Posted by Ted Getchell on September 24, 1998 at 16:23:55:
In Reply to: Re: Tire Pressure Question posted by Rob on September 24, 1998 at 16:04:39:
Thanks, Rob. Rather than autocross, though, I'm in driver's school on race tracks so speeds are up to 125 mph or so. Do you think going 4 psi over mfg's max is OK? As I think about it, I guess the higher pressure would tend to make the tires run cooler, so less stress. Hmm, speed isn't the problem, it's impact as you mentioned. So I best keep it on the track. ;-) Am I thinking right, here?
: To minimize rollover you'll need to run a lot of PSI. For the duration of the autocross, I would not be afraid to go over the max specified on your tires. I'd feel safe going up around 48 PSI. Just make sure there are no pot holes on the course. ;-) As you stated, the idea is to keep your side walls from rolling over. You can mark the sholders of your STREET TIRES with shoe polish to see if they are rolling. (This won't work with R tires.)
: Varying PSI front to rear will have the following effect:
: To increase oversteer/reduce understeer - Raise the rear PSI/lower the front PSI
: To reduce oversteer/increase understeer - Raise the front PSI/lower the rear PSI
: So, the factory PSI settings is used to balance your BMW's handling and keep it from pushing too much. I'd start with the 7 PSI
: difference as a starting point and work from there.
: : Hi 5ers!
: : BMW recommends 33psi front and 41psi rear on my 95 540iA. I currently am running Michelin Pilot XGT V4s with a maximum allowable pressure of 44psi (225/60 R16).
: : When on the track I would like to take the front tire pressure up 5 - 6 psi based on observed scuffing a little farther onto the tire shoulder than you would want. Because of the 44psi max, I can't go up that much and maintain the 7 psi front to rear pressure differential.
: : My question to the group: How important is the 7 psi front to rear difference? I realize I could do lots of testing to figure this out, but I am hoping that someone out there is willing to share their experience.
: : Thanks.
: : Ted
: : '95 540iA