rear wheels trying to keep forward movement, front wheels fighting to stop

[Bdc101]

Originally Posted by oldskewel

The pre-xdrive E53 uses a NV125 transfer case. I'll say it is so different vs the xdrive one that discussing the xdrive here adds nothing other than confusion.

As said, the NV125 can be considered to work as an open differential does. That's really all you need to know.

It is designed to nominally operate with the front and rear drive shafts turning at the same speed; just as a differential is designed to have both left and right wheels turning at about the same speed. Due to the geometry of the gearing, there is a 38% front : 62% rear torque split. Pure mechanical geometry of the gears gives this split ratio. No clutches, circuits, computers, etc. It's like having levers of different lengths.

If you jack just the front, so the rear tires are on the ground and the front ones are in the air, it will work just like on a 2WD car when the driven wheels are jacked. For example, you can turn the left front wheel forwards, and the right front wheel will turn backwards at exactly the same rate and angle. So you cannot turn both front wheels forward at the same time.

The front differential enforces this, and the front driveshaft (from the TC to the front diff) is fixed due to the rear tires being on the ground. That front driveshaft cannot move because the rear tires are on the ground, so the rear driveshaft cannot move, and then the TC will not allow the front drivehaft to turn while the rear is fixed ... because it's just like an open differential.

This is all a carefully explained description that just says that the NV125 TC acts like an open differential. It really does.

The traction control on these things work with the purely mechanical TC+diffs by applying braking to the slipping wheel(s), sending torque to wheels that still have traction. So disabling that system would remove that as a possible source for the OP's problem.

Yes. The pre-X-drive E53 just has a planetary differential with no slip-limiting mechanism. Like you said, it can be considered identical to any other open differential. The sum of the front and rear driveshaft speeds will be twice the speed of the transfer case input shaft, at all times. The center diff may send more torque to the rear driveshaft, but they are mechanically geared together just like any other differential.



Likewise, if you were to disconnect the front driveshaft, the car would go nowhere. You would have to weld or otherwise lock the center diff.