Quote:
Originally Posted by issabmw
He is correct. The x-drive can shift completely to the front axles, in cases where the rears are stripped of traction.
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Wrong. He is quoting marketing. There is no center diff. There is only a clutch pack hanging off an uninterrupted rear drive shaft. So all that happens is you variably engage or disengage the front. The rear cannot be disengaged.
This has been discussed at length here, do a search. Look at a cutaway diagram of the transfer case, it is clear.
On ice where both the rears have absolutely zero traction, and the front have traction, technically in a limited physics sense no torque can go to the rears for drive purposes. The rears will still spin (torque is required for that) as they cannot be disengaged from the tranny. The xDrive cannot dynamically shift any more than 50% to the fronts in any kind of normal real world sense, all that happens is the front and rear shaft lock up and turn at the same speed.
This is BMW marketing trying to imply there is a center diff, as in better AWD systems, when there isn't one.
edit: Another way to look at this is, jack the rear wheels off the ground, start the car and voila! the fronts get all the power. The rears will turn at the same rate. That is not the same as having a true center diff capable of transmitting 50-100% to the front dynamically depending on traction. But BMW marketing wants you to believe it is.