Posted by Ted Getchell on September 25, 1998 at 10:40:58:
In Reply to: Same recomendation, but why not buy R tires? posted by Rob on September 24, 1998 at 20:22:46:
Rob, you are right about the R tires, I know. I have just been kinda easing into this track stuff. I guess I could get two in the trunk & two in the back seat (my instructor buddy used to manage to do this in his 944 so surely I can get there in the 5). BTW, when I rode with him at Roebling he managed 142 indicated at the end of the front straight in that '89 4 cylinder turbo, all stock except for a chip and R tires - had me sucking in deep breaths! :-).
Thanks for taking time to help me with this.
: If you wind up off track you're going to suck grass down the bead if you run 35, or 48 PSI. ;-)
: Yes, I still think 4 PSI over will be fine; however, I can't say if that will give you your fastest lap times. I'd ask other BMW drivers at the track what PSI they're running. I'd also ask the instructors what they recommend. (My high speed experience has not been behind the wheel of a BMW.)
: Have you given any thought to buying a set of R tires for the track? I ran Roebling Road in my '94 Corvette last year using street tires. I was hitting about 135 - 140 MPH on the straight. The corners were pretty lame though. The difference between street tries and R tires in the twisties is stunning. Ride along with me on street tires, and you'll think "cool...kinda fast". On R tires you'll think "MY HEADS GONNA FALL OFF!"
: Perhaps the following will help you make the jump:
: 1) Your street tires will last a lot, lot longer.
: 2) You'll find you will be much safer.
: 3) You will oodles quicker through the curves.
: 4) The car will be a *lot* easier to drive.
: 5) You'll grin ear-to-ear as you take those corners at speeds you never dared to before.
: 5) Used wheels should be cheap and easy to find. They don't have to be pretty, just round.
: 6) R tires can last a long time. Especially if you don't drive around town on them.
: 7) You don't need a tow vehicle. Surely you can fit 4 mounted tires and a floor jack in a 5. (Try that with a 'Vette!)
: : 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?
: : Cheers.
: : Ted
: : '95 540iA
: : : 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