C4RACER

Does anyone on this forum have experience setting
up a C4 for road racing? I've decided to convert
my part time track car to full time track car
and the stock springs and bars have got to go!!

I'm leaning towards seeing how far I can go with stock
type springs instead of going to coil overs, but I'm open to
coil overs from the start IF there is a clear advantage to them.
My concern is that the adjustability won't do me much good
beyond corner wting the car, since I won't know how to set them
up very well....

I have koni yellow shocks and a '91 car with the
base suspension (93nm front, 40 rear - springs)
and stock sway bars - 26/24. I do have a ZR1
rear bar available which is a 26.

Any suggestions on spring rates and bar sizes
to start with? Also sources for springs if non GM?

I also plan to lower the car about 1" or so.

Any other suspension suggestions? Eventually
I will convert the bushings in the front to GW.
The rear arms all have poly bushings, not sure
if that's good or bad - doesn't seem to be a problem.

I run on 275/315 tires on CCW wheels currently and
plan to stay with that set-up because it works well.

Taijutsu

The best advice I can offer is a quote from a "name" tire tech ". "Wheels and tires will do more to effect the handling of you car than anything else you can do!" Sorry, this is the best that I can offer.

John Shiels

30 MM front bar poly bushings tires as said above

C4RACER

OK - perhaps I wasn't clear. I realize tires make a lot of difference in the handling of a car, that wasn't my question. I'm converting my car to a off road only track car to compete in SCCA ITE class and I want to be reasonably competitive. I need to find about 7-8 seconds on Thunderhill for example over my street set-up. I'm already running 275/315 Kumho V700's on CCW 10.5" and 12" wide race wheels. I may switch to hoosiers once I dial the car in, but I know this stock suspension is holding me back some - I get tons of lean in the corners. The stock 93n/m springs are not the greatest for road racing.

I will try the 30S front and 26 rear bar combo which should help. And I plan to go to GW bushings in the front suspension and already have poly in the rear. After that I need to dial in the spring rates or switch to coil overs so I was wondering if anyone here with a competitive road race C4 had any input to get me in the right ball park w/o a ton of experimentation.

I also plan to change the brakes to GN 6-piston fronts and move my 13" GS brakes to the rear and add to my ducting too.

Scott

h rocks

Scott, the GW bushings Rock. Do the uppers and lowers. (lowers most important.) If you're going to be on relatively smooth tracks, stiffer springs a la the '89- '91 Z51/ZO7 springs would be fine. The 30/26 bars are good. You can improve the end links front and rear. Peter has a line on some hard core units, much stronger than the Guldstrand pieces. Make up you own heim jointed trailing arms, and camber arms. The toe rods are a little trickier to make, but doable. The trick is getting the correct taper to make to the rear knuckle. (DON'T use a bolt and install the heims flat to the knuckle. Tragedy will strike.) Guldstrand has a decent toe rod kit. IMPORTANT, bump steer the rear suspension when you install it. You can end up worse off if you don't.
The big poly diff carrier bushings make a huge difference.

As Jim Minneker once told me, "the car likes to be low". Lower the front and rear. Get the rear within about 9/16" (higher) of the front with you in it. I don't know what the shock valving of the Koni's looks like, but if they are the twin tube jobbies, you will want to "trade" them for some type of a monotube shock. A good base alignment to consider is: Front: 1.75* neg camber, zero toe, ~5.5 *pos caster. Rear:1.50* neg camber, 3/32" toe in per side. The car should be very stable, fast, and forgiving.

There a jillion little tweaks, but if you do the above, you will be amazed at the cars you can embarrass. Good luck!

M2001

I also have a C4 and I made a few changes to the suspension which I felt greatly enhanced it's handling characteristics. Those suspension changes were as follows:

Poly bushings on all the suspension bits and pieces including the diff carrier: great bang for the buck, it removed a huge amount of suspension slop

Dial in as much negative camber as possible on the front wheels, I was able to get near negative one degree, more would have been nice but I ran out of shims to remove.

Bought a set of custom valved Bilstein shocks from DRM, a lot stiffer than stock but then again, great improvment in suspension action. I wish I had adjustable shocks because one shock setting is not always the best for all tracks.

Installed spun aluminum, heim jointed rear camber and toe rods. I would rate these pieces as making smaller amounts of stablity improvements to the rear suspension. I think I need to be a better driver to realize it's full potential.

I left the stock sway bars and springs in my car. I did put AFS 5 spoke wheels (9.5" front & 11" rear) on my car which allowed me to use 315 tires on the rear. The tires I used were Pilot Sports but those tires tend to get greasy when they heat up on the track. I hear BFG KD is a better track tire.

I passed on lowering the entire car because I drive my car on the street. I think you could get positive results by lowering just the rear end as little as an inch. I found my cars rear to feel a little light with respect to the front/rear weight bias. If I was going to keep my car, I would settle on lowering just the rear an inch.

My base alignment was almost one degree negative camber with zero toe for the front end. The rear end was set at 1 & 1/2" degrees of negative camber with 1/16th of an inch of toe-in (per wheel).

The biggest gripe I had with my car was the body flex but then again, I have a ragtop. The roll bar helped but it did still flex. I will say that the car was very forgiving and never gave me the feeling that it would break traction abruptly. I always felt the car giving me signals of its limits and when I went beyond my own ablities by getting a little sideways, the car allowed me to make corrections without severe consequences (read crash).

I'm sure others may have their own thoughts on the subject which may differ from my opinion. All in all, the suspension changes I made were not terribly expensive and I could still drive my car on the street with reasonable comfort.

I understand you want an exclusive track car but I figured you might want to get an idea of what others were doing to their C4 to prep it for the track. Good luck with your project.

M2001

BTW, I received a great deal of valuable information from H Rocks when I was in the decision stage of what to do to my suspension.

1991Z07

The DRM suspension brackets are another good investment. They change the geometry of the rear suspension to keep it planted firmly on the ground throughout the suspension arc.

DRM also does a rear knuckle "tweak" in the hydraulic press. They move the ears that the toe-rods attach to. This basically gives you the same rear knuckle as the '96 cars (GM changed it the final year.).

Bilstein makes a set of special shocks they produced for the Corvette Challenge series. They are 3x stiffer than the base shocks on the car. I just don't remember what the P/N was on the.

You didn't say if you had FX3 or not. If you did, there are a lot of other improvements you can make to get the performance you are looking for. Re-valve the shocks, change out the controller and get some of DRM's track-specific PROMS and you won't believe the difference. This single change was by far the best thing (in combination with the DRM brackets) I ever did to my car. When I go around the corner, it is flat. The signature shot was taken at TWS through the carousel doing about 65 MPH. I have very little body lean anymore. Sure makes a big difference.

I would suggest you get some of the professional books on suspension set-up. I'm just not anywhere that I can look up specific names for you. With some good pre-knowledge, you could get a set of coil-overs dialed in pretty quickly. A good g-analyst and a few consistent laps would tell you what the changes did to the performance envelope.

I understand where you are going. I'm going in a little different direction with my car. I'm getting it ready for top-speed open dessert racing in West Texas & Nevada.

All I can say is change 1 thing at a time and test the results. It is the only way you will ever know for sure what works and what doesn't. Methodical, calculated changes and recognizing what is over/understeer goes a long way.

Good luck...

C4racing

C4RACER, nice Forum name! :cheers:

I realize that your original question was about how to be more competitive via suspension (only) tuning but since you mentioned SCCA ITE I'd like to suggest that being competitive is a lot more than just suspension. Maybe you have the rest of the car already sorted out but I'll give you my 2 cents on the *whole* car and you can take from it what you want. Skip to the last paragraph if you're only interested in suspension. :)

My first piece of advice is to get to know the GCR's (General Competition Rules) and in your case, everything about the ITE class. For example, I wonder if your 10.5" and 12" wide wheels are legal in ITE. You can view the SCCA site at http://www.scca.org

Without getting too far off track I have one other suggestion before mentioning a few car setup specifics. A lot of people invest a lot of time and $$ in go/stop fast goodies for their car and neglect one of the most important aspects of "going fast", namely, the driver. Perhaps you're already a hot shoe but for what it's worth, you can reduce your lap times a LOT by investing in appropriate driving/racing schools. Besides, if you ever sell your car with all your trick goodies, you'll still have the wisdom of what's strapped inside your helmet. :yesnod:

I drive my car to and from every race, and with the stock chip installed it passes CA smog. The engine *may* have a couple of mods but of course they're all CARB legal. :D I also run DOT street tires strong enough to withstand road hazards (read Yokohama and Kumho, not Hoosier). So given how 'mild' of a race car this really is I'll give you a couple of lap times at tracks near you and then you can compare them to your times and see if my ranting is worth listening to. :eek:

I drove my car at Sears Point once about 1-1/2 years ago before they started renovating the track and I qualified my "street car" outside pole for an amateur club race with a 1:52. Last month I was at Laguna Seca with a club that didn't have a race class and had strict passing rules. Even with some traffic here and there I was able to post consistent 1:43's, and that was with cracked front brake rotors (only found that out when I got home....I just thought they were warped at the track so I couldn't brake as hard as I wanted to). If these times mean anything then read on..... :rolleyes:

SAFETY--ITE requires a cage, not just a roll bar. Check for regs on seats, window net, emergency kill switch, safety wiring of drains and fasteners, and catch tanks.
WHEELS/TIRES--Wider is better only up to a point. Make sure tires are available and affordable in the widths you want to run. C4's seem to like narrower fronts & wider rears (not equal on each corner). Pick your tire first, then match the rim to the tire.
ENGINE--Do everything within the rules that you can afford. This topic alone could go on for days but I'd suggest focusing on better breathing (intake and exaust), fuel pressure, ignition components, and heads/cam if you can afford it. Also, racing raises every temp imaginable so I'd suggest a better radiator and external oil cooler. Synthetic engine oil. Don't go nuts with a loud exhaust if you want to run at Laguna though.
DRIVETRAIN--Synthetic fluid in trans and diff.
BRAKES--The biggest and best you can afford and still be legal in class. Ducting is also VERY important so add as much as you can in the front AND the rear. Brake bias spring, race pads, hi temp brake fluid, SS brake lines.....and finally.....
SUSPENSION!--Lower is better to a point but you still want to have shock travel. Maintain some rake to the car, i.e., if you measure the gap between the lower frame rail and the ground the gap should be bigger in the rear. I think DRM suggests about 1/2" but it's worth calling them for their opinion and reference points (don't use the underside of the fender arches). Coilovers are great for lowering the car and fine tuning ride height but they're not really used for jacking weight....that's the job of adjustable end links on the sway bars. 30mm/26mm bars with greasable urethane bushings. Synthetic grease anywhere there's a grease fitting. DRM rear trailing brackets. Adjustable rear toe and camber rods. Shock preferences vary greatly....from revalved OEM to triple adjustable Penske's with remote reservoirs.....lots of opinions here. You might want to try the R-D camber brace if it's legal in ITE. If you stay with the leaf springs then try the Corvette Challenge springs (I have a pair for sale :) ). If you go with coils then listen to folks and make up your own mind. I've heard people use anything from 300#/in to 1,000#/in spring rates in the front, rears always lower than fronts.
SUSPENSION SETTINGS--Different tires like different pressures and camber settings so check with the tire manufacturer before starting your own testing. Use a probe style pyrometer (not infra red) to measure temps *inside* the tread just outboard of the plies to fine tune pressures and camber. If you can get it I'd suggest starting with -2* ft, -1.25* rr camber and up to 6-7* pos caster. Front (total) toe can be up to 3/16" OUT if you want the car to turn in quicker on a tight road course, all the way to 1/8" IN for very high speed straights with subtle turns....just stay away from dead zero....pick a little on one side or the other. Rear toe about 1/16" IN per side.

Hope this post wasn't too long. :seeya

(Btw, now I have to ask a question...what the heck are "GW bushings"? That's an acronym I don't know.....) :)


[Modified by C4racing, 12:58 AM 5/16/2002]

Hot Rod Buick

GW= http://www.globalwest.net

I have these bushings on my GN and all I can say is they are awesome! Are there any other bushings ;) :D Global West has some good stuff, but they do take a while to get to your mail box. The good thing is they are here in So Cal so you could always just go pick the stuff up Bill.

C4RACER

thanks for all the great insight, guys - I do apreciate it greatly.

Hey C4RACING, is your car a '89 or '90 - dark blue w/ tan interior?

If so, you are the man - I saw you kicking butt at Bwillow at the Worth event a few months back. In any event, those are fast times - I'm about 9 seconds behind you at LSIR so far. But that was my first time with this car, all previous rr experience was in a Camaro and the vette is a whole different animal to drive. That was also on street tires. I'm running aroun 2:15 at T-hill, but I don't know that track very well. Still working on the driver and my plan is to get as much seat time in the car as possible over the next two years as I develop it and start in ITE in 2004.

One comment about ITE - at least as relates to the SFR region, there basically are no rules. It's a run whatcha brung class for certian car types. Main requirement is that it must be a stock tub body / frame. Almost everything else is totally open.

My engine is already good enough for now, that is the last place I need to turn to make me any faster. It's a L98 with a SuperRam and full length headers and puts out around 310hp or so. I expect to get the car down to 3000lbs even with the cage.

I do need to lower the car now that I'm taking it off the street.

The rear suspension already is done up in all poly bushings by the previous owner. My next suspension move will be 30mm front bar.

I'm pretty happy with the 275/315 tire combo and I also have a set of GS wheels in addition to the CCW's. I should pick up one more set of cheap wheels for storage and transport maybe....

Thanks for the suggestions everyone!!

Scott

C4racing

Greg, Global West was my best guess for "GW" so after I posted I looked at their site and found *nothing* listed for Corvettes. :confused:

I suppose I could always call them but, for the inside scoop, can you tell me what components of theirs are worth looking into? (and don't say "everything") :lol:
Thanks. :seeya

kkonen

I only autox but I have the Koni Yellows and VB "Sport" springs front and rear. I also have the VB 32mm front bar. My car handles extremely well. There is very little body roll. These springs also lower the car about 3/4" or so. I didn't measure the before and after, this is just my guess. VB also sells an "Extreme" front spring that has adjustable spring perches that can lower the car even more. I know MSR is running this spring and loves it.

BrianCunningham

watching this thread, nice to see all the C4 coming out to the track :cool:

I've started doing more time trials with my daily driver car.

BTW what do you think of this?
http://www.vettebrakes.com/detail.asp?PRODUCT_ID=42940


I'm interested in the front spring
Be nice to be able to crank the ride hieght down for the track, and then put it back up for driving on the street. You can already do it in the back, this would complete the package.

kkonen

My thought on the kit is don't mess with it. I have heard bad things about their custom valved shocks. Look under Reviews on the Forum. I absolutely love my setup, although I can truly see the benefit from the stiffer from spring. They want $1660 for this kit. I would go with their "Extreme" front spring ($550), "Sport" rear spring ($280), Koni Yellow shocks ($480) and extended rear bolts for the rear spring ($30). The total for this is $1340. With those spring ratios you probably can use the stock sway bars. I can almost guarantee that it will be worth trying without new sway bars. They don't say what rear sway bar comes with the kit but the stock bar is probably too big if anything. I have had a problem planting power coming out of corners on autox.

I also cannot understand going with a non adjustable shock on a race car unless you have the time and money to truly develop a custom valved shock for your car. Adjustable shocks allow fine tuning the suspension to get just the right feel.

When you adjust ride height your alignment changes. It affects both camber and toe. For this reason I do not recommend doing this, although I am sure there are a lot of people that do.

Zpeedstr

re: Global West delrin bushings: work great but tricky to install properly and get no binding
GW's instruction were wrong in 2 places: zerk thread is 1/4 28 not 20, and top plastic washer
goes to the outside. Also, right side gets overheated from exhaust manifold, and requires heat shield.
Guldstrand makes a ball bearing setup.

re: VetteBrakes: Avoid their Xtreme shocks, they are 3X as stiff as needed.
And the Heavy Duty Toe Rods did not fit because of poor machining.
I like their springs k=1200 front, 590 rear. The front height adjustment is a real improvement
The problem with leaf springs is that you cannot adjust the ride height and corner weights independently

I am now using Koni sports and they feel better than the 3 different Bilsteins I have tried.

The big issue with the Vette is the weight and chassis flex, conflicting problems.

C4RACER

I should fix the chassis flex once and forall when the full cage goes in. I'll be doing bars extended to the front shock tower / frame area as well.

The 1200, 590 you mention are lbs / inch? Is that the exteme spring?

Scott

C4RACER

One more point - why spend $1K on leafs when you can go coil overs for the same price? I can run coil overs in the class I'm going into. Am I better off just switching so I can dial in corner wts and ride heights and more easily (anc cheaply) switch spring rates???

Scott

kkonen

With the adjustable perch on the Vette Brakes Extreme spring you can adjust corner weights. You can also adjust corner weights with the rear spring bolts. I am not saying this is better than coilovers, just that it can be done. Coil overs are superior if they are done right. I don't know if you can get a good set of coil overs for the price of leaf springs. I know kits are available in the $1500 range, but I don't know how good the quality is.

The main reason I run leaf springs is because my class requires me to.

h rocks

Brian, what's your point? Must you comment on every friggin thread on the Forum? I can't believe that you didn't give your standard "I love my HAL shocks" with the googly eyes pitch.

The VB shock valving, based on some personal calls I have received from some that have bought them, is a total mismatch. The valving on the on the ones' brought to my attention were so far out on rebound, that the suspension would have a hard time ever leaving full compression. The camber rods shown are, let us just say, not premium quality. I've seen them new and I've seen them after they crapped out. (and bent on the threaded end) The heimed units with the grease fittings they offer are of the same quality, IMO. I'm under the impression that some of the C5 stuff is pretty decent. But remain unconvinced on their C4 components. Strictly my opinion.




[Modified by h rocks, 10:25 PM 5/17/2002]