xDrive

[DinanX5]

Originally Posted by asawadude

The description of xDrive II that you cited is actually a description of xDrive I first introduced on the 2004 X3's and X5's...

BMW invented a new four-wheel drive system dubbed xDrive shared both in the X5 and X3 in 2004. Instead of using the previous X5 system which consisted of power being split 60-40 (rear wheels-front wheels) and DSC to brake wheels losing traction, xDrive could variably cut off power to the front or rear axles in a matter of milliseconds, while transferring 100% of the engine power to either, thus allowing the vehicle to regain traction quickly."

Yeah, you gotta love Wiki...

But the Wiki description you cite is exactly how I understood the current xDrive (and described it above)... it just stops short of explaining how it manages traction side-to-side (either within a given axle, or diagonally). If what I read previously is true, this is what's improved upon in xDrive II.

As for the DSC button... I JUST got back from a romp in the snow to/from the supermarket, and now I wish I'd checked this thread before I left so I could have tried the press-the-DSC button-for-10+ seconds-trick!!
But I have experimented a bit with just pressing the DSC button once to disengage the DSC, but retaining the DTC. My impression is that, on an xDrive vehicle, the DSC is much less invasive than it is on typical 3/5/7 rear drive models (certainly much less so than on my old 328Ci sport). So there's not a huge difference in 'normal' snow driving with it on or off... but when things get really slick, the DSC will cut engine power and engage ABS as it sees fit to stop what it perceives as an over-/under-steer loss of control. For this reason, in the snow I prefer to drive w/ DSC disengaged at all times (with the exception of high-speed highway travel) to allow for a little xDrive-controlled yaw play. The mix of just traction control - which only limits wheel spin - and xDrive is a phenomenal combination for the snow IMO.

But there are times - when things are REALLY deep and messy - when if you don't just let the wheels spin a bit and keep the vehicle creeping forward, you can still get stuck... as I understand it, this is because DTC is cutting wheel spin, so if none of the 4 wheels are getting sufficient traction, then... well, you get the idea.
Now, the part I'm not sure about is: If you hold down the DSC button for 10 seconds and disengage DTC as well, without traction control's influence, would the ABS still intervene to manage side-to-side wheel spin? (Probably... ) Or would the xDrive's eDiff lock-out and let the wheels spin unencumbered? [Not sure, but it sounds like this is what Audi's mechanical diff does (?)... and I think the Cayenne has a manual lock-out control to accomplish this.]
Most likely, I'd think that xDrive would still function in full, managing torque between axles, and calling upon ABS to manage wheel slip within a given axle.
But to be sure, someone should go out in the snow tonight and test it out!

At any rate, I think xDrive is a great balance between dry-road performance (where Audi's heavy mechanical system is losing ground to xDrive - and some of the other new eDiffs out there), and off-road traction. But if xDrive II will in fact add the capability of shifting torque from side to side as well (rather than limiting wheel slip with ABS) then it should be a nice improvement.

[B&DCalgary]

Originally Posted by DinanX5

But there are times - when things are REALLY deep and messy - when if you don't just let the wheels spin a bit and keep the vehicle creeping forward, you can still get stuck... as I understand it, this is because DTC is cutting wheel spin, so if none of the 4 wheels are getting sufficient traction, then... well, you get the idea.

You have a slight misperception about traction. Once a tire starts slipping (by spinning) you lose traction because the coefficient of friction changes. Slowing the rotation of the tire to prevent spinning actually maintains traction, and a non skidding tire will always provide better traction and better acceleration than a spinning tire, the same way that threshold braking will always stop a car faster than if you lock the wheels up. The only time spinning the tire might be an advantage is if the snow is so deep and soft that you can't get traction, then spinning the tires can sometimes burn down through the snow to the point where it hits pavement.

[DinanX5]

Originally Posted by B&DCalgary

You have a slight misperception about traction ... Slowing the rotation of the tire to prevent spinning actually maintains traction, and a non skidding tire will always provide better traction and better acceleration than a spinning tire ...
The only time spinning the tire might be an advantage is if the snow is so deep and soft that you can't get traction, then spinning the tires can sometimes burn down through the snow to the point where it hits pavement.

No mis-perception here - that's an example of a situation I had in mind... seen it happen several times. I've also (in my snow-tired 328Ci) had TC actually slow the vehicle to a stop when climbing an icy hill b/c the wheels could not find enough traction to keep them from spinning. After switching off DSC/DTC, from a dead stop, I was able to scoot right up and over the hill by keeping the wheels carefully and slowly spinning (like 2-3K RPM).
I'm aware of the physics of traction as well, but I've found that sometimes the real world deviates from academic theory.