Tire Pressure and Chalking the tires? As a baseline on a 70 degree ambient day how many lbs over stock would you set air pressure for a autocross run? Tires are Kumo's.

Exactly how should you mark chalk on the side walls and how do you interpret what it is telling you about your tire pressure?

Thanks

Art

 

Kumho streets, or Kumho R-compounds? V700's, or V710's? Shocks? Springs? I run *lower* than stock in the rear on V710's on my '96. Chalking isn't so reliable on the stiff-sidewall tires either. The only accurate way to gauge pressure is with a pyrometer.

In any case, you will probably get a better response asking this over in the Autocross/Road-Race section of the Forum.

 

Definitely need a pyrometer to know the whole tire is working. You may get enough contact to wear the chalk, but may still not be bringing that area of the tire up to temp.

 

What is done to the car? It will depend on your particular chassis, wheel, tire, and engine set up and to a great extent your driving style. Then you can get into humidity, track surface type and condition, et al.

As noted above chalking really only gets you in the ballpark. It's not exact.

And the autocross / roadrace section has a lot of info (not necessarily all good for your application) on tire pressures.

 

To help answer your question, try 38psi on the front and 34psi on the rear. That should be a good sarting point. Now, your chalk, place marks on the tire, where the sidewall meets the tread, around the tire in several spots. A good reference on your Kumho's will be some triangles which point to the edge of the tread where it meets the sidewall. What you are looking for is how much the tire breaks over during cornering. If you break the tire over and remove the chalk, you should add air pressure. If you are not removing getting close to the chalk, you may need to remove air or drive harder. This is to get started, also an alignment can impact how your tires will react. If you are just starting, then go and get some good seat time.

 

Thanks everyone. I am a newbie to autocross so I don't know what my driving style is. Figure that will developed with some guidance from the instructors, what I observe and learn through study. The car is a 1990 Z51, with 6spd. She stands 43" from ground to top of the roof. The wheels are 1990 ZR-1. The tires are new Kumho ECSTA SPT, equal size front & rear. Not sure of the details, but the suspension has Koni Adjustable schocks, and was prepared by Gulstrand Racing. The prior owner campaigned the car with Team Gulstrand here in So Cal and won 4 championships with the car in the late 90's early 2000's. He and the car were sponsored by Yokahama at the time. He has been less competitve and had less time to race her over the last few years. For the most part, other than the suspension tuning, lowering, aftermarket PROM, K&N filter, and 3" cat back she is stock.

Unfortunately due to slow recovery from knee surgery I am missing the Solo2 school this weekend. My expectations are to run two events in December, one with Cal Club (SCCA), and the other with Speed Ventures. Both will be at AutoClub Speedway in Fontana. Surface is asphalt with some up hill and down hill. I figure being a newbie, I can take advantage of the instructors and get some practice before the season starts next year.

My desire is to protect the car, apply what I have studied and perpare for some fun. to that end seals / gaskets have been replaced, new brakes with cooling ducts, new tires, new hoses, belts, plugs, wires, coolant, fuel regulator, starter and fluids. The car had new battery and clutch just before I bought her from a co-worker. Everything that wasn't replaced has been inspected or tested.

This weekend she gets a harness bar, 5 point, tow hook (future time trials with SV) and new headlights (nothing to do with racing).

 

Sounds like a great car. If Dick and his crew were working on it and it was good enough for a sponsorship the chassis is probably dialed in about as much as it can be. Seat time, as you mention, is the best thing you can do starting out. As you develop your skills you'll be better equiped to decide what you want the car to do. You're taking the right approach, IMHO, learn the car as it is then tinker with it if you want to.

Back to your original question, the post above on reading the chalk is pretty complete. I would add that different tires have different sidewall strength so it gets you started. You'll have to experiment and see where you (and the car) like the pressures to be. Another generic rule of thumb is to run a couple pounds less in the rear than the front. Again, this pressure differential varies from driver to driver and car to car.

 

jwt1603, thanks for the input. I believe the car is a lot more capable then I am and don't foresee the need to do much with it other than what I have detailed. That mainly because I am going to drive it to and from the events, so want to make sure it holds up. Although I am competitive by nature, my goals is to better myself in the first year of competition. If we don't win (and I don't expect to in class) it isn't the car. Looking at the sheets I am going up against long time regulars in BSP, so I am really playing against myself. Can I improve each outing and at least be respectable. Tire pressure seems like a big area of tailoring.

I assume since the tires heat with each run, checking / adjusting pressure between runs makes sense? Of course I then need to understand what more or less air does to the car. Assume you run more pressure in the front to improve turn in and less in the rear to prevent the car from skating and oversteering unexpectedly?

 

L98TPI...you still selling your other car with that set of wheels?

 

Here's a link to some baseline information to get you started.

http://www.tirerack.com/tires/tirete...jsp?techid=58&