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      08-15-2018, 09:38 PM   #23
rjdnyy224
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So here's my question now. I understand that the rolling diameter of all wheels on an xDrive vehicle have to be within 1%. I think I'm just going to do two new tires on the 20's, the damaged one and the one with the most wear. That way I can have at least matching sets on each respective axle. But, the tires currently on the wheels had 9/32s of tread 6,000 miles ago. They are MPSS's so lets assume they wear fast and are now at 4/32nds, absolute worst case scenario, even though they look better than that. Will I still be within tolerance if I replace 2 tires with brand new Michelin Pilot Sport 4S's? I would do more Super Sports but they don't make 'em anymore (also decided to just pay up and get the good ones instead of anything else). What do you guys think would this work, and would the xDrive be happy with it?
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      08-15-2018, 09:52 PM   #24
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Quote:
Originally Posted by rjdnyy224 View Post
I understand that the rolling diameter of all wheels on an xDrive vehicle have to be within 1%.
Not really. The xDrive is electronic. It uses the rotation information from each wheel provided by the ABS sensors to determine how to apply power to each wheel, so the need for relatively exact tire matching that you have with purely mechanical AWD systems doesn't exist. All I'd be concerned with is having the same model tire on each axle. It would take quite a bit of difference in the front and rear diameters to cause a problem, a lot more than 1%.
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      08-15-2018, 10:01 PM   #25
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Originally Posted by Billfitz View Post
Not really. The xDrive is electronic. It uses the rotation information from each wheel provided by the ABS sensors to determine how to apply power to each wheel, so the need for relatively exact tire matching that you have with purely mechanical AWD systems doesn't exist. All I'd be concerned with is having the same model tire on each axle. It would take quite a bit of difference in the front and rear diameters to cause a problem, a lot more than 1%.
So I'll be fine with the same model/wear on each axle but slightly different rolling diameter front to back? Interesting. I just figured that because the rolling diameter will be different from axle to axle, even slightly, the smaller diameter ones will spin faster, thus messing up the AWD system. This thread discusses what I was talking about: https://f30.bimmerpost.com/forums/sh....php?t=1142647 I guess it is a myth after all...
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      08-15-2018, 10:47 PM   #26
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Like all myths based in fact. With a purely mechanical AWD system that tied the front and rear differentials together you could have problems with different size tires, among other things, like going around corners at higher speeds. That's why they disengaged the AWD for normal driving, engaging it only when they needed it.
You wouldn't want to have decidedly different tire diameters front versus rear with xDrive, but a very slight difference would be OK. With 26 inch diameter tires 1% would be only .26 inches, which is about 1/4 of an inch. You could have that much variation from tire to tire just from differing air pressures. I can't imagine even 3% variations being a problem. 5%, maybe.

Edit: Math corrected. Note to self: Don't do calculations in head after the sun's gone down.

Last edited by Billfitz; 08-16-2018 at 09:48 AM.
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      08-15-2018, 11:05 PM   #27
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Quote:
Originally Posted by Billfitz View Post
Like all myths based in fact. With a purely mechanical AWD system that tied the front and rear differentials together you could have problems with different size tires, among other things, like going around corners at higher speeds. That's why they disengaged the AWD for normal driving, engaging it only when they needed it.
You wouldn't want to have decidedly different tire diameters front versus rear with xDrive, but a very slight difference would be OK. With 26 inch diameter tires 1% would be only .026 inches, which is about 1/40 of an inch. You could have that much variation from tire to tire just from differing air pressures. I can't imagine even 5% variations being a problem. 10%, maybe.
Gotcha. Thanks for explaining that. I was planning on just doing two tires before, and now you've reassured me.
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      08-20-2018, 08:47 PM   #28
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For anyone that cares: two brand new PS4S's fitted and mounted on the rears, old PSS's on the front. All is well, no issues. Thanks for everyone's help
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