Matt Cramer

I took my '86 out autocrossing recently for the first time, and found it was in serious need of grip. I was definitely a bit in over my head. So I want to get some better rubber. Being a cheapskate, I've decided I'll get some used race tires and scrounge up some used rims somewhere or buy some cheap stock car wheels.

Right now I'm running in Stock and 17" rims would bump me into Street Prepared. Any thoughts on the handling difference between 16" and 17" rims? How's the availability of used R-compound tires in appropriate sizes?

Bill Dearborn

Do you have the stock 16x9.5 rims or do you have the 16x8.5 ones? The 16x9.5s came on the Z51 cars in 86. In 87 they came on Z51 and Z52 cars. The 16x9.5 rims work a lot better but tires to fit them are becoming scarce (255/50x16). R Compound tires on those rims work pretty good. I ran them on my 86 for 5 years. By the way my car originally came with the 16x8.5 rims and I just used the 9.5s for autocross but nobody ever really knew but me since the tire size is the same for both. Very hard for somebody who isn't knowledgeable about C4 Corvettes to detect. Especially at a local club autocross.

Bill

z28cp

Kumho 265/45ZR16 and Hoosier 275/45ZR16 both fit on the 16x9.5 nicely.

Matt Cramer

Thanks. Mine has the 16 x 9.5" rims. I'll have to check with used tire suppliers to see if they've got a good supply - it may be cheaper for me to move to SP.

AUTO_X_AL

I say go with the 17's and pick up scrubs from the T1 guys. More racing is always better! Unless you have a national level AS car I would go where you can have the most fun for the buck.

Matt Cramer

It's not likely to ever be a top national level car... it definitely doesn't have a national level driver in it right now. I may add some mods that bump it into SP later anyway.

c4cruiser

One of the best things to do if you have just started autocrossing, is to get seat time. You really need to work on how to drive a course effectively where you can learn things like braking points, apexes, being smooth on the gas and brake, and getting seat time. The more practice you get, the faster you will go and after awhile, you cn start to concentrate on mods for the car.

I would start working the suspension before jumping in to race tires. Good shocks, good suspensions bushings, sway bar changes, and maybe a autocross-specific wheel alingment will do a lot more than just bolting on some slicks. And with all of these upgrades, get some seat time with each change and see what kind of a difference you see/feel.

Did I mention getting some seat time??

So on to tires; there is really not a lot of difference in tire diameter and tread width between a 275/40-17 and a 255/50-16. So going to a 275/40-17 should not be a factor in changing the existing gear ratio and overall performance should be the same.

With a 17" wheel, there is a much better selection of tires (either race rubber or good street tires) compared to 16" tires.

With my 87, I have been running autocross Hoosier's in 255/50-16 but I have also run the Kumho Ecsta MX in 275/40-17. The Kumho's have decent traction in wet or damp courses and I use them for track days. And with the Kumho's I can also run them on the street.

I trailer my 87 to events so I can mount my Hoosier's and don't have to worry about changing tires on site.

You can find replica ZR-1 wheels with the proper offset for the 84-87 cars and run a 275/40 or 285/40 tire. This would bounce you out of stock class but if you do other mods, you could easily wind up in B/SP or some other class anyway.

I'm going to be doing a C5 brake upgrade on my 87 and the ZR-1 17" replica wheels are required to clear the C5 rotors while factory sawblades will not. So with doing that, my 16" wheels are not going to be useable at all.

Solo2GS

Something I did not see mentioned what what class you are in. Often changing rim sizes from original will bump you to another class. Check the rule book for what your are running SCCA or NCCC.

If your SCCA AS and change rim sizes you bump to BSP. Not a bad class, but likely more preparred cars to compete against.

I don't know about the earlier C4, but later models can run 17 X 11 rims with 315's on all 4 corners just fine.

Matt Cramer

I definitely need to learn how to control this thing - that last autocross was my first time out in a rear wheel drive car, and it had about 100 hp more than anything else I've ever autocrossed. Just kind of figured race rubber would help more than anything, particularly since it's my daily driver and has 400-treadwear tires for holding up to long commutes.

AUTO_X_AL

Earlier ones can run 315's all around and 335's on the rear. I like the 315x4 setup a lot but the 295/315 setup is a good economical choice and the car can be made to handle very well in that configuration.

CJR2

A lot of guys I know run the Falken AZENIS RT-615 which still qualifies for SCCA Street Tire Class competition. They're a great tire at a great price.

http://www.vulcantire.com/azenis_rt615_t.htm