Does anyone know if front tires with an overall diameter that is 1.4% smaller than that of the rears would cause xDrive to think that the front tires are slipping or loosing traction? Front tires on our 328Xi are 225/45 x 17 and 235/45 x17 on the back - there is a story there, but will spare you the details. I keep DSC activated, and there are no indications that it is trying to compensate for anything, but not so sure I would know if xDrive is getting active. Hope someone out there can help ...
it could, it could also cause damage to the drivetrain, I don't know if xdrive uses a transfer case but you can destroy the transfer case internals by running mismatched tires
I don't know what the tire rolling diameter tolerance range is of the various BMW x drive models but it's not unusual for there to be a 2% allowance for variations in tire wear and static loaded radius. BMW NA should be able to answer this question accurately?
That's actually one of the things I was trying to find out, to tell you the truth. I have read that the Haldex system used on Volvo's and I think on some Audi's tolerates up to a 5% speed differential before it starts to do anything. And that is one of the things that is supposed to be different about DTC as compared to DSC - when you engage DTC, you get a larger tolerance for speed differential between wheels before it starts to do it's thing - it allows more wheel spin before it starts to intervene. So apparently some degree of what could be interpreted as a spinning wheel and loss of traction is OK. I did try to call BMW NA about this, but the customer service guy I talked to said the technical people only interact with the dealers and that I should talk to the dealer about this. Which I did and was told "no worries". But I'm still uneasy - 'cause I can easily imagine the potential for drivetrain damage (as tiFreak suggested). While some degree of wheel speed differential is apparently OK, I don't know what the threshold is and whether a permanent condition might be harmful. I wonder if this is something one of the club's ombudsman could help with.
My e-mail and phone inquires to BMW NA have been similar where the CSR acts like they are from some other planet than earth and they have no technical clue. Obviously if the dealers had any clue they'd show you a TSB or other technical reference to support their statements. I doubt that this is a national secret so I'd expect a timely written response from BMW NA as you have a need to know to protect your investment in a BMW. Hopefully the club ombudsman can help?
I recall this topic coming up in a recent Roundel issue and the answer was for xDrive, the 4 wheels/tires have to be the exact same or there could be serious consequences.
The OP is trying to quantify what "exactly" means as there is no such thing as "exactly the same" when it comes to tire diameters new or after use. From an engineering perspective you must allow for a slight differential in tire speeds. The OP is trying to get an accurate answer to what the tolerance is on the tires. As an example new tires of the same size from different brands can have a different diameter of .5" or a different static loaded radius. Tire wear can easily vary this much also, particulalrly front to rear. BMW should be able to advise what the "exact" requirementreally is on tires for the xDrive.
I would hope BMW has taken into account the tolerances for 4 of the same tire but there must be a limit. And then having different tires just gives the dealer a reason to void a warranty on the drivetrain. Not sure if I'd be willing to play with that.
According to the experts at TireRack, the tires on xdrive cars can be different sizes from original equipment (but must be within 3% of original tire diameter), and they need to be within 1% (not to exceed) diameter size of each other to not cause problems. Here is a link to handy little calculator they sponsor on E90Post forum: http://www.e90post.com/forums/showthread.php?t=35769
