xDrive

BMW xDrive Technology

BMW xDrive is the marketing name for the four-wheel drive system found on the BMW X1, X3, X5, and X6 crossover sport activity vehicles. It is also optional on the 3 Series (2000–present), 4 Series (2014), 5 Series (2005–present), 6 Series (2012-present), and 7 Series (2010–present).

Instead of a permanent torque split (which the earlier systems feature), xDrive provides variable torque split between the front and rear axles through the use of a multi-plate wet clutch located in the gearbox on the output to the front drive shaft. This setup allows xDrive to modulate the torque split between the front and the rear axles, which is normally split at 40:60 ratio. If wheel slip is detected by the ABS/DSC system, xDrive can react within a tenth of a second to redistribute up to 100% of the engine power to the front or rear axle. The wet clutch is applied through a high speed electric servo motor turning a cam shaped actuator disc.

xDrive is connected to the ABS and DSC systems. In the case that wheelspin or directional instability still occurs while xDrive is or has been modulating the torque split, DSC will brake independent wheels to regain traction and improve directional stability without driver intervention.

The front and rear differentials in xDrive vehicles are an open differential design, thus relying on brake application by the DSC system to transfer power from the slipping wheel to the wheel with traction.

BMW’s Dynamic Performance Control torque-vectoring system works in unison with xDrive all-wheel drive and Dynamic Stability Control. DPC is a drivetrain and chassis control system that works to regulate traction and especially correct over- and understeer by actively spreading out drive forces across the rear axle. Torque is split not only between the front and rear wheels (xDrive) but also from side to side at the rear for improved agility and added stability (through the DPC rear axle).[4] DPC works at all speeds and normal driving, not just during hard cornering, in order to steer the car. DPC also interfaces with DSC to initially try to correct the vehicle’s path using torque vectoring which results in smooth turns, whereas DSC by itself would only reduce engine power and apply the brakes to steer the vehicle.

The DPC differential features clutch packs on both output sides that are actuated by an electric motor. The clutch pack activates a planetary gearset which causes one wheel to be overdriven. A conventional control system will use the brakes to reduce the speed of the faster-moving wheel (which is the one with less traction) and reduce engine power. This leads to increased brake wear and slower than optimal progress. In contrast, the DPC system speeds up the slower moving wheel (the one with the most traction) in order to maintain stability when needed. For example, while turning, the outer wheel is overdriven to provide greater acceleration, using the traction advantage through the dynamic loading of the outboard wheel in cornering. Meanwhile, in an oversteer situation, the inner wheel is overdriven to regain traction balance.

The first application of DPC was introduced on the BMW X6, which began production in 2008 as a 2009 model, where it is a standard equipment.A test found that “the X6 feels hundreds of pounds lighter on its feet than an X5 [without DPC]” due to the benefit of DPC even though both vehicles have similar chassis dimensions. The BMW X5 M high performance variant also has standard DPC.