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89 6 Speed, Base Suspension. I run in B Street.
Car has 18" RE71r's, Adjustable Shocks and autocross alignment.
I have the shocks set 80% stiff in rear and 25% front
Car is great in transitions, but on corner exit as all C4's do it tends to push (under steer) making me have to lift.
I just ordered a 1" solid rear sway. I have not put it on yet.
My question is what adjustments do you who run a C4 see me making if any with the shock settings?
The reason I ask, is I have a Divisional coming up on a course that is wide open and I am sure I will see a ton of sweepers.
Wondering if I should just drive the car as is, since I know it or add the bar and then could quickly make adjustments to shocks or driving style throughout my runs.
What did a bigger rear sway do to your car? Magical? I don't want to go into snap overseer.
The shocks are the last item to touch. Shocks are for fine tuning after you get everything else right.
I asked Danny Kellemyer the same question about sway bars a few years back. His response was:
All Corvette understeer on entry.
All Corvettes oversteer on exit
What is the car doing mid-corner? That's the guiding principle.
Also, I have no idea what an 80% stiff shock means. . Get a dyno sheet.
I've used the Bilstein rebuild service over the years. They do my custom valving and then send me a dyno sheet. I have a tremendous amount of rebound. I do know that.
I use an adjustable rear bar.
You can go crazy with this arrangement. You'll need an engineer to figure out all the adjustment.
The shocks are the last item to touch. Shocks are for fine tuning after you get everything else right.
I asked Danny Kellemyer the same question about sway bars a few years back. His response was:
All Corvette understeer on entry.
All Corvettes oversteer on exit
What is the car doing mid-corner? That's the guiding principle.
Also, I have no idea what an 80% stiff shock means. . Get a dyno sheet.
I've used the Bilstein rebuild service over the years. They do my custom valving and then send me a dyno sheet. I have a tremendous amount of rebound. I do know that.
Thanks for the reply Richard, To be honest I have no idea what 80% means either, just that I have room to go stiffer.
Mid corner is under steering as well, I have another autocross this weekend, so I will pay close attention to mid corner.
I know there is a lot more to fine tuning everything, but I am learning.
I can only change one sway, so the front would remain factory. Just don't want to put a huge rear sway on the car and make it worse.
Is your front bar still OEM that came with the base suspension? What are alignment specs? Have you corner weighed your car yet and if so what are the corner weights? You do have some adjustment on the rear, not much but, some adjustment to jack some weight around legally.
Whate size tires are running and what are your tire pressures? What shocks are you running? If you're running a good autocross shock like a Koni, JRI, or a Penske, you may need to have them revalved/reshimmed.
Somebody like a Mike Maier, Inc. or Strano Auto Parts can help you with your shocks. If you're running something else that is off the shelf that is something else that I didn't mention, you're not running with a good shock and that may contributing to your problem.
If your adjustment adjusts the rebound adjust the rebound to full rebound and see if that gets better and than try full soft and see if that gets better.
Is your front bar still OEM that came with the base suspension? What are alignment specs? Have you corner weighed your car yet and if so what are the corner weights? You do have some adjustment on the rear, not much but, some adjustment to jack some weight around legally.
Whate size tires are running and what are your tire pressures? What shocks are you running? If you're running a good autocross shock like a Koni, JRI, or a Penske, you may need to have them revalved/reshimmed.
Somebody like a Mike Maier, Inc. or Strano Auto Parts can help you with your shocks. If you're running something else that is off the shelf that is something else that I didn't mention, you're not running with a good shock and that may contributing to your problem.
If your adjustment adjusts the rebound adjust the rebound to full rebound and see if that gets better and than try full soft and see if that gets better.
Let me know if I can be of further help.
Thanks for the reply
Shocks - Single adjustable QA1's
Alignment
Front -1.5 Camber - zero toe
Rear -.75 Camber - 1/8 toe
Tires
275/35 - 18 RE71R
Pressures - Start at 40 PSI all the way around ended last weekend at front 40 rear 38, but it was a wet track.
Yes
Stock front bar - 26mm
Stock rear bar (Currently) - 22mm - Have a 25.4 on order to try
have not corner weighted it yet and yes there is room to jack the weight with the spring legally.
I guess my question would be what is typically experienced going to this bigger bar. or it would almost be the same size bar front and rear with leaving the stocker up front.
I am still fine tuning the car, running at least top 10 pax of late but chasing C5's in BS.
It seems the C4 is long forgotten, C5 is the easy button.
I have not done any autocross in my 91 but I'll say that the factory setup was very balanced on a tight road course. According to the HH chart the only difference between our suspension setup was the rear bar, the 91 has a 24s rear bar.
The 1" bar is is pretty stiff but with a 39.9 rear spring it might work well. I would keep some body shims on hand to loosen the bushings just in case. (If your club allows it) I don't know how hard it is to find a 24mm bar but that is an option too.
is that all the camber you can get on a b street c4? More camber all around would be better. And you sound way high on pressures for the RE71r. I would think you'd be 4-5psi lower. But it's best to test and compare times or look at tire wear.
Also, every c5 or c6 street class corvette (BS, AS, and SSR) runs a larger front bar. Is the C4 that much different?
is that all the camber you can get on a b street c4? More camber all around would be better. And you sound way high on pressures for the RE71r. I would think you'd be 4-5psi lower. But it's best to test and compare times or look at tire wear.
Also, every c5 or c6 street class corvette (BS, AS, and SSR) runs a larger front bar. Is the C4 that much different?
They are pretty camber limited in the front, I pulled out all the shims in the front and that is about all you can get. I might have a touch more than that, but it is maxed so nothing I can change.
The rear I can set to whatever, I set it at .75, to be honest I don't remember exactly as I just set it half of the front.
Tire pressures have been all over, I used to run 71R's on my STR Miata and would go as low as 27, but when I went lower on the C4 I was wearing the outside. I plan on checking temp across them this weekend.
for an easier question..
What is your opinion of maxing the rear shocks vs a sway bar, or running a bigger sway with less shock.
just a discussion, I don't think I will try the bigger rear sway until after the divisional at a local event.
Chasing with a C4 you're going to have your hands full because you are under powered with the L98. Anyways, how much caster do you have? The QA1 shocks do adjust rebound along with the bump. I would work on the problem end first. That is what I did when I ran my 93 in stock now called Street. I had to have my front shocks revalved in order to get the corner exit problem resolved.
A bigger rear bar may loosen you up on corner entry or quick transitions. The QA1s should be rebuildable. I would have the shocks rebuild by someone who knows autocross shocks such as Mike Maier or Strano and see if they will rebuild your shocks.
The QA1s will also accommodate the Penske shock components. I would seriously think about putting Penske valving in your front QA1s if you want your shocks to work properly.
Based on what adjustments you have available, and if what NoKones describes with your shocks is correct, then take out rear toe or add more rear bar. That car needs to rotate on power, and it's currently not doing it.
They are pretty camber limited in the front, I pulled out all the shims in the front and that is about all you can get. I might have a touch more than that, but it is maxed so nothing I can change.
The rear I can set to whatever, I set it at .75, to be honest I don't remember exactly as I just set it half of the front.
Tire pressures have been all over, I used to run 71R's on my STR Miata and would go as low as 27, but when I went lower on the C4 I was wearing the outside. I plan on checking temp across them this weekend.
for an easier question..
What is your opinion of maxing the rear shocks vs a sway bar, or running a bigger sway with less shock.
just a discussion, I don't think I will try the bigger rear sway until after the divisional at a local event.
Shocks control the car in transition. Swaybars control steady state grip. Unfortunately the corners happen so fast in autocross that it's hard to differentiate between corner entry, mid corner, and corner exit and what the car is doing at each.
Personally I'd be doing whatever you can to get more grip at the front end of the car which will then inherently shift the balance to more oversteer. A front bar will help you keep what little front camber you have in roll. Looks to me like the car could be looser on corner entry as others have said. Adding rear bar will do it but I'm not sure that's the best way to go about it.
Again... IDK if the C4 is different enough from the C5/C6 to warrant a different setup. But literally every competitively autocrossed (national level) C5/C6 running in street is running a larger front bar.
In the past with stock setups, I have found that a 1/4" of toe out in the front definitely will help. First, I would look into the front shocks, insofar as the valving. I would have a shock expert that knows what an autocross shock should be doing, put your shocks on a dyno and see what they are doing.
I don't know for sure if the single adjustable QA1 shocks will accommodate the Penske valving but, I do know that the non-adjustable QA1 shocks will. I did that with my road race car because of the spec shock rule. It was a way to skirt the spec shock rule.
Also, another trick is take your car to a body shop that is an expert on frame straightening. Ask him to bend the frame tabs that hold the front upper control arms, to edge of the factory spec, in the direction to get more negative camber. If my memoory serves me right, that would be in the direction of car's centerline.
This is legal as long as the tabs are within the factory specs. You'll be surprised on how much negative camber you legally gain by having that procedure done to those tabs. Back in the day we use to see at the minimum about 2-2.5 degrees of neg camber.
With the extra camber and the right valving shocks you'll be surprised on the handling in the turns, especially corner exit.
Another stock trick, since the rule says you can only touch one bar and if you decide to play/changeout with the rear bar, shim the front swaybar bushing mounts that attached to the frame. Put something to shim up in between the mounts and the frame and this will help soften up the front bar action some.
The front spring on a base suspension for an 89 C4 is a 797 pound spring rate. That is not too shabby for a front stock spring. That is 214 pounds at the wheel and with the stock .9" swaybar, I think I would concentrate on the front shocks at this point, then work on the rest.
Also, driving style change may help. I'm not saying that you're a poor driver, a stock C4 has to be driven differently than other cars. It just takes some work at it.
I hope this helps.
Oh, by the way, the frame thing and shimming the front swaybar mounts was an old C4 secret for stock and street Prepared cars, so don't tell anyone.
Jstockton, I think you're getting lots of good advice. To go back to your original question, in general that bar will of course reduce understeer a lot or even induce oversteer. You're making a huge difference in rear roll stiffness with that big a jump in diameter. So if that actually gets you where you want to be in steady-state cornering and entry/exit, then you will likely struggle with oversteer in transitions. Your rear damper settings will need to be reduced quite a bit to compensate, and/or you will also want to increase the stiffness in the front dampers. Your current damper settings are surprisingly biased toward rear roll stiffness! One nice thing is that it's really easy to swap rear swaybars, so there's no harm in giving it a try.
I am surprised also to hear that your car understeers heavily upon corner exit. That hasn't been my experience with C4s in general, but my sample is very low. My own car is too heavily modified to be a useful comparison, but it tends to oversteer a bit on exit even though it pushes on entry (that entry push is a function of the high amount of anti-dive built into later C4s, I think). I, too, would recommend a bit of toe out since you have Street class limitations on mods.
They are pretty camber limited in the front, I pulled out all the shims in the front and that is about all you can get. I might have a touch more than that, but it is maxed so nothing I can change.
The rear I can set to whatever, I set it at .75, to be honest I don't remember exactly as I just set it half of the front.
Tire pressures have been all over, I used to run 71R's on my STR Miata and would go as low as 27, but when I went lower on the C4 I was wearing the outside. I plan on checking temp across them this weekend.
for an easier question..
What is your opinion of maxing the rear shocks vs a sway bar, or running a bigger sway with less shock.
just a discussion, I don't think I will try the bigger rear sway until after the divisional at a local event.
When you say you pulled ALL the front shims did you also remove or modify the spacers!
I had to cut the spacers way down and even eliminated the rears in order to get higher negative camber numbers. I think I'm set around -2.5.
While you do remove caster by doing that there is still lots and it was very stable even up to 155 mph. This is on a 88 and I ran with 315/30/17 (on 11x17 A spoke from the GS rear) on all 4
I'd send a picture but don't know how from IPad.
Cheers Rh with almost no shims and cut down front spacer
Lh with cut down front spacer
Close to stock with spacers
Last edited by yellow c5r vette; 04-06-2017 at 03:42 PM.
Jstockton, I think you're getting lots of good advice. To go back to your original question, in general that bar will of course reduce understeer a lot or even induce oversteer. You're making a huge difference in rear roll stiffness with that big a jump in diameter. So if that actually gets you where you want to be in steady-state cornering and entry/exit, then you will likely struggle with oversteer in transitions. Your rear damper settings will need to be reduced quite a bit to compensate, and/or you will also want to increase the stiffness in the front dampers. Your current damper settings are surprisingly biased toward rear roll stiffness! One nice thing is that it's really easy to swap rear swaybars, so there's no harm in giving it a try.
I am surprised also to hear that your car understeers heavily upon corner exit. That hasn't been my experience with C4s in general, but my sample is very low. My own car is too heavily modified to be a useful comparison, but it tends to oversteer a bit on exit even though it pushes on entry (that entry push is a function of the high amount of anti-dive built into later C4s, I think). I, too, would recommend a bit of toe out since you have Street class limitations on mods.
Thanks!
That is the info I was looking for, If I get the bar on and its loose, I can reduce the rear shock settings to get me through the day.
I think it is worth a try, like I said previously the car is pretty good now and I could always switch it back.
I have 2 more events before the switch so I should have some pretty good data by then.
When you say you pulled ALL the front shims did you also remove or modify the spacers!
I had to cut the spacers way down and even eliminated the rears in order to get higher negative camber numbers. I think I'm set around -2.5.
While you do remove caster by doing that there is still lots and it was very stable even up to 155 mph. This is on a 88 and I ran with 315/30/17 (on 11x17 A spoke from the GS rear) on all 4
I'd send a picture but don't know how from IPad.
Cheers
Would like to see a picture of this.
It was said before to pull all the spacers out thus changing your caster.
Out of the top arm, the two sets of shims I pulled all of them out of the rear of the arm and the same amount in the front of the arm on both sides, if that makes sense? I still have a few shims in the front of the arm. I should probably go ahead and pull those out as well. But that would change caster, which the way I understand it, I don't really care. Do I?
04-05-2017, 10:47 AM
C4 Autocross Setup (Rear Sway)
. Get a dyno sheet. 
