The xDrive all-wheel drive system is a development of the BMW Group and belongs to the permanent all-wheel drive systems. The system provides infinitely variable, continuous and variable torque distribution between the front and rear axles depending on driving conditions. Currently, the xDrive system is installed in sport off-road vehicles (SAV, Sports Activity Vehicle) X1, X3, X5, X6 and passenger cars of the 3, 5 and 7 series.
The history of all-wheel drive development by BMW includes four generations:
Generation 1, since 1985 – torque distribution between the axles during normal driving in a ratio of 37:63 (37% to the front axle, 63% to the rear axle), inter-axle differential locking, rear inter-axle differential with a viscous clutch (viscocoupler)
2nd generation, since 1991 – torque distribution between axles under normal driving conditions in the ratio 36:64, locking the inter-axle differential with a multi-disc clutch with electromagnetic control, locking the rear interwheel differential with a multi-disc clutch with electrohydraulic control, the possibility of redistribution of torque between axles (wheels) in the range from 0 to 100%
Generation 3, since 1999 – torque distribution between axles under normal driving conditions in the ratio of 38:62, free-type axle and inter-axle differentials, electronic locking of inter-axle differentials, cooperation with the system of dynamic directional stability control
4th generation, since 2003 – torque distribution between axles during normal driving in the ratio 40:60, the function of the inter-axle differential is performed by a multi-disk frictional clutch with electronic control, the ability to redistribute torque between the axles in the range from 0 to 100%, inner wheel differentials electronic lock, interaction with the system of dynamic stability control
The xDrive all-wheel drive system is based on BMW’s traditional rear-wheel drive transmission system. The distribution of torque between the axles is carried out using a transfer case, which is a cogged front axle drive controlled by a friction clutch. In the transmission of sports SUVs, a chain transmission is used instead of a cogged transmission.
The xDrive system is integrated with Dynamic Stability Control (DSC). In addition to the electronic differential lock, the DSC system integrates DTC (Dynamic Traction Control), HDC (Hill Descent Control) and other systems.
The xDrive and DSC systems interact with the ICM (Integrated Chassis Management) chassis management system. The ICM system also provides connections to the AFS (Active Front Steering) system.
How the system works
The drive all-wheel-drive system operates in several distinctive modes determined by the friction clutch actuation algorithm:
- starting from a place;
- cornering with oversteer;
- understeer; understeer;
- slippery driving;
On start-up under normal conditions, the friction clutch is closed and torque is distributed over the axles at a 40:60 ratio to give maximum traction during acceleration. When reaching 20km/h the torque distribution between the axles is carried out depending on the road conditions.
When cornering with oversteer (the rear axle skids to the outside of the curve), the friction clutch closes with greater force, and more torque is applied to the front axle. If necessary, the DSC system is activated to stabilize the vehicle by braking the wheels.
When cornering understeer (the front axle is drifted to the outside of the curve), the friction clutch opens and up to 100% of the torque is sent to the rear axle. If necessary, the DSC system is activated.
When driving on slippery surfaces (ice, snow, water), slippage of individual wheels is prevented by locking the friction clutch and, if necessary, the electronic inter-axle locking system DSC.
When parked, the friction clutch opens completely and the vehicle becomes rear-wheel drive, thus reducing stresses in the transmission and steering.