93Rubie

My 1st full season of auto-x is nearing a close and not only am I starting to run pretty well on street tires I am also getting a "feel" for what the car is telling me while I'm driving it.

A little back ground first:
1993 LT1 ZF6 FX3 Coupe 275/40/17's at all four on stock sawblade wheels
Running worn street tires, old, hard.
Alignment: -.5 degress of camber at all 4. About 5.5 degrees of caster in front. Toe: Zero in front. Slight Toe out in back.

I've driven it a lot (obviously) had a few others drive it as well all fairly experienced drivers.

We all agree car is very neutral in slalom, handles very nicely.
Has slight oversteer (throttle induced to some extent) in higher speed sweeping auto-x corners. I actually like this, the car is fast this way and its really fun. Slight drift angles, nothing out of control. Easy to control. To keep it in check a light touch on the throttle plants rear end while cornering. This light touch also helps the rear end to rotate as long as you don't get on it too much. Honestly,I find the car is under steering then over steering its hard to drive it neutral in any place other than the slolam. It teeter toters over neutral back and forth.
I've noticed the car has less understeer and less tail happy with full tank of fuel compared to half a tank.

The problem is low speed turns it understeers a lot, I have to be very patient and not over drive the car just let it do its thing and try to keep up momentum and stay smooth.

Remedies I've thought of: Get more camber, this is hard as I have about all but 1/16" of the shims pulled out on the left side.
Get more caster, again maybe a bit more but not much to play with shim wise.
Ditch stock FX3 shocks (which I love FYI) for a set of Koni's?
Run full tanks of fuel when auto-x???

Your thoughts on how to get it to turn a bit better in slow speed conditions WITHOUT hurting the neutral feel in transitions and the gentle oversteer in higher speed sweepers.



Also, your thoughts on running more rear camber say -.75 degrees to help it plant a bit better of the corner. Its does pretty good, but it could be a bit better especially in off camber surface situations.

PS. Thoughts on Star Specs on a C4.
PSS. I want to try and stay in stock class and run street tires for the time being. Obviously, I could dial out a lot of understeer with 315's, bars, springs, etc...but would have to run BSP.

FTD today was 31.0 I ran a 33.6. Keep in mind R-comps versus worn, hard street tires. FTD was a CP car with pretty good driver.

THIS CAR IS FREAKING FUN TO DRIVE!!! My co-driver and me today where trading who was going faster I beat him by a tenth or two.

Solofast

First you need to get all of the front end grip you can get. The real problem with the later C4's is that they changed the front suspension to the "zero scrub radius" design when they went to 17 inch wheels.

What happens is that the king pin angle is such that in low speed corners the outside front wheel loses all of it's camber when you go to high steering angles. Often you can feel the grip go away and if you "unsteer" a bit the grip will come right back.

To fix this you need to start with as much camber as you can get. Take all the shims out of the front end, you will have a bit over -.5 degrees in the left front, and about -.75 (or even more) in the right front. If you didn't do anything else, the car would turn left better than right, but that's what you get with a stock C4. They are all built like that. You can address that if you use the rear spring adjuster bolts to jack some weight off of the left front. Tighten the left rear bolt as much as you can and loosen the right bolt till it hits the cotter key. The end result is a good bit more front end grip, but you still maintain balance since you jacked the weight.

Next you need to get as much caster as you can get. Caster does the opposite of king pin angle, the more you steer, the more negative camber you get. So push the front upper arm mounts as far back as as you can get them before you tighten the bolts. Another thing you can do is make sure the front arms are pushed as far back, relative to the bushings as you can get them, that will give you more caster.

You really need about 1.25-1.5 degrees of negative camber in the back. You are giving up a good bit of grip with your present settings. Also you need to lose the toe out in the rear. It feels good, but it slows you down, since it doesn't let you put down as much power coming off of corners. Oversteer feels fast, but it isn't.

A properly set up C4 will have a bit of push. It won't plow, but the front end should hook and you should feel the front tires working a bit harder than the back. The front should bite and the rear should just follow and the car should go anywhere you put it.

The Koni sport single adjustable shocks were built for this car and are a lot quicker than the FX3's

If you don't have the Z07 springs and bars, you should, that helps the car roll a lot less and that helps it keep the negative camber. You are giving up a lot with the softer springs and bars.

Crepitus

Solofast

Oh yea, I forgot to mention that you run full fuel all the time. As you have noted it will put down more power with the weight behind the rear axle. Also run with the spare and jack in, if you take them out you won't put down as much power and the car will push more. I don't normally advocate ballast, but in some cases, this is one where it helps.

Moreover, the fuel tank on a C4 doesn't have baffles and the fuel pickup will run dry and the engine will stumble if it isn't about 1/4 to half full. But with that much fuel in the tank, the fuel will slosh over and give the back end a "whack" and it will feel like the car wants to oversteer just about the the time it should be getting settled. In slaloms there is a big pendulum effect, so you need to run full fuel all the time.

larryfs

you need toe IN in the rear, and toe OUT in the front. Not too much.

then you need to mill the spacers on front left control arm so the camber matches the right side.

The rear, I disagree with anything over -0.5 camber. My car just started oversteering so bad, and felt terrible when I went crazy with the rear camber.

Solofast

Milling the spacers isn't legal in stock classes. Not sure where the OP is running, but in SCCA or NCCC you can't mill the spacers.

Depends on the tires as to how much rear negative camber you should run, but I never ran less than -1.25 degrees. Rear toe should be about 1/8 of an inch total toe in. Rear toe out gives you roll oversteer and that's not a good thing.

larryfs

I've seen 4 different size spacers in C4 upper control arms. You can swap spacers as needed.

Solofast

There are different spacers for the 16 inch wheel cars, but you can't use those on the 17 inch cars. In stock classes you can't update and back date between years. I'm pretty sure the 17 inch cars have the smallest spacer in the rear of the upper control arm mount anyway (which gives you max negative camber and max caster), so it doesn't do you any good.

larryfs

If your that serious about it, yes. But on a local level, I have never seen anybody so picky about little tidbits. And you need a knowledgeable person who knows the details about specific years.

Local = don't matter.
National = yes does matter if you get real good.

Solofast

Well, maybe to you it doesn't matter but if you aren't legal, you know it.

And you have to ask yourself, did you really beat somebody or did you cheat. It isn't like you are doing this for money, it's just to prove you are the better driver. If you aren't legal, maybe you aren't so hot a shoe after all.

93Rubie

First off I appreciate the info on the spacers would do me more good if I was running BSP.

Using SCCA rules here folks and I try to stick to the spirit of them. B Stock class. Technically, I'm not stock class legal as I'm running a square 275 setup on a factory 255/285 car. However, everyone knows a 255/285 set up is crap, and if I beat you only having a little wider (stock size if 88-92 or 93-95 Z07 package) front tires, I doubt it was my wider tires on a otherwise completely stock car. So bite me on that one. Machining spacers that is a different cookie.

Run FULL tank=Check
Tweak my alignment=Check, I'll get this set next spring.

I'm going to check and see if the spacer is on the front or rear of the control arm. I don't remember off hand. I would assume the rear for more caster?

I'm curious about adding more rear camber. Saturday my co-driver and I had 20 runs on the car. You can clearly see on my very worn out tires that the inside edges front and rear have a bit more wear than the outside. Would adding more rear camber not increase this?
Him and I discussed adding a bit more like -.75 instead of -.5 on the rear for more bite as you suggest. This is a street car too so I worry on tire wear. I don't want to get TOO aggressive with the alignment.
FYI, my street tires that I don't auto-x on, they are wearing pretty good with my CURRENT alignment specs.

I'm surprised at the spare tire thing, I hate to throw more weight in the car. Full Tank in "race trim"=3300lbs. In street trim with full tank I'm close to 3400lbs. Yikes, I have the spare, jack, jack handle, spare carrier off the car, right now. The spare doesn't hold air.

If you (or anyone else) knows of some Z07 springs FSK/NYU and a 30mm solid front bar for sale, I'm all ears. I've been looking but are hard to find. The rear spring is easy, the front is hard so is the bar.

I think the Koni Shocks are a good idea. I'll put it on my list of wants. Are the sports the red ones or the yellow ones. If I remember right stock class your allowed 2 external adjustments on the shocks?

What do you think on the tires issue? I want to stick with streets while I'm still learning. They don't hide crappy driving the way R-comps do. Best to learn good habits. Just my thoughts on that one. Everyone is saying I'm doing pretty good for what I'm running with so evidently I'm learning.

Star Specs are my current lean but a little more money. DW=less dry, GREAT wet. XS=GREAT dry, crap wet. Star Specs=a very good compromise in dry and wet capabilities. Just not sure how the Star Specs would do on a heavier car. I hear they can get greasy on a hot day, some have to mist them down. Not a big deal but something to think about.

P.S. that Zero Scrub Radius is good for the ABS on split friction surfaces. Gravel/pavement. Dave McLellan's book Corvette from the Inside was very informative. Just wish he would have discussed the handling of the car more and why they went with different spring rates/bars etc....

Rob31

Would anyone really notice if you took a couple mills off the rear control arm spacer ? Do they look the cars over that much ?
Xs tires arent that bad in the wet . Its when its under 50 degrees that they suck .

larryfs

Depends on the local club. 1 driver in class. makes no difference. It's a fun day. There's nothing to prove running 5 laps in a parking lot, on worn street tires.

When you start worrying about tuned PCM's, milled spacers, fuel pressure regulators, TB air foils, then the fun is out of it.

Rob31

I come from an oval track background ,so Im used to cheating on a much bigger scale .

Crepitus

93Rubie

I typically do not auto-x under 50 degrees so I might be ok then?

Yes, I could probably get away with tweaking this and that, however, I personally want to know I beat/ran a good time in stock trim not a tweaked and messed with trim.

93Rubie

I appreciate all input especially Solofast's. Bump for any additional thoughts/details especially on tires.