The Saab XWD "Cross Wheel Drive" system was developed in Sweden with Haldex of Stockholm and designed to provide optimum vehicle handling and stability in any type of driving conditions. The same system is also equipped in the 2008 Saab Turbo X and Aero version of the 9-3 Sport Sedan and SportCombi with high quality and offered in combination with an enhanced 280hp (206kW), 2.8 V6 turbo engine made 15 percent more powerful with maximum torque of 400Nm/295 lbs/ft.
The Saab XWD "Cross Wheel Drive" system is offered in a choice of six-speed manual or automatic transmissions. It also includes two unique features the pre-emptive engagement of the rear wheels that optimizes the traction at take-off and an active rear limited-slip differential (eLSD) that allows variable torque transfer between the rear wheels.
In addition, the XWD system is a fully-automatic, on-demand system that is capable of sending up to 100 percent of the engine torque to the front or the rear wheels whenever it is required. Aside from offering owners of Saab Turbo X and 9-3 Aero XWD with efficient handling in conditions where gripping power is required, it's highly sophisticated operation also helps in enhancing driving experience in various road conditions. The perfect balancing of the drive torque between the front and the rear wheels increases the limit that triggers the ESP throttle and braking interventions which in turn more options for driver involvement.
The system is controlled by an electronic control unit which functions in harmony with the engine, transmission, and ABS/ESP control modules. The hardware is consists of a Power Take-off Unit (PTU) in the front final-drive that transmits engine torque through a prop-shaft to the Rear Drive Module (RDM) that incorporates a Torque Transfer Device (TTD) and an electronically controlled Limited Slip Differential (eLSD)-both are wet multi-plate clutch units from Haldex.
When starting from rest, the TTD is activated when clutch plates are forced together under hydraulic pressure engaging the RDM---this is also considered as the pre-emptive function and its vital for the improvement in current technology which calls for the detection of wheel slip before the TTD is set off. The pre-emptive function also provides maximum traction for smooth and strong acceleration even when vehicle starts from rest.
Once on the road, drive torque remains flawless and varied between the axles controlled by the valve in the TTD. This increases or reduces the pressure on the wet clutch plates. The Saab XWD is simply remarkable when it comes to cornering since it provides a balanced and a more efficient chassis dynamics. Furthermore, the data from the vehicle's ABS/ESP sensors that measures wheel speed, steering angle and yaw rate together with highly sophisticated programming of the XWD enables the application of the rear drive that balance oversteer and understeer thereby improving stability and road grip.
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