aggie88

I have a stock C6 Z51 MN6, with Bilstein Sports and 18 inch Kumho V710s tires, 285 F and 315 R.

At this weekend's event, after I got the tires warmed up, I found the front end getting light under power in the turns. I could feel the front tires skipping. Anyone recommend a remedy for this? It was a cool day in the 50s and I ran 32F/34R cold tires.

JCD

Skipping? Is something broke/out of alignment? Sounds like you have a good setup. Perhaps it was bad pavement?

Any more info?

racerwannabe

sounds like pushing to me..maybe more camber??in the front

mountainbiker2

I know everyone has there preference on tire pressure, but I think yours are to high. Somewhere around 30F HOT, 28R HOT.

Steve

Bill Dearborn

On my C5 Z I am running my V710s at 28 front and 26 rear cold. Hot pressures are tough to work with in an autocross because the tires never get up to temperature and the temps will be different from one run to the next so you really do not have a good basis to work from. Baseline everything at cold temp and adjust based on that temp. A pound of air added or subtracted after the tires are warm will be equivalent to a pound of air added or subtracted to a cold tire.

Bill

mountainbiker2

I'll quote my own.

Start with 30f cold. 28r cold. After each lap, bleed down to those pressure again. Keep doing this after each lap. Now you have hot pressures at that psi.

That's what I really ment.

Steve

aggie88

Surface is worn off asphalt but no ruts. I discounted understeer because the car does turn in, but the steering is really light when powering out of the turn. I guess its power understeer with a RWD???? Funky.

I will try keeping the pressures lower... I really am not sure what the V710s like. I do like to run more air in the rear because of axle hop out of the gate.

GTB/ZR-1

I agree w/ the others that you should run more pressure (2-3 lbs) in the front--hot or cold.

Have you had this condition before?
Obvious question: Were you getting on the gas too soon possibly? The lower front pressure could be contributing to the condition as well.
My .02...

gonzalezfj

That's the wrong thing to do. Learn how to control the axle hop by not spinning your tires. Don't just rev the engine up and dump the clutch. I autoX a C6Z and the fast launch is accomplished with relatively low RPM 2000 to 2200, slip the clutch for just a moment and don't stomp on the gas pedal. Roll the power on slowly as the car gets grip.

My co-driver and I ran 710s for 2 years in a C5Z. 28F, 24R hot, although some folks ran up to 6 PSI more. The car was competitive at the national level with those pressures. Don't follow misguided advice to set your tire pressures cold (that's fine for road racing but not for autoX). As the tires warm up (with each run they will get hotter, especially if you have a co-driver) you need to check the pressure before EACH run. You will find that the tire pressures jump 2-4 PSI after your first run, then 1-2 PSI per run afterwards until they get to full operating temp (4-5 runs in our car). Then the pressures will stabilize and you typically will not need to bleed air from the tires after that.

If this is a dedicated autoX car, you will need to get a competent alignment.

Solofast

As others have said, you are way too high in pressures, with that much air on a cold day I would expect the fronts to "skidder" across the pavement with little or no grip.

If you start too high (and in particular if it is a cold day), you don't get any grip. If you don't have any grip you won't distort the tire and guess what, you won't generate any heat. No heat = no grip, so you are stuck sliding around... Your rears were lower, and they got warmer, so the started to work, but the fronts weren't there and you just skated around...

You also didn't mention the age of the tires. Older tires aren't as grippy, and generally require less starting pressure (a pound or two) lower than newer tires to start out.

On a cold day I would expect to start at like 27/25, and let them heat up to 28 or 29 in the fronts and 26 in the back, and then bleed them off to stay no higher than that.

XPC5R

The 285/18 kumho is a short, very low profile tire compared to the 26" tall 315/18. It will make the front end feel light in almost all conditions. Add more front camber and toe out, and it should track and hold better under power on corner exit, as well as feedback more.

aggie88

More Negative camber?

Silverton

Aggie88: I have a C6 Z51, and I'd like to run 285/35-18 tires (or even 295/35-18) on the fronts. May I ask you what is the offset of your front 18" wheels? Also, do you have any rubbing at, say full lock or full compression? Thanks!

JiminVirginia

On a cold day you need as much tire squirm as possible to heat the tires, and as guys have pointed out, this is best accomplished with lower tire pressures--like as much as six pounds less than you were running. If the tires stay cold they will never provide the traction you need.

The other factor that occurs to me is to be darn sure you're not over driving the car: try slow in, fast out approaches and see if it helps. (This is not criticism; some times I need to follow my own advice better, I'll be the first to admit.)

aggie88

They are Fikse FM5s 18X10 custom made for a C5 Z06. I bought them used so I am unsure of the exact offset, but they measure between 7.25 and 7.5 backspace - no rub ever under any conditions with the 285 30 18 or 285 35 18s. The outer sidewall sits flush with the fenderline.