SAWD: description, features, and operating principle

SAWD: description, features, and operating principle

Symmetrical AWD or SAWD is an all-wheel-drive system in Subaru cars that has been used since 2003. It is installed on all models of the make, except BRZ and kei car models. It improves steerability and stability of the vehicle on slippery surfaces. The system allows faster acceleration on dry roads and improves off-road capability.

How it works

The distinctive feature of SAWD is its symmetrical layout. The engine crankshaft, the gearbox, the propeller shaft, and the centre and axle differentials are all arranged along the longitudinal axis of the vehicle. This helps to lower its centre of gravity and ensures better weight distribution. This results in better vehicle handling and increased comfort for the driver and passengers.

SAWD is available in various designs. Versions with a manual gearbox are usually equipped with a centre differential with a bevel gear and a viscous coupling for locking. In conditions of equal traction on all wheels, the engine torque is distributed equally between the axles. The viscous coupling engages if one of them starts slipping and the majority of the torque is redistributed to the axle with better traction. Versions with more powerful engines feature a rear self-locking differential with a viscous coupling.

On cars equipped with an automatic transmission or CVT, operation of SAWD is performed with the help of a multi-plate clutch with a gerotor pump. It distributes torque between the axles according to the road conditions. Sometimes an electronically-controlled multi-plate clutch with positive locking is used for this.

SAWD features

Gearbox type Subaru models Features of the differentials
Manual Outback BL, BP, BR, BS; Impreza GR, CH, G3, GG; GJ, GP; Legacy BL, BP, BR, BM. The centre differential has a bevel gear. It locks with the help of a viscous coupling. Often there is a reduction gear. The rear differential can also have a lockup with the help of a viscous coupling.
Automatic Legacy BL, BP, BR, BM; Tribeca B9. An unsymmetrical differential with a planetary gear set and a plate-type clutch is used as a centre differential. If necessary, all the engine power can be supplied to one wheel thanks to the vehicle dynamics control system (VDC).
Lineartronic CVT Outback BL, BP, BR, BS; Impreza GR, CH, G3, GG; GJ, GP; Legacy BL, BP, BR, BM. A multi-plate clutch with electronic control is used for distribution of torque between axles instead of a centre differential. The axle differentials are of open type.


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