mbfan72

Originally posted in "General discussion" - didn't know this section existed

I co-drive my street-driven 1987 Corvette coupe (automatic with Z52 package) with my son in BSP with street tires (put on 17" wheels this year which bumped me into prepared). This is our second season. Car was originally purchased as daily driver/restoration project until I discovered my son was interested in autocross. I have done some work to the car (see below). Car is run on a hobby budget (read low $)

I am looking for advice from other C4 owners who autocross.

1. Am I on the right track? With limited budget what should I do next? G-force tires are new so they'll stay on until they wear out (although advice is that they were not a good choice). I was thinking Hawk street brake pads and slotted rotors.

2. Last year with old shocks and 255's the car was called "tail happy". This year gas shocks and 275's were added before first event. We noticed a huge improvement. Car was driven this past weekend by a driver who had been to SCCA Nationals who still described it as tail happy. He suggested that I go back to a smaller rear sway bar. Any thoughts appreciated.

Work done

New polyurethane bushings all-around, 30 mm front sway bar, 24 mm rear sway bar, adjustable camber struts (originals damaged), fabricated X brace (original missing), 17"x 9.5" wheels (with correct offset for an 87), 27540ZR17 BF Goodrich G-Force Sport tires (run at 37 psi front and rear), Bilstein Z51 gas shocks (correct for car), alignment front camber -0.5; rear camber -1.0, front toe 0.0 and rear toe -1/16". Front was low when car was purchased but seems to be stock – rear was lowered (clearance to lower frame rail at jack point is 7.25” front and 7.5” rear). Everything else is stock.

Jason

Tail happy in autox is not necessarily a bad thing. You've got a lot of variables there. I'd fix the alignment first (easiest). Look up a C5Z06 factory alignment settings as a good place to start. Ask around the other autocrossers about a local shop to get a corner balance and autox alignment. With the ride height being changed from stock you need to get a fresh starting point. Then start experimenting with rear bars. Make small changes. Keep lots of records. One change at a time. And keep in mind your skill level will be changing drastically as you get more experience.

Slotted rotors won't gain any time. If you can hit the ABS and don't get any fade at the end of a run, you've got plenty of brake pad.

Alot depends on what you're looking to get from this. Wanting to teach your son car control? Get the car balanced and predicitable, stay on street tires and thrash the hell out of it. Wanting to be competitive? That's a whole 'nother can of worms.

Solofast

Increasing the rear roll stiffness is what is causing your problem.

Here is a link to Hib Halverson's C4 suspension chart.

http://www.corvetteactioncenter.com/...usp_chart.html

For your basic setup a 22mm rear bar is what GM put in there in the first place. Going to a 24 is likely going to make the car more tail happy.

Shocks have a big role and you have the Bilstein's that are correct for the car. Not sure if you have the Bilstein sport's or the street shocks. The sports are a lot better for what you are doing with the car than the street shocks. Adjustable shocks go a long way in dialing in a different setup, but in your case it is most likely the larger rear bar that is the issue.

First thing to do is fix the imbalance in the roll stiffness that you have created. Either stiffen the front bar or soften the rear bar. You can go back to the 22 mm rear bar if you still have it, but to match the front roll stiffness you probably want to add poly bushings on it as you did the front.

Another thing that is really important with these cars is the sway bar bushing stiffness. You have poly bushings in the back bar and that stiffens it up significantly. As a first step I'd put softer (and that means 24mm RUBBER rear bushings) in the rear bar. If you don't have the 24 mm rubber rear bushings, you can drill out the 22 mm bushings, but in no case should you use 22 mm bushings on a 24mm bar without drilling them out. We used to use rear sway bar bushings to tune our BSP car and going to rubber was equivalent to about 2 mm in bar diameter, depending on the clamping load on the rubber. Even our full tilt BSP car we had rubber bushings in the rear bar so that we could tune the front to rear balance with bushing stiffness. Once you start changing things, like going to poly bushings, changing sway bars and springs, it helps a lot to have something that you can use to make fine tuning changes to get the balance back. That's why a rubber bushing that you can soften or stiffen the clamping load on is a good thing.

To go stiffer in the front you need to calculate the linkage ratio for the front and rear bars and then you can readily figure out what you need to match the increase in stiffness that you added to the rear. If you don't want to do that you can look for an adjustable front bar that is a bit stiffer than what you have now and use that to get the back end under control.

If you are on a tight budget I'd go with the rubber bushings in the rear bar as a first step. If the car is still tail happy you can soften the clamping load on the rear bushing by putting a washer between the sway bar clamp and the frame. This softens the preload and makes the bar work like a softer one. If the car pushes with the rear rubber bushings, put a 1/16 of an inch shim between the rubber and the frame to increase the clamping load on the rubber.

Forget the slotted rotors, you don't need them for autocross, just a waste of money. Do pads when the stock ones wear out, but be careful. You can do more to upset the car by going to different pads. Get a set that are made for autocross work. That is, the have the same bite cold and hot, so that you aren't having changes in brake bias as the pads get hotter on each run, or you aren't dragging the brakes to the start line to get some heat into them.

Getting the car corner weighted is probably the next thing to do, and it doesn't cost much. With a C4 you can adjust corner weights to an extent by adjusting the rear spring bolts. That works to the limit of travel that you have in the rear bolts. If one rear bolt is fully tight and the other full loose, you can put in a stock rear bolt to replace the tight one and jack some more weight if you have to.

Get it corner weighted, get some more negative camber in the front and you will have something that you can have some fun with.

BrianCunningham

Street prepared allows a lot of mods

offset bushings to get more camber
better spring and shocks
also you run much bigger tires than stock.

here's mine

here's a national car, aero mods, flares, custom exhaust, full cage, stand alone engine management.

mbfan72

Thanks for so much info!

The plan is to stay on street tires - hopefully better ones next year.

I have the 22 mm rear sway bar and the urethane bushings for it so I'll put it back.

I'll shelve slotted rotors but go for Hawk HPS pads

Hate to show how little I know but what is a corner balance??

I found a shop that does alignments on customs, rods and is happy to put on whatever settings I want. Did I misread the advice or is the C5Z06 alignment good for a C4?

BrianCunningham

C5's and C4's a different animals

here's a good start

http://www.vbandp.com/View-document-...ent-Specs.html

notice the difference between street and competition

l98tpi

Check out the chart above for a good reference. Also, you could check with a local Corvette Club and see if they know where you could get an autocross alignment and corner balance (adjusting the car to have as close to same force applied to ground at each wheel as possible). I am not saying anyone is incorrect by the suggestion to decrease the rear anti-roll bar, but to achieve the best handling you want to have high spring rates and the total spring rate is the spring itself plus the stiffness of the anti-roll bar.

I wouldn't touch the anti-roll bars. Leave it alone. Also, don't change your shocks either. They are just fine. If I had your car and wanted to get the most for my money, I would get a good alignment and corner balance first. You said you don't want to change the tires and that is not a bad thing for learning since you need to get more seat time and learn the limit of your car. Street tires are more forgiving in that they will loose traction more gradually than a race compound tire. What you do need to do though as far as the tires you have is lower your pressures and start reading the tires. Learning to read the tires is one of the most important things you can learn. Start off with 36psi front and 32psi rear. C4's like different pressures in front and rear. After your runs think about how the car felt and look at how far over the tires rolled on the shoulder. Most tires will have a small triangle at the edge of the sidewall pointing at the point where the shoulder meets the sidewall. If the tire rolls over past the point onto the sidewall, add pressure and if it isn't getting close enough to the point then decrease pressure.

It takes time to learn the limit and sometimes you have to go past the limit to know where the limit is. If you feel comfortable on a course you run on, let the tail come out on you some and try to keep it going and when you want to straighten up, stab the throttle and see if you don't straighten up.

As far as moving your car to BSP, I don't think you need to put your car in that group. C4's, maybe not your year, but later years did come stock with 17" wheels, so I think you should run your car in B-Stock until you do additional mods.

Good Luck

Crepitus

Excellent thread! good advice from some very knowledgeable people.

mbfan72

Thanks for the advice. I actually wanted to stay in BS but local club wisdom is that 17" weren't available in 87. They also don't like the fabricated X brace or the aftermarket adjustable camber stuts or the ride height. I also have a brace when the roof is off - they don't like it either.

Right now we're just after seat time and to be competitive a local Corvette Club events so I'm not too worried about classing with the sports car club

Kubs

Like others have mentioned the alignment is off, but contrary to others I think the 24mm bar is fine. I would say leave the rear but tighten up the front to say a 32mm bar from VBP and fix the alignment and see how you like it. On factory springs and a 24mm bar your rear roll stiffness is not close to being too high to worry about.

For alignment specs I would try this:

Front - Make camber as high as your can get it (if that -0.5° is all you can get leave it) but make it even side to side. On street tires you shouldnt need more than -1.5° on the front. Go 1/16" total toe OUT (-1/16") and between +5-6° of caster.

Rear - Make the rear compliment the front camber. If all you can get out of the front is -0.5° then go -0.25 or -0.5° in the rear. If you can get over -1.0° in the front go -1.0° in the rear. Again on street tires -1.0° should be pleanty. Toe in the rear should be IN not out. I believe that is where most of your tail happy feeling is coming from, because the rear tires are pointed in the wrong direction. Go as much as 3/8" total toe IN on the rear.

Before you buy or change out parts, get an alignment and see how it feels. If you feel like you need to change sway bars, go bigger on the front and leave the rear. My car is a roadrace, not autocross, but I still have bigger sway bars than you and my car is neutral. I have 26mm in the rear and 32mm in the front and it feels great.

Also, try running about 2 psi less in the rear tires than the front. Try to end up around 36 front and 34 rear HOT.

John B

What he said about your alignment!!

mbfan72

Thanks for taking the time to share. It was tough for me to guess where to start. Wish I had used the forum sooner!

mbfan72

Thanks to everyone who posted. I have a clear plan for this year.

The car will be aligned with more front camber (shop will work with camber, caster and toe to give me the best compromise) and a little toe out 1/16th(to see how it feels)
The shop will also scale the car for me to see what the balance is now and what can be done.
I will try the car with the new alignment and see how it feels.
I have the 22 mm sway bar with urethane bushings and i'll try it, after trying the alignment, to see how it feels. Easy to change.
Brake pads need replacement so I have ordered HAWK HPS pads.
Will try 36 psi front and 34 rear
Get as much seat time as possible

Next year a set of grippier tires!

Kubs

Sounds like a good plan! Let us know what you think afterwards.