GKN Driveline adds to performance of BMW ‘M’ cars

As is the case with all BMW ‘M’ (Motorsport) models, BMW’s dynamic new M3 Coupe recently revealed at the Geneva motor show, is fitted with GKN Driveline’s Visco Lok limited slip differential (LSD). A completely new design and the first of a four-model line-up, the new BMW M3 Coupe has “embodied the athletic heart of the brand…”.

Visco Lok is a unique, speed-sensing, progressive-locking differential developed to optimise traction and handling on performance cars such as the new BMW M3. The benefits include all-round enhanced dynamic performance with improved cornering characteristics. The Visco Lok offers vehicle manufacturers a performance LSD that is compatible with ESP systems, but easy to integrate as it does not require an electronic control.

Graeme Walford, Managing Director of GKN Driveline’s Torque Technology Group: “ Visco Lok has a progressive locking characteristic that is tunable providing maximum torque transfers in excess of skid torque”. Continues Walford, "Compared to torque-sensing LSDs, this allows even better acceleration and less understeer in cornering, a feature particularly attractive to OEMs such as BMW M, seeking optimized, controlled performance from a passive LSD."

Visco Lok operation: Hydraulic pressure proportional to the speed difference between the two rear wheels is provided by an integral silicone-fluid filled reservoir and shear pump. In cornering, the speed differences between the wheels and the fluid-filled channel of the pump disc and resulting fluid shear forces, has the effect of moving the fluid from the reservoir to the apply piston. This hydraulic pressure creates an axial force via the apply piston compressing the clutch pack. The resulting locking torque through the clutch pack transfers torque to the wheel with the greater traction. There is also a variable volume reservoir to compensate for thermal expansion of the fluid.

To enable bi-directional operation, the pump disc automatically indexes to access the reservoir. The steel/Molybdenum clutch pack is lubricated by the axle hypoid gear oil while tuning is achieved by changing silicone fluid viscosity and the size of the clutch pack.