xDrive: Variable Torque Split with Quick Reflexes

Both the new X3 and X5 use an innovative all-wheel drive system called xDrive.

Detailed cutaway of the xDrive system

Rich Brekus, manager of product planning and strategy, says, “Most companies show you how their four-wheel-drive systems go through the mud. We show you how well ours goes around corners.” With xDrive, driving torque is always transmitted to the rear wheels, while the portion sent to the fronts is controlled by a multi-disc clutch that can be fully open, fully engaged or at any level in between. Engagement pressure is applied by a servomotor through a linkage of levers, a ball ramp and a disc cam, directed by an electronic control system receiving inputs from the DSC via sensors for the rotational speed of each wheel, steering angle, vehicle yaw, lateral acceleration and the brakelight switch.

In normal driving, the split ranges from a nominal 40/60 to a maximum of 50/50 front-to-rear. In tight curves (including parking maneuvers) with low to medium torque, during heavy understeer and when antilock braking is active, the system is 100 percent rear-drive. Conversely, the rear proportion approaches zero when both rear wheels have near-zero traction. The DSC’s traction control can reduce engine torque and apply individual wheel brakes to achieve maximum possible force to move the vehicle.

Transparently to the driver, xDrive also optimizes handling agility and stability on any surface by adjusting the torque split to correct for understeer or oversteer by directing more torque to the non-slipping wheels. It also increases torque to the rears under heavy acceleration and decreases it for light acceleration. All this happens so quickly (reaction time from sensing to control actions) that the system almost operates pro-actively, BMW says.

The system operates seamlessly. We drove an X3 on a stretch of undulating, twisty mountain roads outside of Carefree, Ariz. When the vehicle was pitched hard into a tight corner, the expected understeer wasn’t there.

The vehicle also showed remarkable balance while driving on loose gravel roads, correcting steering input and keeping the vehicle going in a straight line.

While xDrive can handle torque split front to rear, side to side duties are handled by the DSC which is electronically linked to xDrive. The xDrive system is manufactured by Magna Steyr and is available on both the X3 and X5. Though no official announcements have been made, BMW engineers say that the system is easily adaptable to any vehicle in the BMW line-up.