Symmetrical All Wheel Drive
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The Symmetrical All-Wheel Drive (also known as Symmetrical AWD or SAWD) is a full-time four-wheel drive system developed by the Japanese automobile manufacturer Subaru. The system consists of a longitudinally mounted boxer engine coupled to a symmetrical drivetrain with equal length half-axles. The combination of the symmetrical layout with a flat engine and a transmission balanced over the front axle provides optimum weight distribution with low center of gravity, improving the steering characteristics of the vehicle.[1] Ever since 1986, most of the Subaru models sold in the international market are equipped with the SAWD system by default, with the rear wheel drive BRZ and kei cars as the exceptions.[2]
The earliest version of the Subaru SAWD debuted in September 1972 as an optional part-time mechanical four-wheel drive system for the first-generation Subaru Leone Wagon.[3] In 1986, the automatic non-turbocharged version of the Subaru XT was equipped with the first full-time SAWD system, with an electronically controlled version introduced in 1987. Variable Torque Distribution (VTD) AWD was first presented in 1991, with the addition of Vehicle Dynamics Control (VDC) in 1998, allowing for better control of the all-wheel drive system.[4] Modern implementations of SAWD are used in conjunction with VDC, ABS, and traction control for enhanced handling performance.