Saab Xwd "cross Wheel Drive" System

By: Evander Klum

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 9-3 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 all-wheel drive system will be available with either a six-speed manual or six-speed automatic transmission. It will provide to impressive features, namely: pre-emptive engagement of the rear wheels to optimize traction at take-off; and an active rear limited-slip differential (eLSD) to allow variable torque transfer between the rear wheels.

The Cross Wheel Drive is a fully automatic on-demand 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 9-3 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 state-of-the art all-wheel 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 final-drive, the hardware comes with a Power Take-off Unit (PTU), which transmits engine torque through a prop-shaft in the Rear Drive Module (RDM). It works with wet, multi-plate clutch units from Haldex, namely a Torque Transfer Device (TTD) and an electronically-controlled Limited Slip Differential (eLSD).

In the process of take-off, 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 electronically-controlled, 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 , the vehicle can be kept balanced and be tightly controlled.

Fitment of a new rear sub-frame, 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.

Currently, Saab, as a division of General Motors Corp. Saab Automobile USA, is providing premium automobiles for Saab Automobile AB, Sweden.

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