XWD, an acronym for Cross-Wheel Drive, is an advanced all-wheel drive system designed by Haldex in partnership with Saab. Also known as Haldex Generation 4, it is an intelligent permanent all-wheel drive system that can pre-emptively and continuously change torque distribution before wheel slip occurs.
To achieve optimum take-off performance, Saab XWD is capable of completely locking the front and rear axles, but can also adapt as little as 4% of torque to the rear wheels during highway cruising, thus maximizing fuel economy. To ensure immediate traction, torque distribution can be transferred to the wheel with the most grip in fractions of a second, and using the two couplings the XWD system can send 85% of available torque to a single rear wheel. The ability to transfer torque laterally between the rear wheels is similar to Mitsubishi's Super Active Yaw Control - (Mitsubishi's S-AYC can also control power to both left and right FRONT wheels as well).
An ECU continuously collects various data from the car's onboard systems, and in conjunction with the ESP, ABS and TCS calculates the best torque distribution in the driveline.
The XWD system debuted on the 2008 limited edition Saab Turbo X, equipped with an electronically controlled limited slip differential (eLSD) and was gradually made available through the rest of the 9-3 line (in sedan and combi body styles). XWD will be the standard drivetrain for the new generation Saab 9-5, expected as a 2010 model year vehicle and will also underpin Saab's upcoming cross-over utility vehicle, the 2011 Saab 9-4X, as well as the Saab 9-3X, an off-road modification of the 9-3 SportCombi. This XWD system is also used in Opel Insignia.
While the Haldex Generation 4 system will be made available to other automobile manufacturers starting 2009, the XWD moniker is a trademark owned by Saab Automobile AB, and thus cannot be used by any other make.