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4th gen. Haldex AWD system
For the new SAAB 9-3 XWD

Saab new 9-3 midsize car gets the new Haldex all-wheel-drive system, the Saab XWD - 'cross wheel drive' system.

XWD on Aero versions of the 9-3 Sport Sedan and SportCombi.

APN,
12 June 2007.

Swedish all-wheel-drive specialist Haldex systems launched the fourth generation of its limited slip coupling with the all new XWD electronic limited slip differential in the All-Wheel Drive versions of the new SAAB 9-3 model, introduced as the Saab 9-3 XWD for Cross Wheel Drive.

Employed in conjunction with the Haldex coupling, the electronically controlled add-on module integrates in the power and control systems of the longitudinal device, offering a flexible and cost effective solution for further enhanced vehicle dynamics. The add-on module can be optionally used in the vehicle product lines to control the side-to-side torque distribution, with limited weight and size for better performance and fuel economy.

“The cooperation with SAAB Automobile showcases our consistent and continued commitment to meet the needs of OEM customers through innovation. The Haldex XWD equips the driver with improved dynamic safety, broadens the performance scope of the vehicle and generates superior traction. The new business consolidates our leading position with European car manufacturers in the area of controllable and intelligent systems for All-Wheel Drive” says CEO and President of the Haldex Group, Joakim Olsson.

Haldex currently supply three of the major OEM groups, VW AG, Ford Motor Company and General Motors (owner of Saab Automobile AB, amongst other brands) with proprietary and advanced systems and solutions for AWD.

Launched to mark Saab's 60th anniversary, the new Saab 9-3 offers the active, all-wheel-drive system (Saab XWD) on Aero versions of the 9-3 Sport Sedan and SportCombi, in combination with an uprated 280 hp (206 kW), 2.8 V6 turbo engine which delivers almost 15% more maximum torque (400 Nm). It comes with a choice of six-speed manual or automatic transmission.

Manufactured and supplied from a new Haldex production facility in Guanajuato, Mexico, the new system includes two innovative features:

- pre-emptive engagement of the rear wheels to optimize traction at take-off; and,
- an active rear limited-slip differential (eLSD), allowing variable torque transfer between the rear wheels.

Saab XWD is a fully automatic, on-demand system capable of sending up to 100% of engine torque to the front or rear wheels whenever necessary. Its operation 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 harness 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 from rest, the TTD is initially activated when the clutch plates are forced together under hydraulic pressure, thereby engaging the RDM. This pre-emptive function is an 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 the possibility of any initial hesitation.

On the open road, drive torque is 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 provides 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 roadholding.

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 wheels. This helps provide the driver with a measure of greater stability, while helping to save fuel.

The eLSD option is the first application of an electronically-controlled, rear limited slip differential in this segment of the market. Installed alongside the RDM, it operates via pressurized clutch plates on a principle similar to the larger TTD. In icy or wet split-friction conditions, for example, it uses inputs from the rear wheel speed sensors and can transfer up to 40% of torque between the drive shafts, to whichever wheel has more grip.

The eLSD also improves 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. This yaw damping effect 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. Saab XWD will be available from the beginning of next year.

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