Posted by Ted Getchell on September 26, 1998 at 10:20:31:
In Reply to: Re: Tire Pressure Question posted by EM on September 25, 1998 at 17:05:16:
I am not sure of the weight distribution on the 540iA. Something I should know, though. I have checked the tire pressures after some hot laps and they still maintain a front to rear pressure differential although it drops to more like 4 or 5 psi.
To further complicate my thought process, I would think that a front-heavy car would naturally tend to understeer. Lower front pressure tends to increase understeer. And yet, BMW recommends this 7 psi differential. Experience tells me the car is about neutral but with too much roll-over on the fronts. Shoe polish indicators on the rear tires show that they arn't working very hard compared to the fronts. Go figure!
Anyway, thanks for your detailed thoughts on all of this. If you happen to come up with a Michelin contact, I will really appreciate it.
I'm going back to Roebling in November and will look around for someone with a pyrometer - good idea. It sounds like I am obsessing on this, but not really - just a sort of interesting technical question.
: Is the reason that BMW recommends the 7lb differential because they're accomodating a front/rear weight distribution that causes the front tires to increase in pressure (when hot) so that all 4 tires (eventually) have equal pressures? So, maybe you start out w/a 7lb differential, but they all end up around the same pressure?? The reason I ask this is because I'm assuming that the 5-series is much heavier in the front than the rear (I'm guessing 65/35 perhaps...? I don't know), and, within my limited racing experience, we try to get the tires to be the same pressure all around when hot. Have you taken the pressures when hot?
: There's a group in this area (mid-atlantic) that's running a couple of M3's in one of the IMSASportsCarNATCCSCCAorwhateveritscalled now series. I'll try and see whether they have a Michelin contact that might offer some factory advice. If I can get in touch with them, I'll post the results...
: At the risk of being long-whinded, one more thing... Is there a way you can get tire temps? That'll tell you a lot about how the suspension is performing. If you have an opportunity, take them during a practice session after about 5-6 good laps. Usually, someone will have a pyrometer handy and will be happy to take them for you. Have them write the temps down -- outer, center and inner for each tire. Ideally, they would all be the same ax the tread and between tires.
: Apologies in advance for anyone who thinks I'm droning on too long about this. Sorry.
: [email protected]
: : Good advice and you are right. Blowing a tire at speed in a fast sweeper would be very ugly. I AM talking cold pressures in my question and understand how the pressure builds. I wish I could find a way to talk to some tech person at Michelin, but there doesn't seem to be any access at their website. Any ideas?
: : Ted
: : '95 540iA
: : : I'd be VERY careful about exceeding a manufacturer's maximum recommended pressure -- especially at the speeds you mentioned!! You certainly don't want a catastrophic failure or you might damage your car (or yourself). Also, you don't say whether the pressures you're running are taken hot or cold. Remember, if you use air (not nitrogen), the pressures will increase as the tires heat up.
: : : Keep in mind that sidewall rollover is not only a function of tire pressure. It results from the interaction of your other suspension components with each other. What about your tire speed rating, shocks/struts, camber settings, sway bar size, tire temps, etc.? Granted, if you need to do something here, it isn't as easy as changing tire pressures (or as inexpensive), but you have to consider the whole package to remain safe.
: : : : 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