Saab XWD Cross Wheel Drive System

Saab XWD Cross Wheel Drive System

Saab, the Swedish automaker, has developed the XWD "Cross Wheel Drive" system, with intention to optimize vehicle handling and stability in all driving conditions. Currently, the system is being offered as an option in the 2008 Saab Turbo X and Aero versions of the 93 Sport Sedan and SportCombi. It is being partnered with a 280 hp (206 kW), 2.8 V6 turbo engine that produces almost 15 percent more maximum torque of 400 Nm/295 lbs.ft.), to enhance the available traction.

The allwheel drive system will be available with either a sixspeed manual or sixspeed automatic transmission. It will provide to impressive features, namely: preemptive engagement of the rear wheels to optimize traction at takeoff; and an active rear limitedslip differential (eLSD) to allow variable torque transfer between the rear wheels.

The Cross Wheel Drive is a fully automatic ondemand system. It could send up to 100 percent of engine torque towards the front or rear wheels when necessary. Its sophisticated operation adds further sporty dimension to the driving experience with the new Saab Turbo X and 93 Aero XWD. It produces fine balancing of the drive torque between the front and rear axles that raises the threshold where ESP throttle and braking interventions are triggered; thus, provides more scope for intimate driver involvement.

The stateofthe art allwheel drive system is being dominated by its own electronic control unit that works with the engine, transmission and ABS/ESP control modules. In the front finaldrive, the hardware comes with a Power Takeoff Unit (PTU), which transmits engine torque through a propshaft in the Rear Drive Module (RDM). It works with wet, multiplate clutch units from Haldex, namely a Torque Transfer Device (TTD) and an electronicallycontrolled Limited Slip Differential (eLSD).

In the process of takeoff, the TTD is being activated as the clutch plates are forced together under hydraulic pressure. In effect, the RDM becomes engaged. In such process, detection of wheel slip before the TTD activates is required, making it a significant improvement in the technology.

In open road cornering, Saab XWD offers enhanced and more finely balanced chassis dynamics. The data from the vehicle's ABS/ESP sensors are being utilized. And, the programming of the system permits the application of rear drive in balancing both oversteer and understeer characteristics to optimize stability and roadholding.

For highway cruising, there is only about 5 to 10 percent of the engine torque is basically used in the rear wheels to provide a measure of greater stability and enhancement of fuel consumption.

The eLSD holds the most significant role in Saab XWD. It is the first application of an electronicallycontrolled, rear limited slip differential in the segment. Along with RDM, the eLSD operates through pressurized clutch plates under a principle same as with the larger TTD.

The eLSD also provides the driver better control on hard cornering and high speed maneuver. It happens through the application of either more or less torque to the wheels, helping the rear follow the direction of the front wheels. Without requiring 'outside' intervention from electronic stability aides and quality Saab struts, the vehicle can be kept balanced and be tightly controlled.

Fitment of a new rear subframe, in the process of installing the Saab XWD, is required to carry the RDM, revised rear suspension geometry and new wheel hubs for the drive shafts. Wheelbase and rear track dimensions are kept unaltered.