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      03-21-2009, 05:25 AM   #1
E90M3CDFR
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Plus 100mph driving & 'Ring Tire Pressures

The BMW recommendations for sustained high-speed driving are confusing me a bit, and maybe someone can clear this up for me: the owner's manual says that for "traveling speeds up to a max. of 100mph", you should have 35psi front and 36psi rear (for the 19") for a lightly or heavily loaded car.

For "traveling speeds including thos exceeding 100mph" in a lightly loaded car, you can use the same pressure in the front, but slightly less (35psi)(?) in the rear. For plus 100mph driving in a heavily loaded car, you are instructed to increase the pressures to 41psi front and 44psi rear.

On the other hand, the yellow sticker on the door jamb seems to recommend going to the 41psi/44psi pressure for any plus-100mph driving, regardless of weight loading. To make matters somewhat more confusing, for 18" wheels, the recommendation for over 100mph driving for a lightly loaded car requires a slightly (33 to 35psi) increase in front tires (as opposed to the rear with the 19"), but leaves the rears unchanged.

Normally I set my pressures at the "less-than" 100mph pressures (we're limited to 130km/h in France), but when I drive to Germany, I have been resetting them to the over 100mph pressures (41psi front, 44 psi rear) if I have passengers/luggage in the car (usually the case). If I don't have passengers in the car, I set the pressures halfway between the two extremes, ie 38psi front, 41psi rear. Based on the limited knowledge I have of the relationship between tire/speed/temp and the preference for higher pressure to stiffen the sidewall. On the other hand, the 19" tires have a pretty stiff sidewall to begin with, so maybe not as much of an issue. At any rate--would welcome any advice as to whether to go with the numbers in the manual, the door sticker, or something else.

On another tire topic: when I was the 'Ring recently, a couple of German BMW drivers (hard-core 'Ringers) were taking about 10percent reduction in their tire pressures before heading out on the track for a long series of laps. They were very emphatic that adding more air--as you would do for an autocross--is a bad practice for the 'Ring if you are going to a string of laps because they said tire heat buildup is a rear problem, and in any case causes loss of grip because of the lessened contact patch of a higher-pressure tire . Now clearly running with a very low pressure will lead to problems as the tire overheats from deforming, but this very slightly pressure decrease was something I had not heard of before (and they weren't pulling my leg since I saw them taking the air out). Thoughts?
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      03-21-2009, 10:59 PM   #2
JAJ
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It works like this:

- if you're running on the highway at high speed, essentially just cruising, you need more pressure when you start because it reduces flex in the tires and that actually reduces heat buildup and the associated increase in pressure. The tires run cooler under these conditions.

- if you're on the track, scrubbing when cornering and braking hard builds up heat fast, regardless of the initial tire pressure. You need to lower the starting pressure so the tire doesn't develop so much added pressure it loses grip ("getting greasy" is the term we use).

It's logical - the two situations are different - stiffening up the tire reduces heating on a highway cruise, while lowering the pressure keeps a tire that's destined to get really hot from getting over-pressured.
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      03-23-2009, 04:20 AM   #3
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Thanks, that makes a lot of sense. So you would recommend that for extended high-speed driving, interpret BMW's conflicting recommendations in favor of the higher pressure settings (41psi front/44psi rear)?

Much appreciate the advice.
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