to have to run more pressure to keep the tire from rolling over, and that seriously reduces rear grip. Spinning is often a result of a sharp breakaway, and that is often characteristic of too much pressure. If you have too much pressure the car will let go suddenly and you won't have much warning that it is about to get loose....

3.) Not sure what springs and bars you have on the car, but I am assuming that you still have the stock stuff on it. If you go to

http://www.vettenet.org/susp_chart.html

you can compare your springs and bars with those of the Z07, which was balanced with the 9.5 inch wheels and the same size tires all around. The Z07 had a larger front bar, and was balanced with the bigger wheels and tires in front. I suspect that you have gained grip in the front, but you likely need more front roll stiffness to get the balance back. It would be better to change over to the Z07 springs and bars as a package. Then, with the wider front rims you would have basically a Z07 and that would likely be a better starting point.

It is very difficult to get the balance right without an adjustable shock. Even if it is one way adjustable, you can alter the balance enough, generally speaking to divorce steady state from transient response. Not sure how the shocks are valved, but since you are dealing only with sway bar stiffness as a tuning tool, you should add some front roll stiffness (or soften the rear) to get it back under control.

The beauty of adjustable shocks is that you can dial in the stead state response with the bars and work on corner entry and exit with the shocks and get the car dialed in...

astock165

Thanks for the tip, I'll drop it a bit and try it out.

I was looking for that chart but couldn't find it. I do have the stock FE1 package on it. I would like to upgrade to the Z07 package and definately feel like I need some kind of adj shock.

Questions about the Z07 package:
Would a used set be OK?
Is there an aftermarket or non-GM equiv?
How would 11" wheels work with it? Or should I stick with the 9.5's that it was designed for?

Thanks again for all the feedback

Solofast

I would look for an astute wrecking yard and get the springs and bars from them. Make sure that the spring codes match the Z07 numbers and measure the diameters of the bars and you are good to go. I don't know of any aftermarket packages, but I am sure they are out there. Problem is they will be expensive and that isn't really necessary.

The 11 inch wheels will help, but you should make sure that you can get the correct negative camber, and that likely means offset lower control arm bushings.....

crznZ06

I actually have a set of '88-'90 Z51 springs sitting in my garage. The front spring was almost impossible to find. I don't really have any use for them, so I would like to sell them. Please let me know if you're interested. Good luck.

Bill Dearborn

Some number of years ago a friend of mine spent a lot of money putting in poly bushings in his 88. When he was done the car felt very tight on its suspension and the back end would barely move when you pushed sideways on it. After some initial handling problems he took it to Kim Baker (BakeRacing) for an alignment. Kim knows his way around C4's since he competed in Show Room Stock and the Corvette Challenge series in them. The first thing he did was put stock rear Z51 control rods (the two rods on each side that go back to the knuckle and hold the wheel in place from a fore/aft perspective). His explanation was that as the suspension travels up and down these two rods need to also pivot sideways just a bit to account for wheel travel which isn't straight up and down but in a slight arc. If they are too tight (as poly bushings make them) the rear suspension cannot work properly since the rods bind. If you used poly bushings in these rods this could cause the rear to break loose sooner.

Bill

MightyZR-1

I found that this adjustment works well on my car, to maintain control I check air pressure after each run.

trackdayc5

Bill is 100 % correct - back when I was autocrossing an '89 I picked up a loose condition after installing poly bushings in the rear suspension. After calling around to people who might know about such things, and spending some time under the rear watching the suspension move through it's travel, I determined that the bushings in the control rods must be allowed to twist as the suspension moves through it's arc in jounce and rebound. I replaced the rods with stock pieces and my oversteer issues were gone. I left all the other poly bushings in the car.

Gary

HammerDown

I'll just throw in my experience here, too. I have a '94 6-spd with the base suspension (FE1?). Anyway, I changed to Z51 Bilstein shocks all around, and 9.5 inch Collector's Edition 5-spokes with 285 tires all around. Both of these changes were at about the same time.

Following these changes, I too noticed exactly what you are describing. The car turned in much quicker, but after it did the back end felt like it got extremely "light" and wanted to come all the way around. Scared the crap out of me the first couple of times. The car turns very well now, but the back end never feels planted in the turns, almost like the rear suspension is becoming unweighted as the car rotates toward the apex. Needless to say, I cannot really turn the car in hard anymore for fear of the spinning it.

I've been running 30 psi all the way around on the street. The tires are Kumho Ecsta Supra 712s. Alignment is stock (whatever that is).