Contour Enthusiasts Group Archives Forums General Autocross and Open Track Tire Pressure

Does increasing the tire pressure on the front and rear tires have the same effect on grip? i.e. To increase rear tire grip to reduce oversteer should I increase tire pressure?

Most have found that increasing rear tire pressure INCREASES oversteer. I personally find 45/45 front/rear works for me. Rrxp683? (Chris) has been very successful using over 50 lbs. in front. I don't know what he uses in the rear.

The increased oversteer with higher rear pressures seems to be a function of reducing rear tire rollover thereby causing less tire grip and increased slip angles. You want the tire to rollover a bit for maximum grip. You must have at least 45 lbs. or so in the front tires or they will rollover too far causing less grip and more tire wear. Remember that there is much more weight on the front tires than on the rear tires. Then you have acceleration forces and the brunt of the braking forces there as well so front tires are the workhorses.
Dave Andrews what are you running and any others?

Are tires not rated for less pressure then that? I thought the max pressure is around 45?

You'll need quite a bit of pressure in the front to keep the tires from rolling under simultaneous braking and turn-in. I think the static weight bias is about 60%+ on the front. The dynamic load under hard braking is probably close to 75% (I'm guessing). What has worked for me is to find a front pressure that allows the car to turn-in under braking without the front end scrubbing. I've done it by feel and not by chalking the tires. Once I found that pressure I tune with the rear pressure. Last weekend I started with 50psi in the rear. The car was very loose through one section of the track so I lowered the rears to 48psi and the balance was better. My theory (not saying it's right) is to start 52/50psi and tune from there. Want more steering go a bit higher in the rear. Want less take some out. I usually end up with a different rear setting each weekend.

I had a thought the other day about my extreme front pressures. My gauge might not be accurate? The tires on my truck look very low even though my trusty, el-cheapo gauge says 60psi. I'm gonna find a good gauge and see how the readings on this one compare.

Chris

And you aren't supposed to remove the tags from mattresses under penalty of law. Your results may vary.

For the most part this is going to be a factor of your driving style. Some like to enter a corner fast, and carry the speed...Others prefer to come in slow and exit under acceleration. Either way, the tire pressure that suits you best is something that you'll have to toy with over time.

That said, I can offer my experiences as a baseline. When I ran street tires, I usually started at about 45/35. Track conditions would dictate from there (i.e. damp/wet = lower hot/dry/grippy = higher). I have tried as high as 52, but 45 seemed to work best for me.

The front is usually the easiest to get right for me. The back steps out only under really hard cornering or slolams, and that can be tricky to get just right before my last run.

I have had no problems running my tires at above 50psi. I don't know if there is a max psi rating for my tires.

Good Luck,
Mark

40 psi is what I'm running. sometimes a bit less. The surface that your on has a major influence on this. The surface I deal with is pretty rough for asphault. Probably why I only need 40 psi. If I put in anymore its like being on ice. The car actually likes less air pressure but (better grip) the tires roll onto the sidewall then.
I've cut down to 35 psi in the rear tires and it was my best overall performance yet last time I was out.

You should almost always use as little as possible at first to keep the tires from rolling under. Learn how to control the car, then start playing with tire pressure. I know I'm still learning how to drive a FWD in autoX. Tough lesson coming from my old RWD big V-8 car. I also seem to go quicker when my tail end never comes out. The times is does, I end up going slower. T

im pretty new to the auto-x thing but last run i did. i guy that does this for years. recommended to run 40 front 44 rear. (which doesnt make sense to me) but i followed his advice and my time dropped by 6 secound. all that crappy understerr and pushing was gone. the tail wasnt too loose but it was loose enough to get the front end in the direction i needed it in and at a much faster speed. i droped from 53 sec to 47 secs.
go see for yourself. http://autox.carlc.com may 19 time trials G-Stock. (ps. that was my 1st time auto-crossing) my next will be june 9th.

Breidy, you didn't say what pressures you were running before. Because it was your first autocross, a lot of things could explain a six second drop in time other than tire pressures. We don't know what kind of surface you are on, either, but I suspect you could probably get away with adding 5 pounds more to the front, to about 45 lbs. I have tried it at 40 and 50, and think 45 is about right...but that is on concrete. I have autocrossed once or twice,too, and my pressure choice was tested on a Contour! Your friend may know more, but try it both ways.