Originally Posted by Kolyan2k
everyone and their moms know its staged. there are no rollers in real life. torque sensing diffs need friction to work. (not to mention other tricks that could possibly effect the results)
Oh? The q5 has a torsen center diff and a 'sport diff' on the rear axle (regular open+EDL on the front). In theory, it should be sufficient to immobilize the car that all but one rear wheel are free, since the car can't divert more than 80% (I think) to the rear, and of that, there's no full lockup on the rear diff to put it to the one wheel. I THINK that it's actually sufficient to have one wheel on each axle free.
On xdrive, no trouble. Computer senses all wheels spinning w.r.t. the one rear, fuly slides the clutch to the rear and brakes the free wheel to send 100% torque minus losses to the one good wheel.
The idea here is that BMW 'cheated' by picking one very specific situation in which xdrive is advantageous and focusing on it.