STEVEN13

Hi,

Thanks in advance for your help!

Car: 1992 FX3 6Spd

I used to run stock springs with the Z51 sway bars, revalved FX3 shocks, and Nitto R2s (285x18s and 275x17s). Hp-266 RWHP. Alignment: almost -2.0 camber zero toe front -1.5 camber 1/8 toe-in rear. With this combo-my best lap at LimeRock was a 103.9 on the old surface (10/2007) a cool day 50-60 degrees.

Current set-up: VBP adjustable front spring 1074lbs rear spring 411lbs. Also relocated the from swaybar mount and took out all pre-load. R888 scrub tires (date code 2007) 305x18s and 275x17s. Hp-289.7 RWHP. Alignment: -2.5 camber 1/16 toe-in front -2.0 camber 1/4 toe-in rear. The weather was around 85 degrees and sunny. Best Lap 106.1

The car was also very loose and got slower with each lap for all sessions-I was only able to get a total of 22 laps.

Here are my questions-
With the stiffer springs should I run a R6 or Equivalent?
Are the scub tires I purchased just used up?
Can I run an Nitto R2 or NT01 with this set-up?
Was the track just slow-Although the ST1 cars were fast
Or maybe I can't drive anymore-from my almost 3yr absence

Anyone running stiff springs with R2s or R888?

Thank you!

Steve

P.S. As slow as I was in my class (EMRA ST2) still managed to finish 1st in class-pure luck!!

Solofast

When is it loose, under braking, corner entry, mid corner or corner exit?

How do your spring rates compare with the Z07, stiffer or softer, front and rear?

You have modded the front bar mount, what bushings are you using?

Second, which rear bar are you using, you said Z51, but which year, or what diameter? There are several that you could be using.

-2 degrees of negative camber in the back is probably more than you want. With stiffer springs you have less roll and you added more negative camber????

How much rear travel do you have. Lots of people lower C4's in the back and cut the rear bump stops. Make sure you have enough travel back there, it can lead to nasy things on corner exit when it squats under power...

grantar2

The other question that should be asked in tire pressure. If it's really loose, might take some pressure out of the tires especially rear. Also did you do testing as you added each piece or did you just do a winter mod. If the later it's going to be a lot of work identifying what is giving you fits.

Sidney004

Its the tires. Just get some scrub R6's, soften them up with Formula V, run them about 32 psi hot and try it again. I have never seen any club racer go fast on scrub R888's. I don't know about the 275/17's up front; I really like the square setup on mine using Z06 18 x 10.5's with the best scrubs I can find. Phil has scrub R6's and he is in New York: http://philstireservice.com/#javascript:;

rfn026

Think Hoosier

Richard Newton
My Speed TV Page

Rob31

Where is it Loose ?
Ft. bar preload ? how much
-2 in the rear is alot . stock location on camber rods ?

STEVEN13

My response in Caps.

Thanks, Steve

STEVEN13

I did take out air in the rear-did seem to help. These mods were all done before taking it to the track. Thank you!

This is what I am hoping it is!! Thank you!

I may try new tires...Thanks!!


Loose in entry. Front bar has NO preload. Not using the stock camber rods (AKA strut rod bar). Thank you!

Solofast

Ok, this one is pretty easy.

What's happening is you have too much rear roll stiffness and it is leading to jacking and oversteer on turn in. The suspension geometry of these cars can only take so much rear roll stiffness before this happens. The total roll stiffness is a combination of spring rate AND rear roll bar stiffness. When the combined rate is too high the rear suspension jacks. This is most noticible under braking and on corner entry (depends on your driving style, if you trail brake it is worse). As you get off the brakes and get past initial turn in, the rear sets back down, and when you add a bit of power it squats and then behaves itself.

If you look at the C4 suspension rates ( http://www.corvetteactioncenter.com/...usp_chart.html ) you will see that the 26mm rear bar is only used with softer rear spring rates on the ZR1. They used it on the back of the 91 Z07 but that car had the same problem when coupled with the stiffer Z07 rear springs.

We had the same problem with our BSP car when we put 84 Z51 springs with a 26 mm rear bar. If your springs are stiffer than the Z07 and you try to put a 26mm rear bar it's going to happen and there isn't much you can do about it. Since you have stiffer sprngs than a Z07 you can't use any more rear roll stiffness than a 24mm bar, and even then you may have to reduce the rear stiffness by using rubber bushings and softening the preload with a couple of washers under the mounting bracket. I don't know how stiff your rear spring is, but if it's a lot stiffer then you have to reduce your rear roll stiffnes more to get it back under control. Once you reduce the rear roll stiffness you will have to adjust the front roll stiffness and/or spring rates to get the car balanced again.

Just like you we initially thought that going stiffer in the back would be better, so we went to stiff springs and a stiff rear bar and when we did skid pad and drove the car it was well balanced in mid corner and on exit, and a little squeeze on the throttle would settle it down, but it was loose if you weren't on the throttle at least a little bit.

STEVEN13

Thanks for this detailed response.

I think I will call VBP and see about another rear spring-this is the 2nd one I used. The 1st was a 488lbs and this one is 411lbs. I never road raced it with the 488 spring-but realized just driving on the street it was too stiff. This spring does fell softer-but as you point out, combined with my roll bar may be too stiff. I think for me-keeping the rear bar I have with a softer spring will be the way to go. I am also going to go with a square 275x17s tire size.

I will update my results-even if its a while from now!

Thanks again,
Steve

Solofast

I'd probably go with a 24mm rear bar, it's easier to change and you can still tune it a bit with bushings. With the Z07 rear springs a 26mm bar was still too much. The only way you can get the 26mm rear bar to work is by going to a ZR1 rear spring and that's too soft for serious track work.

JMHO, but I'd change the rear bar out first and see if that gets you where you need to be. If you go with a 26mm rear bar put in rubber bushings and space the brackets from the frame by 1/16 of an inch on both bolts. That will soften the preload and soften the bar a bit and that might be enough to get you into the ball park.

John Shiels

don't say that I'll be in the same boat! Why rear spring so soft?

STEVEN13

I called VBP and their response is similar to yours. Funny-because I thought my rear spring was on the stiff side. They are saying that it is on the soft side for a sport/track spring and would not recommend going softer. I gave away my stock sway-bars so I may try your recommendations on my current bar or get a different one.

Thanks again, I appreciate the help!!

Steve

STEVEN13

You will drive fine!! I'm a slow learner.

Compared to my stock spring-it is stiff.

Steve

P.S. When are you going out?

rfn026

I use the Corvette Challenge springs from Hyperco. These are coupled with the Z-51 bars. I got the springs from Corvette Central.

Richard Newton
My Facebook Page

Kubs

I dont have much experience on the Corvette leaf springs, but on my '91 Camaro I was actually faster on softer springs and big bars, than stiff springs and smaller bars. I believe (and this is just my opinion and works for the way I drive) that softer springs help absorb bumps and the anti sway bars and shock are stiffer to help in roll stiffness. When I had stiffer springs and smaller bars the Camaro seemed to hop over bumps and skate around. With the softer springs and bigger bars it seemed more planted (really good feel for a solid rear setup), easier to drive, and faster.

When I do springs on my car Im keeping the rear under 400lb and only going like 750lb on the front. I know it is more money and harder to change, but I would go the route of a softer spring but maybe thats just me.

John Shiels

next season.

Solofast

The problem with a C4 is that there isn't much travel available in the rear if you lower the car at all. You can cut the bump stop and gain maybe a half an inch more, but there is precious little travel to begin with and if you are lowered you can easily get into the bump stops on acceleration when the rear end squats and you are cornering, like on corner exit under acceleration. For track work you want to lower the car some and keep the springs stiff so that you stay off of the bump stops. You just have to soften the rear bar some to keep the total roll stiffness under control and the rear end doesn't jack on you.

I agree that often people set up their cars with too much stiffness, but in this case you just don't have a lot to work with.

ZR1 MK

Its seems there is a contradiction regarding the above, or maybe its me.
I think the spring rates a fine based on my conversion factor to change them to NM. Front a bit stiffer and rear a bit softer than 1991 Z07 springs.
Id try a 32 mm solid bar in front and get 315 tires on the rear for a better balance.


What are the spring rates? I checked your catalog and dont see spring rates listed for the different springs available.

Kubs

I see. I will have to look at my bumpstops. I have my car pretty low on stock springs/shocks/bars, and have a corner exit oversteer condition, but that is because the rear has heim joints and the front is all sotck rubber bushings. It was pretty neutral before the heim joints so once the front is as stiff it should be neutral again. It may be due to the bumpstops also, but it seemed neutral before when it was lowered.