racebum

I have a spring rate question for my c4. I just installed a VBP 491lb/in rear spring and love it, now it's just what to do with the front? I'm using the Koni yellow shocks, the car has a hot motor & runs 12.3@115 in the 1/4. The most likely tire i will run will be a Nitto 555R 315 in the back and a 555RII 275 in the front. The theory with the tires being that they are still streetable; yet quite sticky. VBP has said a 700-800lb spring seems about right. What do you guys think? They have a 770 on the shelf, or i can have a 700 made. The car is also still using tame sway bars with 26mm on the front and 19mm out back.

racebum

Perhaps i should add what it is i am after. I run lap days and am looking for a neutral balance. My front FHA spring is a little soft with this new rear, the FHA's are aprox 530lb, the car also gets quite a bit of street driving. I guess, more than anything i am seeking input from C4 drivers who have modified cars and run above stock spring rates.

96GS#007

I run '89/'90 Z51 springs on my GS so I can't comment on what spring rate you should chose other than to say that I'd lean more towards 700 up front vs 770. I would think you have a lot of oversteer with the FHA spring and 700 should balance things.

I will suggest you toss the idea of running the Drag Radials in the back. If pushed hard I think you'll find the sidewall rolls over too easily. It's unfortunate that the 555RII doesn't come in the 315 size.

Might also want to list the year of your car since '84-'87 front suspension is different than '88 & up.

racebum

I'm sorry, you're right, i should have mentioned that, it's a 1990 car. You are also correct on the FHA being waaaay too soft, you can feel the back end in corners letting you know when it's close. The front just kind of hangs out up there. The 700 range was exactly my plan, induce understeer, or a balance. It's almost silly right now with nearly equal spring rates. Have you tried the 555R personally? good on the street but i have never pushed them on the track doing a road course. 35-36psi seems to help somewhat with the soft sidewall. the NT01 was the other option but people say they don't live long on the road. With the KD's gone, very few 315s come up on my radar. And I HATE those Sumitomos worthless tire if ever there was one. Also aren't the 89-90 z51 springs basically the best of the c4? like 650lb ish? 115nm if i remember right which is somewhere around 650. Can i ask what rear spring you are using? If i remember right the only stiff factory rears were very early in the c4 production run.

96GS#007

I have the FHB spring up front and NYU in the rear. They're rated at 115.5n/mm and 57.2n/mm (~660 lb/in and 327lb/in) respectively. This combo was also used on the 88 Z51, the '89 & '90 Corvette Challenge and Z51 cars, and the '91 Z07. Is it the best? I think so, at least as far as factory setups go

I have not personally used the drag radial. I chose not to based on feedback from those that have and from Nitto. While increasing pressure may address the sidewall issue, you'll then have to deal with overinflation as the tire heats up. I use Hoosier R6s

racebum

How do you like the car with those springs? Is it fairly easy to control? Fairly balanced? I'm starting to wonder about the 770 beings my rear is over 100lb more than what you are using. I don't want an oversteer nightmare ride like i have now. Beides hopping on the gas out the apex is a wonderful way to oversteer !! LOL Also the Hoosiers are night and day a world above a dual use tire like i'm thinking of. Short lived but the level of grip i've heard is great.

Slalom4me

I have an '89 FE1, a dual-purpose car with std ride height and 275/275
tires on 9.5/9.5 wheels. Three of R-D Racing's braces are installed to
reduce chassis flex.

I use the stock FHA at the front (93.1 N/mm 531.6 lbs/in). At the rear,
I was aiming for 330 lbs/in but OEM NYU's were/are rare. I settled for
a VB&P "59-67 N/mm" that was picked from their available stock on the
basis of being near the low end of the limit (61.3 N/mm 350 lbs/in).

This combination of 93.1 fr/61.3 rr compares well with the Z07 rates
of 90.1 fr/57.2 rr.

With symmetric rubber, I found that 32mm fr/26mm rr bars tended toward
understeer in steady state conditions. I switched to a 30mm fr bar and
the balance is neutral now.

If I switched to 275/315 tires, I expect that my current neutral balance would
shift towards understeer. If I had the greater power that your car appears to
have, then more throttle would be available to me to overcome the increased
push.

491 lbs/in (86 N/mm) at the rear and a suitable spring for the front? My vote is
that this would be jarringly stiff for my circumstances.

.

racebum

Stiff it is, almost more than i would like. It's one of those things where i already own the 491lb rear, drive the car rarely and am setting the car up mainly for lap days. You are also correct that the increase in power makes oversteer far more likely. My old '91 was fairly stock and honestly far easier to drive than this modified 90 is. By the way thank you for all the helpful comments everyone. I'm starting to think that i could go one of two ways. A: buy the 700 and if i still have oversteer take my sway bar up to 30 or 32mm. B: buy the 770 and if i have understeer increase the rear bar. My daily driver is a '97 Integra GSR, 2600lb car with 250lb/in front and 130lb/in rears, i'm very happy i have this for doing any type of trip driving. The Vette bounces a guy all over the place on the street.

Slalom4me

Another alternative may be to talk nicely to Pattie Cooke, Sales Mgr.
at VB&P. She is active on the forum as member VBP and her number is
1-800-237-9991.

Perhaps something can be done about the rear spring to get you to the
place you want to be. Ultimately it is up to you to decide where that is,
but my impression is that Pattie and the folks at VB&P will help you get
there if they can.

.

96GS#007

It understeers a bit right now, with the 26mm bar in the rear (30 in the front). I'll be able to correct it by adjusting my alignment a bit. That being said, it is an easy car to control as C4s go.

As Slalom said, that's a heck of a harsh spring you have. My concern with that rear spring (and eventually the new front spring) is how it reacts on a rough road course. Not all tracks are smooth. I'v been on several that'll beat you to death and with super stiff springs I think you'll find the car difficult to drive.

racebum

100% correct on thr rough roads. With just the FHA front and VBP rear i drove a old country road where i live that's very tight and twisty. The hardest part was apex exit. As you all know the car is setup to oversteer like a mad man right now, the nice part is the spring in the back is stiff enough to let me know what the tires are doing. I did run into that skipping along feeling on some of the rough surfaces, but like i was saying, the second i goose the gas i can step the car out. On to the VBP advice, Pattie just left me a VM on Friday suggesting the 770lb front to counteract the stiff rear spring and also the motor the car has. It's a basic solid bottom end 355ci with Pro Action 180 heads, a Miniram and a Comp 224/230 hyd roller cam. Power peaks right at 358RWHP@6050rpm and valve float comes on with any rev limiter above 6500 which is where i now have it set.

STEVEN13

Hi-SORRY to hijack,

What are all of your opinions on the adjustable (ride height only) front straight-(C5 type) spring for the C4 (VBP)? If I understand this spring correctly the rate is about 800-900+lbs however being straight works fine on our C4s.

To the original Poster-

I run stock FX3 springs with revalved FX3 Bilstein shocks and Z51 sway-bars (stock ride height) and Nitto R2s. These tires are a great dual-purpose tire. I don't think I would run the drag radials on a high speed road course.

Steven

racebum

The adjustable one is the one i'm looking at. They call it a sport plus in the rates i'm looking for, at least i think so. From what i was told they can make any rate you are after in this height adjustable setup. That being said someone who wanted a FHB rate but with the height adjustment could easily get one since you would basically ask for a 650lb/in spring. They do make 800-900lb springs and some as high as 1600lb on C5's that run downforce. Anyway, like i was saying the ride height adjustment is available in the 700-770lb rates i'm looking at and i believe lower as well. My other option tire wise is to just run R2s in size 275 on all 4, i doubt it would look that bad stretched on my 11's out back, or keep with the drags, so far i haven't had any cornering complaints with them. If i do wreck at some insane speed and kill myself someone please make a post advising others to avoid road racing that tire LOL>...In all honesty i am going to use the white chalk on them before i do the road course and see what kind or roll over they actually have.