During the press presentation this week of the 2008 Saab 9-3, they revealed their new Cross wheel drive system (XWD) that will be coming to your dealership in early 2008.
The presentation really got interesting when one of their product manager's (Andreas Andersson) who has extensive race and AWD experience, spewed out that the new system was more effective than Audi's quattro and beat a Porsche 911 Turbo through the cones, in the test runs!
He then showed us the final slides with the test data and then gave us all a taste on how the new system performs on a closed 'wet grass' course.
Saab XWD is a fully automatic, on-demand system capable of sending up to 100 percent of engine torque to the front or rear wheels. While offering sure-footed handling in low-grip conditions, its sophisticated operation also adds a further sporty dimension to the driving experience in all road conditions. Fine balancing of the drive torque between the front and rear axles raises the threshold at which ESP throttle and braking interventions are triggered, providing more scope for closer driver involvement.
The system is governed by its own electronic control unit, which functions in harmony with the engine, transmission and ABS/ESP control modules. The hardware 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). This incorporates a Torque Transfer Device (TTD) and an optional electronically-controlled Limited Slip Differential (eLSD). Both are wet, multi-plate clutch units from Haldex.
At take-off, the TTD is initially activated when the clutch plates are forced together under hydraulic pressure, thereby engaging the RDM. This pre-emptive function is a valuable improvement in current technology, which requires the detection of wheel slip before the TTD is activated. For the driver, the enhanced functionality gives maximum traction immediately for smooth, strong acceleration from rest without hesitation.
On the open road, drive torque is seamlessly and continuously varied between the axles by the control of a valve in the TTD, which increases or reduces the pressure on the wet clutch plates. When cornering, Saab XWD rewards the driver by providing enhanced, more finely balanced chassis dynamics. Data from the vehicle’s ABS/ESP sensors – measuring wheel speed, yaw rate and steering angle – is utilized and careful programming of Saab XWD enables the application of rear drive to balance oversteer and understeer characteristics, improving stability and road-holding.
In highway cruising conditions, when traction or optimum grip is not an issue, only 5 percent to 10 percent of engine torque is typically transmitted to the rear wheels. This helps provide the driver with a measure of greater stability, while helping save fuel.
The “icing on the cake” with Saab XWD is the eLSD option. In this segment of the market, it is the first application of an electronically-controlled, multi-plate limited-slip differential in combination with all-wheel-drive. It is installed alongside the RDM and operates via pressurized clutch plates on a principle similar to the larger TTD. In icy or wet split-friction conditions, for example, it uses input from the rear-wheel speed sensors and can transfer up to 40 percent of torque between the drive shafts, sending it to the wheel that has more grip.
The eLSD also gives the driver enhanced control when cornering hard or completing a high speed maneuver, such as a lane change, by momentarily applying more or less torque to either of the wheels to help the rear of the car more closely follow the direction of the front wheels. In this way, the eLSD can keep the car better balanced and more tightly controlled without requiring “outside” intervention from electronic stability aides.
Installation of Saab XWD includes the fitment of a new rear sub-frame to carry the RDM, revised rear suspension geometry and new wheel hubs for the drive shafts. The three-piece prop-shaft runs through two bearings with constant velocity joints for smooth running with minimal “wind-up.” Wheelbase and rear track dimensions are unaltered from front-wheel-drive models.
Take a look at the slides for yourself and let us know if you believe, that what he said could be true and it has obsoleted Audi's famous Quattro system.