Originally Posted by Nick Nazareno
Yes, I recall you posting that but I never saw the tech to back up your claim that E90 AWD doesn't require matched overall diameter of the tires.
Well, wait, now, let's be clear. I didn't mean to suggest that anyone should go mounting wheels of fundamentally different sizes in front vs. back. But what you wrote before was this:
Originally Posted by Nick Nazareno
This isn't just for a Xi BMW. it is for all AWD cars. you must stay within the same circumference/rotating diameter so the differentials do not suffer premature wear and failure.
And I thought you were suggesting something more than that the tires should be the same size. Specifically, based on the parts that I underlined, I thought you were saying that all awd systems require that all four wheels have identical circumfrence/rolling diameter at all times. What I meant was that different systems have different tolerances for variation between front and rear wheel circumfrence. I didn't mean to imply that the OP should go out and buy different sized tires for front and rear. Sorry if my post unclear on that point.
I'll give you an example using Subarus, because they have a few different types of AWD systems within their product line. Subies that use a viscous center coupling (which are generally the base models with 5MTs) require less than 1/4" diameter difference between front and rear wheels because the viscous coupling is designed to always maintain a 50:50 torque split, and its range of power distribution is limited. As Subaru explains:
"A synthetic viscous fluid is sealed in a housing with two sets of plates, one from each output shaft. Under normal driving conditions, both sets of plates and the viscous fluid spin at the same speed, but if one output shaft begins to move more quickly than the other (as would happen if one set of wheels began to lose traction), its set of plates begin to spin faster. This generates heat, causing the fluid to thicken, strengthening the adhesion between the two plates. The fluid tries to keep up with the faster plates, dragging the slower ones along with it. This balances the torque to both outputs, effectively “locking” both driveshafts at the same speed."
When one set of wheels is larger than the other, the viscous coupling gets stuck in a high-friction state longer than it should, and it will, in relatively short order, overheat and fail.
Subarus with a Variable Torque Distribution AWD system are different. The VTD system doesn't use a viscous center coupling- it uses an electronically controlled, heat-resistant multiplate center clutch pack (like xDrive), combined with a planetary gear set. In this type of system, the torque split is not always 50:50 - it's infinitely variable because the center clutch can be fully open, fully closed, or anywhere in between. When you put different sized tires on the front and rear wheels, what happens is simply that the clutch engages either more or less than usual, depending on whether the smaller wheels are on the front or rear axle. But because the clutch is more durable and made out of heat-resistant materials, it can tolerate more stress for a longer period of time than a viscous center coupling can. If you overload it for an extended period, it will still fail prematurely, but this type of system (which is very similar to xDrive) can tolerate a bigger difference in overall wheel diameter for a longer period of time than a viscous center coupling-based system can.
Now, that doeesn't mean that anyone should be running tires of fundamentally different sizes in the front and rear, just like you shouldn't run different size tires on the left and right. But it does mean that with a system like xDrive, you don't have to sweat the small stuff. You don't for example, have to buy 4 new tires when you get a flat at 5,000 miles. With some other AWD systems, you do have to think about that.
And the tech is in the BMW xDrive Dynamics manual. I'm not a BMW tech so I don't have my own copy. But I have seen it before. And I did stay at a Holiday Inn Express last night