coneracer

Have only autocrossed the car a couple of times but the rear end has quite a bit of squat and was actually bouncing off of the bump stop. I shaved the bump stop down and it still barely hit so I took it off of the car. Initial turn in and grip to mid corner is also bad and throttle must be applied to help get the car more neutral, so main complaint is the understeer. Here is the setup

FHB Front spring 115.5 nm around 650 ft lbs
front sway bar 30mm
rear spring NYU 57.2nm around 320 ft lbs
rear sway bar 24 mm
full urethane suspension bushings and batwing etc.
vette brake products revalved bilstiens
Delrin shock mount bushings

Alignment settings
Front camber-2.0 with 1/8th total toe out caster in the 6 degree range. Camber is limited to shim removal.
Rear camber -1.8 with 1/4 total toe in

Any suggestions would be appreciated. It appears that the rear is under sprung but is also slightly under valved as well.

C409

..... The NYU spring is pretty soft ... it was developed for all the geezers that bought these cars new and didn't much care for the sports car ride ... it was even used for several years with the Z51 RPO ....... I drag race , so I can't speak for you left n right guys but I have a BMH and its VERY firm ... I believe the BMF is the stiffest ... someone out there has a chart with all the spring codes and rates ... maybe they'll chime in .............

coneracer

Sounds like you and I need to change springs. That rear transfers alot of weight and hooks up in a straight line well. Don't think it is the fastest around the cones though. Thanks for the response.

mcm95403

I thought only the 84 Z51's had a stiffer spring than the rest of the Z51's: http://corvetteactioncenter.com/tech...l#.UW3nfa_n-Uk

kimmer

Correct, I have the NYU and it's almost straight compared to the arch on my previous spring. To the op, I run a 32 and a 26 and it works out great.

indianavette

FWIW...You might consider a BMH (87.5 N/mm) rear spring, which would put the car very close to '84 trim.

C409

... I threw away an NYR after I put this bad boy in ... car launches great , no squat , no camber change ...... you can keep yours ........ .........

kimmer

He said NYU not NYR, big diff. as far as the BMH it would be too stiff for autox.

coneracer

Kimmer the 32/26 you refer to is the bars I'm assuming? It looks like in the chart the only car/year that had stiffer rear spring was both models of the 84. Would the 72 nm spring work or should I try a larger bar first? I've got enough squat that the inner fender liner is rubbing through, running 17x11 with 315 a6's. Had to up the pressures from 30psi to 36 to get the car to work better and quit rubbing on the rear. The wheels we used might very well be in question as well, very thin shelled centerlines that started loosing air in one at the weld's in second event.
Thanks for all the suggestions.

kimmer

To reduce the squat go with a stiffer spring. The larger bars are to help keep your tires planted and reduce body roll. In the rear I have the 52.7 spring [tried the 72 but was too stiff and bounced the rear around the course] the 26mm bar , run 315 Kumho's , sport shocks and it's lowered. The whole combination works great for me, I win my class 99.9% of the time + I take out most of the stock C5's & c6's.

93Rubie

You want a quick trick to try?

Add some 1/16" alignment washers under the swaybar mounts (between the top and bottom part). This should effectively reduce the front bar stiffness. It reduces the bushing load on the bar. However, this will only work with rubber bushings. Or so I'm told.

leesvet

yeah...BMF & BMH are 411lbs vs 499lbs as hard as a high school boy on prom nite...

stiff...if that sounds better !

the front seems harsh...650 for the front...may be a bit much...it does have to support the motor and all....but I would have thought a450 to 500 would do that and still leave some stiff travel up front. mines already shakes fillings loose and its less than any of those.

coneracer

650 doesn't sound that stiff to me. My previous car was a 93 civic hatch and I ran 800 fronts and 450 rears with a 32mm bar on the rear. So I guess everything is relative to what you've got to work with. I wouldn't think the next spring up would be that bad, but how a transverse leaf works and where it attaches probably makes a big difference.

93Rubie

BSP C4's at the national level typical run something like 1200 front, 600 rear 30mm solid bar front, 22mm solid bar rear.

The problem with the C4 rear suspension is that if you induce too much roll stiffness with bar/spring combo you get jacking. Once you get jacking the car handles funny and won't be fast.

Your right on how the transverse works/mounts.

The front spring acts like a sway bar because the mounting points are spread apart. The rear not so much as the mounting points are close together.

Getting the front to stick on a C4 is critical, you want the rear to follow. If you need to rotate the car, throttle, trail braking. You can trail brake the crap out of a C4.

The small FWD cars that 3 wheel always amuse me. Its cool to watch them tap the brakes mid corner and the airborne wheel stops.

For the most part a C4 is going to do one of 3 three things, PLOW ie. stock FE1/FX3 setups, understeer-push (Z07, Z51, modified setups) or jack.
The goal is to get understeer at a minimum (you'll never eliminate it) and keep the car stable and NOT jack. That is just the way these cars are.

Adjust your driving style to suit. No car is perfect, the driver has to adjust to match the strengths of the car and minimize the weaknesses. I'm sure you know this however.

coneracer

So what about the rear spring mounts that I've seen that have adjustable wider mounting points and where are they finding such High spring rates front and rear. I guess super revalved shocks are in order due to the spring rates. The revalved vbp bilsteins aren't up to the task of the medioker spring that is on the car now.
I've got a good at speed photo of the car and looks like it needs more rear spring or bar and the front is really stiff enough but needs more camber. First thing we are doing is to change the wheels the centerlines are definately an issue to the rate of 1/2 inch of deflection under full load. Then replace the front upper arms to gain extra negative camber and caster adjustability. Then a 26mm hollow bar to help the back. Sound like a skeem???

kimmer

Just a suggestion but run a season, get some seat time, get use to your cars handling and start modding it on the off season. You have not done enough autox to know what to start changing and you may not do the right things and never get it right. I did nothing the first season and started to mod the second, 6 years later I'm still changing things. As far as jacking which is called oversteer you can avoid that by no heavy throttle until your wheels are pointed straight ahead. Understeer or pushing means you are going into the turns too hot. Once you can run hot and avoid doing these 2 things then think about upgrading.

kmillar

I use my '90 convertible for auto-X, here's my setup that seems very competitive
Tires Kumo's 285-17 all the way around
sway bars: 32/28, with grease zerks added to urethane bushings
springs:stock FE2
brakes: DRM bias spring,hawk pads, front brakes upgraded to C4 13"
Other; DRM camber brackets, VBP rear suspension heim joints,
boxed in X brace.
I compete with the local corvette club and am faster than some Z06's in a tight course. Once the course opens up... horsepower wins...

This year I hope to get my '90 coupe with the 396 going, then we'll see about that horsepower thing

coneracer

Sorry to mislead but I'm not new to autocrossing been doing it at a national level for 20 years and have 3 pro solo and 3 solo national championships but the first year in a corvette so some advice from some seasoned corvette owners, I thought would be in order. Can't sit on my lorals, not going to try to learn the car when having these type of issues, want to get up to speed fast and see if I'm missing something. Most that I see reply say they are on these spring rates. This may be a compromise for street use but wanting to do what it takes to get this thing right without going the wrong direction too many times.
Thanks to all that have responded so far
Ron

93Rubie

Some links for you:
http://forums.corvetteforum.com/auto...tte-setup.html

http://forums.corvetteforum.com/c4-t...-c4-build.html

http://forums.corvetteforum.com/c4s-...post1548457243

Also, look up forum member Solofast and PM him. He can help you out, better than I can.

Go get em!!!