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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.
An ECU continuously collects various data from the car's onboard systems, and in conjunction with the ESC, 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 was to be the standard drivetrain for the 2010-2012 Saab 9-5, and would have underpinned Saab's upcoming cross-over utility vehicle, the 2011 Saab 9-4X, if it had reached production, as well as the Saab 9-3X, an XUV version of the 9-3 SportCombi. The XWD system is also used in the Opel Insignia.
While the Haldex Generation 4 system will be made available to other automobile manufacturers starting 2009, the XWD moniker was a trademark owned by Saab Automobile AB, and thus cannot be used by any other make. As of the bankruptcy of Saab Automobile AB in 2011, the XWD name will no longer be used in any production vehicle.