[Tengocity]

Originally Posted by HighlandPete

There is a big difference in slow speed control, than decelerating from high speed.

We've used the descent control (hill assist) in my son's X3, seen its ability on snow, on a steep drive up to the family croft. Clearly a hand's off approach, ABS/DSC is in control, you can clearly see what it's doing, if you stand and watch.

What is beyond dispute about xDrive, it is regulating drive torque (forward traction) to the appropriate wheels on acceleration. I've trawled through all the xDrive data I have access to, and never found specifics on how the system works on the over-run, or engine braking. The critical bit will be how the transfer clutch works and is regulated, to balance the grip between the axles during braking, assisting individual wheel grip levels. No mention of this in BMW technical data. If the clutch opens, (as it does in some situations, like slow speed tight cornering), then all bets are off to how xDrive assists deceleration. What we do know ABS takes control under slip conditions during braking.

We also know, braking alone can use all the grip available, doesn't require engine braking to stop the vehicle. OK, there are times when engine braking retards the car, but "what axles are doing what?" in xDrive vehicles. With RWD we know it is the rear that is using engine braking, leaves the front tyres for modulating the speed of deceleration using braking, or in other words, under the driver's command and control.

An example is in snow. I'm suspicious of how xDrive assists stopping through engine braking. RWD it is clear, engine braking gives control of the rear, separate from the brake pedal. Can allow sensitive modulation of the brakes to retard and control front end slip. You know you have two control systems, either separate, or together, working under the driver's control.

If anyone has reference to how xDrive 'transfer' works under braking... if it does... most interested to understand its function.

And not to forget that most of the xdrive cars are diesel, which has much less engine braking than a petrol engine.