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      11-05-2016, 02:43 PM   #1
x1GTA
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Winter Tire Pressure

I just installed a set of Evergreen EW66 winter tires on my 2012 X1. (Not the tire I would have picked, but they came with the car).

Regardless, what pressure do most people run in their winter tires? I currently set them at 35psi all around. The door sticker indicates 31 Front and 38 Rear.

I'm concerned about running different pressures due to the fear of transfer case failure due to different tire diameters front/back due to tire pressure. Probably insignificant in reality.


Any thoughts on this matter?
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      11-05-2016, 02:55 PM   #2
4llr
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Different cold PSI does not directly translate to different tire diameters when running (different weights front/back, tires can heat differently, etc.).

I would just go with what the door sticker says personally. I don't think BMW engineers would give different PSI front/back if it would cause transfer case failure.

Last edited by 4llr; 11-06-2016 at 08:45 AM.. Reason: Add clarification
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      11-05-2016, 03:07 PM   #3
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Quote:
Originally Posted by 4llr View Post
Different PSI does not directly translate to different tire diameters.
Did you fail physics? Of course it does, at least to a small degree. How on earth can you possible say it wouldn't?

OP, relax. 35/35 is 100% fine. This question is not at all winter tire specific.
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      11-06-2016, 07:40 AM   #4
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Quote:
Originally Posted by PrematureApex View Post
Did you fail physics? Of course it does, at least to a small degree. How on earth can you possible say it wouldn't?

OP, relax. 35/35 is 100% fine. This question is not at all winter tire specific.
Chill a bit, I'm only trying to help here.

Yes 35/35 is probably fine. No, your transfer case won't blow because of this. I should have specified that the rolling circumference diff between the front/back would be very minimal on hot PSI and with all the different variables taking into account (understeer, tire size, weight distribution, etc.). Search on the internet, it's not that simple, and that's why I recommend sticking with what the door jam sticker says, since they have all the car's data to make the right recommendation, and I don't think they would recommend different PSI on a Xdrive if the transfer case wouldn't be able to tolerate the small difference in rolling circ. (if there's any).

more info here: http://f30.bimmerpost.com/forums/sho...d.php?t=760712

If someone has more info on the topic, I would love to learn more, and to be proven wrong if I am

Last edited by 4llr; 11-06-2016 at 07:48 AM..
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      11-06-2016, 08:09 AM   #5
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Guess what BMW's own RPA tire pressure monitoring system (not the TPMS/RDC system) uses to detect low tire pressure? Wheel speed.

It sees a wheel traveling slightly faster than the others, because it has a small diameter, indicating a tire down on air.

You just told everyone tire diameter is not affected by tire pressure. I'm not sure how helpful you're going to be...
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      11-06-2016, 08:33 AM   #6
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Just want to add that the tire pressure system was only able to sense a difference of 5-6 psi on a non-RFT for us and needed about 4km to do it. And run flats are far less sensitive to pressure differences. Also, tire pressure system might only detect side to side differences (I'm not sure if that's the case, but it is possible) so it might not notice front to rear differences.

Don't get me wrong, difference in pressure does make a difference in circumference, and for that reason I would stick with what the door jam sticker says. It's entirely plausible that front tires heat up more than the rear, equalizing the issue a bit. Plus other factors might affect the actual circumference of the tire. And finally, the transfer case might be set up for a slightly larger diameter rear wheel - after all, more power goes to the back then the front.
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      11-06-2016, 08:35 AM   #7
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Quote:
Originally Posted by PrematureApex View Post
Guess what BMW's own RPA tire pressure monitoring system (not the TPMS/RDC system) uses to detect low tire pressure? Wheel speed.

It sees a wheel traveling slightly faster than the others, because it has a small diameter, indicating a tire down on air.

You just told everyone tire diameter is not affected by tire pressure. I'm not sure anyone is listening to anything you say anymore...
Here in Canada we don't have TPMS per tire but the ABS tire pressure system you mention (FTM). I run the recommended tire pressures (higher in the back / lower in the front) and I get no FTM alerts. FTM (iTPMS) is relative to the reset value:

"iTPMS cannot measure or display absolute pressure values, they are relative by nature and have to be reset by the driver once the tires are checked and all pressures adjusted correctly".

When you are driving, and one tire is losing air, diameter will be different than the relative reset value and trigger the FTM (good info on the system here: http://www.bavarianmw.com/guide-4444.html).

And you are right, my initial reply was not specific enough and inaccurate, but AFAIK, different rear and front cold PSI does not directly translate to a significant difference in rolling circumference that could cause transfer case failure. That should have been my initial reply.
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      11-06-2016, 08:38 AM   #8
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Quote:
Originally Posted by vstolpner View Post
Just want to add that the tire pressure system was only able to sense a difference of 5-6 psi on a non-RFT for us and needed about 4km to do it. And run flats are far less sensitive to pressure differences. Also, tire pressure system might only detect side to side differences (I'm not sure if that's the case, but it is possible) so it might not notice front to rear differences.

Don't get me wrong, difference in pressure does make a difference in circumference, and for that reason I would stick with what the door jam sticker says. It's entirely plausible that front tires heat up more than the rear, equalizing the issue a bit. Plus other factors might affect the actual circumference of the tire. And finally, the transfer case might be set up for a slightly larger diameter rear wheel - after all, more power goes to the back then the front.
Exactly, we have no idea how the transfer case is set up internally. I've read that xDrive is 60% rear and 40% front, so that makes sense.

I don't see how one can go wrong by sticking to the recommended value. See Garry's reply here: http://blog.tirerack.com/blog/make-d...run-flat-tires

Last edited by 4llr; 11-06-2016 at 09:00 AM..
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      11-13-2016, 10:36 PM   #9
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Personally, this is the first vehicle I've owned where I actually felt the factory recommended tire pressures work best. I stick w/ what the door placard says. This is for a 35i M Sport w/ 18" X-Ice 3's.
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