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Query on recommended tire pressure and weight front to rear?

7064 Views 8 Replies 7 Participants Last post by  Nyyyce&Eeeezy
The door card recommended tire pressure is 29 PSI front and 35 PSI rear on the 560SL. The weight distribution is heavier in front, about 55% front 45% rear. I thought that tire pressure should be directly proportional to load. So perhaps there is insightful expertise on this board to explain why the heavier front gets the lower pressure?

To add to this mystery, my 450SL door card said 32 PSI front and 26 PSI rear, with an approximate weight distribution of 57% front. So what is the basis for the OEM pressure differentials between front and rear across the 107 series? Anybody find good results with different sets of pressure?

Thank you for your ideas.
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I decided to run 35 psi on all four corners and the only noteworthy difference so far is it is easier to steer.
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Thanks all for the comments so far. I might be answering my own question with a paste from Bimmerfest. Seems that BMWs also use low front high rear tire pressures. According to Big Money Wasted, lower front pressures in spite of higher front end weight is an intentional way to create understeer. This handling characteristic is of course a matter of personal preference.
After Hours said:
You don't have to use the recommended pressures, but it is best if you don't go below them--low tire pressures lead to heat build-up and tire failure, which led to the mandatory US tire pressure monitoring.

The reason for the difference in pressure is to create understeer. Increase the front pressures and the car will understeer less, or to put it another way, it will turn in more quickly on the entry to a corner. I would guess that if the tire pressures are equal front and back, a BMW with 50-50 weight distribution, front and back, would oscillate disturbingly between understeer on braking and oversteer on acceleration.

I once had an old Porsche with almost 60% of the weight in the back. Porsche recommended higher pressures in the rear tires than the front to help tame the oversteer. I learned to like oversteer, and on my F31 I have increased the pressures in the front tires by 3 psi and the rears by 2 psi, which doesn't create oversteer but at least it decreases the understeer. On an E91 that I had the recommended pressures led to the tires wearing out only along the edges. The tires lasted longer when I increased the pressures enough to bring the middle of the tread into play. The only downside is a slightly stiffer ride.

BMW clearly thinks that you will like the car most with the combination of ride and handling resulting from 32 psi/ 38 psi, but you may prefer some other combination. It is certainly an inexpensive bit of chassis tuning, and if you don't like it it is easy to change back.
Tire pressures different front/rear - Bimmerfest - BMW Forums
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