The Symmetrical All-Wheel Drive (also known as Symmetrical AWD or SAWD) is a full-time

four-wheel drive Four-wheel drive, also called 4×4 ("four by four") or 4WD, refers to a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer ca ...

system developed by the Japanese automobile manufacturer

Subaru ( or ; ) is the automobile manufacturing division of Japanese transportation conglomerate Subaru Corporation (formerly known as Fuji Heavy Industries), the twenty-first largest automaker by production worldwide in 2017. Subaru cars are ...

. The SAWD system consists of a longitudinally mounted

boxer engine A flat engine is a piston engine where the cylinders are located on either side of a central crankshaft. Flat engines are also known as horizontally opposed engines, however this is distinct from the less common opposed-piston engine design, wh ...

coupled to a symmetrical

drivetrain A drivetrain (also frequently spelled as drive train or sometimes drive-train) is the group of components that deliver mechanical power from the prime mover to the driven components. In automotive engineering, the drivetrain is the components o ...

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 In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...

, improving the steering characteristics of the vehicle. Ever since 1966, 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 car Kei car (or , kanji: , "light automobile", ), known variously outside Japan as Japanese city car or Japanese microcar, is the Japanese vehicle category for the smallest highway-legal passenger cars with restricted dimensions and engine capaci ...

s as the exceptions.

History

The earliest version of Subaru SAWD debuted in September 1972 as an optional part-time mechanical four-wheel drive system for the first-generation

Subaru Leone The Subaru Leone is a compact car produced by the Japanese car manufacturer Subaru from 1971 to 1994. The word ''leone'' is Italian for lion. It was released as a replacement for the Subaru 1000 and was the predecessor of the Subaru Impreza. Al ...

Wagon. In 1986, the automatic non-

turbocharged In an internal combustion engine, a turbocharger (often called a turbo) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake gas, forcing more air into the engine in order to pr ...

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 introduced in 1991, with the addition of

Vehicle Dynamics Control Electronic stability control (ESC), also referred to as electronic stability program (ESP) or dynamic stability control (DSC), is a computerized technology that improves a vehicle's stability by detecting and reducing loss of traction ( skiddi ...

(VDC) in 1998, allowing for better control of the all-wheel drive system. Modern implementations of SAWD are used in conjunction with VDC, ABS, and traction control for enhanced handling performance.

Variants

Active torque split AWD

First introduced in 1987 for the Subaru XT, the active torque split AWD variant uses an electronically controlled multi-plate transfer

clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...

for a default torque distribution of 60% front, 40% rear ratio. Torque distribution is adjusted up to a 50:50 split in real time with input from the throttle, transmission,

engine control unit An engine control unit (ECU), also commonly called an engine control module (ECM), is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. It does this by ...

and wheel speed sensors. Active torque split AWD is paired with 4-speed automatic transmissions and Subaru's chain-driven Lineartronic

continuously variable transmission A continuously variable transmission (CVT) is an automatic transmission that can change seamlessly through a continuous range of gear ratios. This contrasts with other transmissions that provide a limited number of gear ratios in fixed steps. ...

(CVT).

Variable torque distribution AWD

The 1991

Subaru SVX The Subaru Alcyone SVX, marketed outside Japan as the Subaru SVX, is a two-door, front-engine, all- or front-wheel drive coupé manufactured and marketed by Subaru from 1991 to 1996 over a single generation. Superseding the company's aviation-i ...

was equipped with the first version of variable torque distribution (VTD) AWD with a normal torque split of 36% front, 64% rear, later implementations of VTD use a torque split of 45% front, 55% rear. The rear biased torque distribution reduces

understeer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occ ...

, which occurs in AWD vehicles. VTD AWD uses a multi-plate clutch in conjunction with planetary gears in the center differential and a

viscous coupling A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. Design Rotary viscous couplings with interleaved, perforated plates and filled with viscous fluids are used in automotive system ...

rear differential to distribute torque up to a 50:50 ratio. With the addition of VDC, all torque can be sent to a single wheel.

Viscous centre differential AWD

Subaru vehicles equipped with manual transmissions use a viscous limited slip centre differential (VCD) AWD system, which by default distributes torque evenly between the front and rear axles.

Multi-mode Driver Control Centre Differential AWD

The

Subaru WRX STI The is a compact car that has been manufactured by the Japanese automaker Subaru since 1992. It was introduced as a replacement for the Leone, with the predecessor's EA series engines replaced by the new EJ series. It is now in its sixth gene ...

exclusively uses the Multi-mode Driver Control Centre Differential (DCCD) AWD system, it uses a torque sensing mechanical limited slip differential (LSD) with an electronic LSD to distribute torque with a 41% front and 59% rear split. Multi-mode DCCD AWD is able to operate in automatic mode or manual mode, where the driver is able to adjust the center differential lockup on the fly.

References

{{reflist Subaru Drivetrain Four-wheel-drive system tradenames