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

four-wheel drive A four-wheel drive, also called 4×4 ("four by four") or 4WD, is 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 case pr ...

system developed by the

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

ese automobile manufacturer

Subaru is the automaker, automobile manufacturing division of Japanese transportation conglomerate (company), conglomerate Subaru Corporation (formerly known as Fuji Heavy Industries), the Automotive industry#By manufacturer, twenty-first largest aut ...

. The system consists of a longitudinally mounted

boxer engine A flat engine is a Internal combustion engine#Reciprocating engines, 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 ...

coupled to a symmetrical

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

with equal length half-

axle An axle or axletree is a central shaft for a rotation, rotating wheel and axle, wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to the vehicle, with the wheels rotating around the axle. In ...

s. The combination of the

symmetrical Symmetry () in everyday life refers to a sense of harmonious and beautiful proportion and balance. In mathematics, the term has a more precise definition and is usually used to refer to an object that is invariant under some transformations ...

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 barycenter or balance point) is the unique point at any given time where the weighted relative position of the distributed mass sums to zero. For ...

, improving the steering characteristics of the vehicle. 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 car Kei car is the smallest category of Japanese expressway-legal motor vehicles. The term ''kei'' is a shortening of , (kanji: ), which translates to English as "light vehicle" (). With restricted dimensions and engine specifications, owners ...

s as the exceptions.

History

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 The Subaru Leone is a compact car produced by the Japanese automaker, 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 Im ...

Wagon. In 1986, the automatic non-

turbocharged In an internal combustion engine, a turbocharger (also known as a turbo or a turbosupercharger) is a forced induction device that is powered by the flow of exhaust gases. It uses this energy to compress the intake air, forcing more air into the ...

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. 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 allows an output shaft to be disconnected from a rotating input shaft. The clutch's input shaft is typically attached to a motor, while the clutch's output shaft is connected to the mechanism that does th ...

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 called an engine control module (ECM), is a device that controls various subsystems of an internal combustion engine. Systems commonly controlled by an ECU include the fuel injection and ignition systems. The ...

and wheel speed sensors. Active torque split AWD is paired with 4-speed

automatic transmission An automatic transmission (AT) or automatic gearbox is a multi-speed transmission (mechanics), transmission used in motor vehicles that does not require any input from the driver to change forward gears under normal driving conditions. The 1904 ...

s and Subaru's chain-driven Lineartronic

continuously variable transmission A continuously variable transmission (CVT) is an automated Transmission (mechanical device), transmission that can change through a continuous range of gear ratios, typically resulting in better fuel economy in gasoline applications. This contr ...

(CVT).

Variable torque distribution AWD

The 1991 Subaru SVX was equipped with the first version of variable torque distribution (VTD) AWD with a normal torque split of 36% front, and 64% rear, later implementations of VTD use a torque split of 45% front, and 55% rear. The rear biased torque distribution reduces

understeer Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of the vehicle to changes in steering angle associated with changes in lateral acceleration. This sensitivity is defined for a level road for a given steady state ...

, 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 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 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