Jeff Eikenberry

Problem: When racing, the rear end feels like it is on marbles and will snap out with little provocation. The general sense from people I compete with is there are needs in the set up. Here is background info: I have an automatic C6 with magnetic ride control. Tires are Kumho XS and stock sizes, pressure 32 front, 30 rear (NOTE: when I purchased my tires in December, the rears were 3.5 and 4 years old. The fronts were 6 mos. The rear tires definitely feel harder than the front when cold. Could this be an issue? A friend I compete with feels the tires have lost some of their traction, as witnesses by the hardness). Brakes are Hawk HP. Suspension front -2 degrees camber, 6 degrees positive caster (couldn't get anymore), close to zero toe in; rear -1.5 camber and 1/8" toe in (note - I have had the alignment double checked). Sway bars were changed to Z06 two months ago, with little difference in the rear's desire to step out. Front sticks much better, tho. I run without any aides (traction control, etc). My experience is about 12 auto crosses, all this season. I did autocross extensively in '70 to 73 ('68 427 with off road suspension). Any thoughts on what I should do to get the rear end to stick better? I'm considering putting the stock rear sway bar back on. Good idea/bad idea. While I prefer not to run with the aides on, I'm open to suggestions. Thanks very much, Jeff

AverageVetteNut

I'd make 1 change at a time and see how things go.

1) Get newer tires. Not sure what to go with since I don't run street tires, but I hear the XS is a good tire, but doesn't like a ton of heat ( or wet weather ).

2) I'd go back to a stock rear bar and see what that does.

3) I'd drop the rear pressures down a few lbs and see if that helps

Don't know much about how the caster change up front would affect handling, and I've not touched mine.


What class are you running in and what organization? NASA, NCCC, SCCA ?

The rear toe in sounds like it's about right. I wouldn't think you'd want more. I DO think that you're running a bit too much camber in the rear. I would try dialing it back to 1.1 or so.

Hope that helps some. Again, I would find a local TNT event or something, where you can run all day and try different things that you can adjust THERE. Sway bar, pressures and so on.

I'm sure guys with more experience and wisdom will chime in

Jeff Eikenberry

Thanks much, AVN. I suspect the tires will have to be replace. Checked back with Tire Rack, but they weren't interested in taking them back..... Said that's whattheygot from Kumho. Will see what Kumho has to say. I'll run the Z06 bar this Saturday and then the stock on Sunday. Will try 28 pounds in the rear both days. I'm running in stock street tire class but have not committed to any organization. I run in the central coast of California, primarily near Monterey (Marina Airport). Do you believe the rear camber may be impacting the footprint? Don't know what TNT is. I assume it's some type of school offered by a club?? Once again, thanks for your help. Jeff

mountainbiker2

Stock bar or no rear bar. A lot more toe in for the rear. I think the tires are fine.
Here I am with Kumho XS tires.



Steve A.

AverageVetteNut

oops... sorry... TNT = Test n Tune. Basically an autocross with no points or classes. You just pay your fee and run as much as you want.

I would think that with less a contact patch in the rear that it would be more prone to moving around back there, so I would back it off some.

Just look into your area's clubs and see what you can do in certain classes. Since you're starting over in the sport you may not care about classing NOW, but you don't want to change things enough to bump you into a class your car won't be competitive in later down the road when you might care.

Example...SCCA stock class rules... you can change the front OR rear bar ( and it's end links and bushings ) but you can't change both. Stock size wheels ( but the offset/backspacing can be different by up to 1/4" ) but the tires are open as long as they aren't a full slick.

Other stuff in there as well, but just to give you an idea in case you haven't looked into things like that yet.

Anyway....

We have a local guy that runs XS's from time to time and he likes them but mentioned that too much heat in them after some back to back runs makes them feel greasy on track.

Again, I'd make changes incrementally to narrow down the cause.

wtb-z

The XS takes a lot of heat to work well, and the age can't be helping either.

Jeff Eikenberry

AVN, thanks again. The suspension settings I started with came from a pretty successful driver, but (here it comes) he has a C5 Z06. I'm not familiar with the differences in suspension, so will look to dialing back the rear camber if I don't have success with changing the rear bar back to stock and lowering the tire pressure slightly.

I'll take your advise re what changes I make to the car to assure I can be in a class I can be competitive. A stock C6 does have it's challenges when competing against Z51's and Grand Sports. Right now I'm really on a learning curve to improve my skills and be in the groove with my car. Jeff

Jeff Eikenberry

WTB-Z, I agree with you. Many of the people I compete with do have concerns about tire age. Thanks much, Jeff

wtb-z

Last year's comp tires were a set of 2010 XS (front) and 2008 XS (rear). Definitely a spooky tire until they warm up, which by the end of their life took multiple laps. I don't think I would have fun autocrossing them - the XS may ultimately be faster than the RE11 but on a stock C6 I might still do the RE11.

That said, on Hankooks this year and they are much the same - slightly faster warm up.

383

You run more than -1/8" in the rear? Is that for street tires only or does it help with something like a Toyo 888
How much toe out do you use in the front with more than 1/8" in rear?
ps. Nice driving!

Jeff Eikenberry

Steve, no doubt you weren't having bite issues. Thanks much, Jeff

Jeff Eikenberry

WTB-Z, So what brand are RE11's? jeff

mountainbiker2

All of our setup is for autocross. This is my friends 04 Z06 and we were running as much as 3/4" toe in at the rear with Kumho XS tires. That was with a stock Z06 front bar and no rear bar. 04 Z06 shocks. We are now trying a 32mm front bar with a stock rear bar. Plus he bought dbl. adjustable Penske's. On Hoosiers and Kumho's XS we have about 5/16" toe in for the rear with this new setup. The video is with the new setup. In my Corvette, I'm running close to 3/4" toe in. The bottom line is, you have to try it at the track. I think his front is 0 toe.

Thrillhouse

I run a stock 08 Z51 with the Pfadt street alignment and RE11s, I don't have any problems with the back stepping out. I'm also going to guess it's the old tires.

l98tpi

"feels like on marbles", definitely the tires. If you paid full price for those tires being 3.5 to 4 yrs old I'd be pi55ed and call back to tirerack and ask for the manager and work my way up the ladder. I think the tirerack will work with you.

With this being your first year, I think your alignment is set up for an oversteer condition, which may be a little much to learn with. Yes, it is a quicker set up once you learn to drive the car, but for starting out, I think an under steer set up would be best.

wtb-z

Bridgestone

Gives a up a bit to the Kumho in terms of speed when things are dry and hot, but is going to work over a much broader range of temperatures, which might be useful for A-X.

Jeff Eikenberry

AVN, I took your advise and changed out the Z06 rear bar for stock. I also dropped pressure in the rear from 30 to 27. The car was totally controllable on both Saturday and Sunday. While the rear end did step out on occasion, I had time to control things and have NO complaints. This was the first time I was comfortable with the car. Thanks very much for taking the time to cooment.. Jeff

Jeff Eikenberry

Please see my comments above to A V N. Thanks for your help. Jeff

Jeff Eikenberry

Please see my comments to AVN, above. While the car is now controllable the rear still has that marble feeling. I'll switch them out for next season. Meanwhile, I'm gaining valuable experience on controlling a rear end thar wants to step out. Thanks very much, jeff

jcarothers

I don't have much to add here, but I believe I meet you (Jeff) at the Marina AAS event last early month. This is James, I had the silver C6. Glad to hear you've finally got things sorted out with how your car was handling also.