Cold Tire Pressure
09-05-2009, 02:29 PM
#31
09-05-2009, 02:42 PM
#32
) but yes, on a stock suspension, increasing your front psi from 36cold and leaving your rears constant, you should reduce understeer somewhat. On my car, with ROW 030, 38/40 cold seems to work well at AX. Very neutral, and can steer with the throttle. However, the suspension tuning is responsible for most all of the reduction in understeer on my car.You may have to experiment with your pressures to find a combination of front/rear pressures that works for your car and your driving style.
09-07-2009, 12:00 PM
#33
Drifting
I don't profess to be an autocross expert either, but I'm doing well in the local clubs, too. Tire pressure for autocross are something you unfortunately will simply have to use trial and error to find, as different tires require different pressures, and no two cars are ever optioned out/trimmed out exactly alike and thus have different corner weights. Not to mention there's a real amount of preference, too- some people like a lot of oversteer, while others don't. You might even find you need to change the pressure as you progress and your preferences change.
I'd recommend scanning through the autocross forum here and try to find someone else running a setup similar to yours, and start there. (Bear in mind anyone on r-comps will be running drastically different pressures, and your gauge is probably reading a couple random psi off from anyone else you talk to.) Chalking your tires will help, too, as optimal inflation pressures are generally around the point where you're using the entire tread and you get back to the pits with just a tiny amount of chalk left above the sidewall. But it will take trial and error to dial in the car from there, and to get it just how you like it. The more experienced guys in your club can help you, too. The best thing you can do is attend a test 'n tune and just play with the pressures, as 3-5 runs per month is going to take a while to dial them in right. Be sure to keep a log, too!
One other question: how aggressively do you drive when you autocross? Have you ever spun? Do you frequently hit cones? (If not, you're not driving fast enough, but that's another issue...) If you're an aggressive driver and always on the edge of traction, you'll probably be able to feel 2psi of pressure difference, especially if you adjust the front while keeping the rear at a constant pressure, and vice versa. If you're not yet driving consistantly on the edge of traction, you probably won't be able to notice small differences in tire pressure, and should just pick a decent set of pressures (ie, use the chalk) and stick with it until you improve a bit. And maybe next season, you can give it another go. Speaking of which, bear in mind you *can* put wider than OEM tires on the stock rims.
I'd recommend scanning through the autocross forum here and try to find someone else running a setup similar to yours, and start there. (Bear in mind anyone on r-comps will be running drastically different pressures, and your gauge is probably reading a couple random psi off from anyone else you talk to.) Chalking your tires will help, too, as optimal inflation pressures are generally around the point where you're using the entire tread and you get back to the pits with just a tiny amount of chalk left above the sidewall. But it will take trial and error to dial in the car from there, and to get it just how you like it. The more experienced guys in your club can help you, too. The best thing you can do is attend a test 'n tune and just play with the pressures, as 3-5 runs per month is going to take a while to dial them in right. Be sure to keep a log, too!
One other question: how aggressively do you drive when you autocross? Have you ever spun? Do you frequently hit cones? (If not, you're not driving fast enough, but that's another issue...) If you're an aggressive driver and always on the edge of traction, you'll probably be able to feel 2psi of pressure difference, especially if you adjust the front while keeping the rear at a constant pressure, and vice versa. If you're not yet driving consistantly on the edge of traction, you probably won't be able to notice small differences in tire pressure, and should just pick a decent set of pressures (ie, use the chalk) and stick with it until you improve a bit. And maybe next season, you can give it another go. Speaking of which, bear in mind you *can* put wider than OEM tires on the stock rims.
09-07-2009, 02:43 PM
#34
Three Wheelin'
I don't profess to be an autocross expert either, but I'm doing well in the local clubs, too. Tire pressure for autocross are something you unfortunately will simply have to use trial and error to find, as different tires require different pressures, and no two cars are ever optioned out/trimmed out exactly alike and thus have different corner weights. Not to mention there's a real amount of preference, too- some people like a lot of oversteer, while others don't. You might even find you need to change the pressure as you progress and your preferences change.
I'd recommend scanning through the autocross forum here and try to find someone else running a setup similar to yours, and start there. (Bear in mind anyone on r-comps will be running drastically different pressures, and your gauge is probably reading a couple random psi off from anyone else you talk to.) Chalking your tires will help, too, as optimal inflation pressures are generally around the point where you're using the entire tread and you get back to the pits with just a tiny amount of chalk left above the sidewall. But it will take trial and error to dial in the car from there, and to get it just how you like it. The more experienced guys in your club can help you, too. The best thing you can do is attend a test 'n tune and just play with the pressures, as 3-5 runs per month is going to take a while to dial them in right. Be sure to keep a log, too!
One other question: how aggressively do you drive when you autocross? Have you ever spun? Do you frequently hit cones? (If not, you're not driving fast enough, but that's another issue...) If you're an aggressive driver and always on the edge of traction, you'll probably be able to feel 2psi of pressure difference, especially if you adjust the front while keeping the rear at a constant pressure, and vice versa. If you're not yet driving consistantly on the edge of traction, you probably won't be able to notice small differences in tire pressure, and should just pick a decent set of pressures (ie, use the chalk) and stick with it until you improve a bit. And maybe next season, you can give it another go. Speaking of which, bear in mind you *can* put wider than OEM tires on the stock rims.
I'd recommend scanning through the autocross forum here and try to find someone else running a setup similar to yours, and start there. (Bear in mind anyone on r-comps will be running drastically different pressures, and your gauge is probably reading a couple random psi off from anyone else you talk to.) Chalking your tires will help, too, as optimal inflation pressures are generally around the point where you're using the entire tread and you get back to the pits with just a tiny amount of chalk left above the sidewall. But it will take trial and error to dial in the car from there, and to get it just how you like it. The more experienced guys in your club can help you, too. The best thing you can do is attend a test 'n tune and just play with the pressures, as 3-5 runs per month is going to take a while to dial them in right. Be sure to keep a log, too!
One other question: how aggressively do you drive when you autocross? Have you ever spun? Do you frequently hit cones? (If not, you're not driving fast enough, but that's another issue...) If you're an aggressive driver and always on the edge of traction, you'll probably be able to feel 2psi of pressure difference, especially if you adjust the front while keeping the rear at a constant pressure, and vice versa. If you're not yet driving consistantly on the edge of traction, you probably won't be able to notice small differences in tire pressure, and should just pick a decent set of pressures (ie, use the chalk) and stick with it until you improve a bit. And maybe next season, you can give it another go. Speaking of which, bear in mind you *can* put wider than OEM tires on the stock rims.
I've only just started autocrossing the 996 though and it is a completely different style of autocrossing. The blue car was very light, very tossable and most of the steering was done with the throttle. The 996 understeers and over the three events I've done and no one could tell me how to tame this. I've got PSS9 coilovers on the way, and of course I'll dial in more camber when I install those, but in the mean time I don't want to be embarrassed by my old air cooled buddies.
Yesterday I applied a lesson learned here and increased the front pressure to 38, dropped the rear to 42 and on a rainy day no less, the car handled beautifully neutral with oversteering tendencies. Finally I some decent 911 handling back and I am now placing back in the rankings where I belong.
Thanks for the help lads.
Last edited by jasper; 09-16-2013 at 02:05 PM.
09-07-2009, 03:19 PM
#35
Drifting
hahaha, sorry, the way you asked the question, I just assumed you were a novice. Glad I could be of some help! What organization/class are you racing in? Sway bar settings can help, too.
09-07-2009, 03:31 PM
#36
Three Wheelin'
Now that you mention it, I forgot that Iinstalled a set of H&R sway bars a few months ago and this was my first time autocrossing with them on....I wonder if this didn't have as much to do with the improved handling as the tyre pressures 
Jeezuz I must be getting old!Either way - I'm happy with the setup for now.
Cheers!
