ICEngineering Subjects

Thursday, August 6, 2015

Autocross Corvette: Introduction

Photo credit: Andrew Wong

It's been a loooong time since I've written anything here. I guess motorsport has gotten pretty routine for me in the last couple of years. I autocross a lot and do ok but not great, and it doesn't feel like there's much of a story there. The RX-8 has been a great reliable autocross car and it just hasn't needed any work. Which is boring. That's about to change, because it's getting transformed into a whole new car, but that's a story for later. This is about what I'm doing during the months that the RX-8 is in pieces.

About a year ago, Allen (who has the cool SSM Miata in #1 here) and I decided to go halfsies on a new autocross car. My motivation was to learn to drive something with some power, because hopefully some day the RX-8 will have power and I'll have to learn how to use the intermediate positions of the gas pedal. Also, I've been autocrossing the RX-8 ever since I started autocrossing back in 2007 so it'd be interesting to try something new. We wanted this to be fairly easy, so we decided to get something that would be fun and competitive to run in the "Street" (formerly "Stock") category, but since it would be a dedicated toy, we wanted something that is actually a sports car. Basically, we didn't want something slow with lots of doors and seats and stuff. But we're also cheap. Our top candidate was a C5 Corvette Z06, but we eventually settled for a non-Z06 C5 Fixed Roof Coupe (FRC) because it was very inexpensive. The FRC is missing 1" of wheel width front and rear, about 50 hp and a little bit of suspension stiffness when compared to the Z06. But it's otherwise a very similar car. We figured that with the street tires that we are required to run, we wouldn't miss the hp too much, and the FRC looks like it's a very competitive car to have for the "B Street" class, while the Z06 looks like it might be lagging a little bit compared to its C6 brethren in "A Street."

I added the vinyl, but the ugly wheels were included with the purchase!

Turns out it was cheap for a reason. First of all, the steering felt really bad, so as we went to adjust it, we discovered that the rack was broken and I had to replace it. Then we bought some wheels and old tires plus a used set of Z06 shocks for it and took it to an autocross in Ohio where it promptly overheated, the clutch slipped, the brakes went to mush and it generally drove like crap. From there, we replaced the engine's harmonic damper (which required removing the steering rack again), tie rod ends, brake fluid, rotors and pads, and upsized the front sway bar and took it to another autocross in Oscoda. It seemed like we fixed all the problems, but the old tires were definitely ruining the driving experience and we weren't buying new ones because we knew that a new crop was coming for 2015. So we played around with it on junk tires and then put it in storage for the winter.

Grip was poor

The first thing we picked up for the car in 2015 was a set of Koni Sport shocks. The "cheap" Konis for the Corvette are actually a pretty nice set of rebound-adjustable monotubes, but they aren't all that cheap at ~$1100 for the set. And because the rules allow it (also because this blog would be boring without some fabrication), I built a set of spherical bearing lower eyes for the fronts.

Obligatory lathe picture

These parts probably don't gain us much, but it makes it more like a race car

We also bought another 1.5 sets of wheels so we could test tires. This year, Bridgestone and BFGoodrich really upped the ante by releasing the unbelievably grippy RE71R and the Rival-S respectively. We're ambitious people, so we bought a set of each. The Corvette wears 17x8.5 wheels in the front and 18x9.5 in the rear, so we ordered each set with 255/40/17's for the front and 275/35/18's for the rear. Interestingly, both manufacturers had some supply problems, which meant that we ended up getting the 255 Rival-S for the front and the 275 RE71R's for the rear while the others were back-ordered. We started the season out that way, running a few local events with mismatched wheels and tires. The car was amazing with pretty much no tuning, and even on the tight courses at our local sites, we were consistently getting great results.

Cool wheels, right?
Photo credit: Andrew Wong

We eventually got our 255/40/17 RE71R's for the front, but after BFGoodrich kept pushing back the ship date on the Rival-S for our rear wheels, we eventually canceled that order and instead ordered another pair of 275/35/18 RE71R's to squeeze onto a pair of 18x8.5 front wheels that we bought. SCCA Solo's Street class allows a +/- 1" diameter change for wheels, so we took advantage of that to get a wider front tire.

It's a tight fit

We took the car to a test-n-tune in Indiana to test our three different front tires, and found that we hated the narrow RE71R's, loved the wide ones and the Rival-S were fast but weird feeling. All ran very similar lap times, but the wide front tires did it most easily, so we decided to use those for events that we care about a lot, and to burn up the other tires at local events because it's more cost effective than selling them.

Since then, we've done a lot of autocrossing with extremely good results, though we weren't able to hit any national events until deep into July when we attended the Toledo Match Tour which was a great success. We managed to take 1st and 2nd in class competition in spite of lots of rain and bald tires, and we both qualified for the "Super Shootout" which is a bracket competition in two senses of the phrase. It's set up in a tournament-style bracket, but it's also run like bracket drag racing where drivers are run against their "dial-in" times from the qualifying round. So you basically want to beat your own dial-in time by more than the other guy beats their dial-in time. I knocked myself out when I was paired with Allen by hitting a cone, but Allen took that opportunity to go and win the whole thing!

This is Allen's winning face
Photo credit: Andrew Wong

Just this last weekend, we ran the Wilmington ProSolo where we faced some more good competition in our class and again managed to place 1st and 2nd on tires that really have no tread on them.

This is before we put 26 more runs on them

So all in all, we're feeling pretty good with one more Champ Tour coming up in Wilmington and then the National Championships in about a month. I think that if I don't screw up too badly, I can take a decent trophy position, and Allen has a real shot at the championship. Let's see what happens!

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