From your Bimmerfest post:

"The Kumhos are 27.3" OD front and 27" OD rear. Could not find Revolutions per mile on this tire anywhere in the specs. The whine sounds like gear whine, short duration for a few seconds at a time and could be present at any given time and speed. Just from standstill or going 65 on the highway."

I'm no tire specialist nor XDrive expert but I think this might have been the problem. As the tires are not the same diameter it may be having an affect in the XDrive system because even going in a straight line the front and rear sets would be rotating at different rates (or Revolutions/Mile) all the time.

You also stated that when you put on identical Michlins the problem went away.

Maybe a BMW mechanic familiar with the XDrive system (say at your dealer) might be able to verify if that's the problem.

I took that into consideration when buying Pirellis. The 245/45/18 front, 275/40/18 rears are identical 26.7" in diameter and identical 780.4 revolutions per mile. Still the intermittent X-drive problem (noise) persists. All I can think of is that the tires are rotation at slightly higher rpm. About 2.7% higher that the car thinks they should be rotating.

The point I don't get is the assumption that the car thinks the tires should be going a certain rotational speed. The only way the car knows how fast the tires are going is by the wheel speed sensors. It doesn't have another reference signal, relating engine speed to true ground speed. There is usually a torque converter in the equation, so it isn't possible to just compare engine speed to driveline speed. You could have an issue with the torque converter locking up, separate from any problem in the transfer case.

The speed difference between different tires could absolutely cause a problem in the transfer case. I know of no factory or dealer programming for that, but I don't work for the factory or a dealer, so maybe it has just never come up.

I don't think the small differences you are quoting front/rear for rotational speed should cause a problem. You are assuming that the actual rolling diameter is as published by the tire manufacturers, and that may not be a good assumption.

If you want to check, it is very easy to drop the tires on one axle by 5 psi, test drive the car, then put those tires back to spec and drop the tire pressure on the other axle by 5 psi. Don't drive right to the handling limits when doing this, but see if the noise is gone in a straight line. You can easily achieve a different effective rolling diameter just by adjusting tire pressures. If the noise is worse in one direction, and goes away in the other direction, you have found the source of the problem.

It might just be me, but I would just put the stock tires back on and enjoy the car.

[quote=JCL]The point I don't get is the assumption that the car thinks the tires should be going a certain rotational speed. The only way the car knows how fast the tires are going is by the wheel speed sensors. It doesn't have another reference signal, relating engine speed to true ground speed. There is usually a torque converter in the equation, so it isn't possible to just compare engine speed to driveline speed. You could have an issue with the torque converter locking up, separate from any problem in the transfer case.quote]

There are accelerometers in the transmission that always know actual vehicle speed. When the abs sensor detects wheel speed that does not correspond to actual vehicle speed it goes into default. That is the only explanation I can come up with.

Sorry, I don't agree. Accelerometers measure acceleration, not speed. As I understand it, the ABS system works by comparing wheel speed sensors to each other, not to a reference signal.

Did you try changing the tire pressures either front or back to see if you could reproduce the noise, or alternatively, eliminate it?

Don't mean to give you a lesson in high school physics but velocity/time = acceleration.

Didn't try doing that yet. Thought about doing that already. Will try as the weather permits. But in reality probably impossible to match front to rear without measuring. Also the tires I have on now, Pirelli Pzero Nero MS are identical diameter and also identical Revolutions per mile specifications front to rear.

:rofl: :rofl:
Don't mean to give you a lesson in automotive mechanics and engineering, but since you asked for it....If you want to measure the speed of a rotating shaft with an accelerometer you will have to mount the accelerometer on the rotating component. How will you then take the signal off of it? Yes, you can get a wireless accelerometer, but that would be silly. The speed of a rotational shaft will best be measured with an optical or magnetic pickup that counts revolutions (revolutions/time is proportional to velocity). The neat thing about this method is that the sensor is mounted in a fixed position, and wiring isn't complicated by the rotating shaft.

This isn't about physics, or design. It is about automotive diagnosis and repair. The best and most suitable diagnostic aid you have available is a tire pressure gauge.

Cheers

:confused:

This is really all new stuff, for me...:rofl:

I feel like a caveman that just walked into a
college level cosmology class.

DoubleOhSeven, I'm with JCL: I think you are slightly
misinterpreting what your car "does", where it gets
the speed info from, and wringing your hands about
a situ that is not really tire size related.
But, that's just me.

Hope ya'll get it solved.
Good Luck, mD