Principle of Operation: Pre-control

The pre-control logic (shared from DSC) reflects the driver's command and is calculated based on:

• Accelerator Pedal Value
• Engine Torque
• Engine RPM

In normal driving, the clutch is operated with minimum slip so that permanent four-wheel drive with a driving torque distribution of 40 percent on the front axle and 60 percent on the rear axle is available. Even when the traction for the front and rear axles is dramatically different, the pre-control ensures that the system responds very quickly.

In the case of the open transfer case, the brake is applied after slip is detected on the rear axle. This takes approximately one half of a second in reaction time. Sixty-two percent of the driving torque is supported on the two rear brake discs and only 38 percent of the driving torque can be transferred to the front axle. In other words, wheel slip must be sensed first before driving torque is transferred through the transfer case by applying the rear wheel brakes. In contrast to an "open" transfer case (differential), the xDrive does not require brake intervention on the rear axle because no slip can occur (permanent through connection). The transfer case clutch is engaging the front axle as the vehicle is accelerating. This takes significantly less time (approximately 1/10th of a second).