Saab XWD: Stretching the Performance Envelope

Saab XWD brings the car and driver closer together for an intimate driving experience. Utilising cutting-edge technology, it has been developed by Saab for its 9-3 Aero and Turbo X models, delivering a level of driver control with all-wheel-drive that promises to be ‘best in class’.

Saab XWD is an ‘intelligent’, active system that continuously distributes engine drive torque between the front and rear axles, giving optimum handling, stability and grip in all driving conditions – from fast bends to slower corners, in dry or wet weather.

Tuning of the chassis and the operating system’s software has been carried out by development engineers based at Trollhättan, the home of Saab in Sweden. In honing the integration and performance of Saab XWD, the team completed more than one million kilometres of testing in all weather conditions across some of the most challenging roads in Europe, as well as the Nurburgring circuit in Germany and test tracks in Italy and Spain.

The result is an active, all-wheel-drive system specifically tuned for the Saab 9-3 chassis. It fully exploits the power of an uprated, top-of-the-line, 2.8 V6 turbo engine delivering 280 hp and an impressive 400 Nm of torque.

To optimise traction at take-off, Saab XWD incorporates an innovative, pre-emptive engagement of the rear wheels. Unlike conventional all-wheel-drive systems, this eliminates the need to detect front wheel slip before rear drive is activated. It is all part of a system which Saab believes sets a new benchmark in all-wheel-drive performance

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Perfect Balance

On the open road, Saab XWD operates seamlessly and is programmed to help the driver keep the car stable, splitting drive torque to counteract oversteer or understeer characteristics when cornering.

The Saab XWD electronic control unit receives inputs 100 times every second from more than 20 sensors in the vehicle, including those for the ABS and ESC (Electronic Stability Control) systems. Data processing includes vehicle speed, wheel rotation speeds, lateral acceleration, throttle setting, yaw rate and the steering angle. The software, developed by the Saab XWD team, ensures that torque is continuously distributed between the axles for maximum grip and roadholding.

When exiting a bend under hard acceleration, for example, up to 80% of torque could be transferred to the rear wheels to keep the car balanced. However, in highway cruising conditions, when traction or optimum grip is not an issue, only 5% to 10% of engine torque is typically transmitted to the rear axle. This helps provide the driver with a measure of greater stability, as well as benefiting fuel economy.

While offering sure-footed handling in low-grip conditions, Saab XWD also rewards the enthusiastic driver. The fine balancing of the drive torque raises the threshold at which ESC throttle and/or braking interventions may be triggered, providing more scope for closer driving involvement.

The ultimate ‘icing on the cake’ with Saab XWD is the addition of an active rear limited-slip differential (eLSD), fitted as standard to the Turbo X Limited Edition. It is the first application of an electronically-controlled, rear limited-slip differential in this segment of the market.

The eLSD can transfer up to 50% of max rear torque between the rear wheels, to whichever has more grip. Under hard cornering, or when completing a high speed maneuver, such as a lane change, the brief application of more or less torque to either wheel helps the rear of the car more closely follow the direction of the front wheels.

The successful integration of Saab XWD has entailed completely new chassis settings and the adoption of a revised rear suspension geometry with self-leveling and a 50% increase in camber stiffness. Other alterations include re-tuning of springs, dampers and roll-bars in both the front and rear suspensions, as well as reworking of the steering system for good feedback. In order to handle the increased performance, the tires are also uprated.

“We have been fanatical in optimising the operation of Saab XWD,” says Anders Tysk, head of the development team. “In view of all our work, including comparative testing with other systems, we have no doubt that Saab XWD raises the bar for all-wheel-drive performance.”

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How It Works

The Saab XWD 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), which includes a Torque Transfer Device (TTD) and the optional eLSD. Both are wet, multi-plate clutch units from Haldex.

At take-off from rest, the TTD is initially activated when a gear is selected, thereby engaging the RDM. This pre-emptive function is a valuable improvement in current technology, which requires the detection of wheel slip before rear drive deployed. For the driver, the enhanced functionality gives maximum traction immediately for smooth, strong acceleration from rest without the possibility of any initial hesitation.

On the move, torque delivery between the axles is varied by the control of a valve in the TTD, which increases or reduces the hydraulic pressure on the wet clutch plates to progressively engage or disengage rear drive. The degree of ‘slip’ dictates how much drive is transmitted. The eLSD, located alongside the RDM, operates on the same principle as the larger TTD. The Saab XWD electronic control unit functions in harness with the engine, transmission and ABS/ESC control modules.

Installation of Saab XWD includes the fitment of a different rear sub-frame to carry the RDM, as well as the 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’. Standard wheelbase and rear track dimensions are unaltered.

Sales Start

Customer deliveries of Saab XWD begin in March 2008. The Turbo X Limited Edition showcases the launch of XWD and the rear eLSD, together with the uprated 2.8V6 engine and a choice of six-speed manual or automatic transmission (see separate release).

Saab XWD is also available for Saab 9-3 Aero Saloon and SportWagon models in combination with the same engine. The rear eLSD will be offered as an option for these models from the start of the next model year.

The successful development of Saab XWD is reflected in the appointment of the Technical Centre at Trollhättan as one of the centres of expertise within General Motors for the development of all-wheel-drive systems.

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