Winter 2009 Forward to a Friend

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SINCE 1997, EVERY SUBARU VEHICLE SOLD IN THE UNITED STATES HAS BEEN EQUIPPED WITH ALL-WHEEL DRIVE. TODAY, SUBARU SYMMETRICAL ALL-WHEEL DRIVE (AWD) IS ONE OF THE SUBARU CORE TECHNOLOGIES.

Subaru AWD

SUBARU SYMMETRICAL
ALL-WHEEL DRIVE

  • Boxer engine. Lightweight, compact, lower center of gravity
  • Linear power flow to all wheels
  • Powertrain balanced left to right
  • Drivetrain engineered as a single unit – not added on

Truck-Based 4-Wheel Drive

TRUCK-BASED 4-WHEEL DRIVE

  • Requires power to flow through several 90° turns
  • Extra parts, extra weight
  • Often part-time 4WD, requiring driver intervention

Front-Wheel Drive

ALL-WHEEL DRIVE ADAPTED FROM FRONT-WHEEL DRIVE

  • Requires power to flow through several 90° turns to the rear wheels
  • Often front-wheel drive until wheelspin is detected

Front-Wheel Drive

ALL-WHEEL DRIVE ADAPTED FROM REAR-WHEEL DRIVE

  • Requires power to flow through several 90° turns
  • Raised center of gravity, extra weight
  • Often rear-wheel drive until wheelspin is detected

In 1972, Subaru introduced its first AWD vehicles. During the intervening years, Subaru has continuously improved its AWD technology. Today, this technology enhances traction, control, and balance, contributing to vehicle safety as well as performance.

Giving the Subaru system an added edge is its symmetry. Subaru designs and engineers drivetrain components for balanced front-to-rear and side-to-side operation.

COMPONENTS THAT MAKE UP SYMMETRICAL AWD

Here is what comprises Subaru Symmetrical AWD, using Outback as an example. The main system components are the engine, transmission, transfer case, propeller shaft (driveshaft), and rear differential – all parts of the drivetrain.

Notice how the engine is laid out longitudinally – that is, in the direction of the length of the vehicle. Two axle shafts emerge from the transmission housing, one on either side. Otherwise, there are no external shafts driving the front wheels outside the transmission housing and transfer case.

SYMMETRY

The two sides of the Subaru layout are mirror images of each other. They’re balanced – symmetrical. However, despite the mechanically attractive appearance of Symmetrical AWD, Subaru didn’t engineer it for aesthetics. There are practical reasons for such a design.

Examination of other manufacturers’ AWD vehicles reveals that a good many of them have transversely mounted engines that necessitate changing the direction of torque from the engine by 90 degrees. These are primarily front-wheel-drive vehicles with rear-wheel drive added, usually as an option.

Even vehicles with longitudinally mounted engines tend to have external driveshafts for the front wheels, along with other components outside the centrally located powertrain.

The Subaru design is simpler and cleaner. It lends to a balanced, predictable driving experience and improved driving control.


Components that Make up Symmetrical All-Wheel Drive

A. The engine produces the power to turn the drive wheels.1
B. The transmission connects the rest of the drivetrain to the engine, determining direction as well as helping to determine speed.
C. The transfer case provides gearing to actuate the front and rear wheels via the front axles and propeller shaft to the rear.
D. The propeller shaft connects the power source to the rear differential.
E. The rear differential passes on the engine power to the rear axles, wheels, and tires.
1 To see how an engine works, go to the article “Active Valve Control System (AVCS)” in the Winter 2005 issue of Drive at www.drive.subaru.com.

ACHIEVING BALANCE

System Components

Starting with the SUBARU BOXER engine and all along the drivetrain, every part of the Symmetrical AWD system has efficiencies built into it that contribute to control.

The entire system lies along the centerline of the vehicle, balancing weight distribution between the two sides.

The engine’s horizontal rather than vertical shape helps to lower the vehicle’s center of gravity.

The self-contained transmission and transfer case are completely enclosed.

The propeller shaft has two sections – it’s not all one piece. That allows the engine/transmission/transfer case to be as low as possible without making ground clearance too low at the center of the vehicle. That helps to lower the center of gravity, which contributes to improved control.

Components are balanced side to side and front to rear. Also, the weight of the drivetrain components is spread out from front to rear. This prevents the vehicle from being excessively heavy in the nose or tail – once again, improving balance.

Engine

The SUBARU BOXER engine is itself a study in symmetry. Yes, other types of engines have to have a certain amount of balance or they’d shake themselves apart. However, the horizontally opposed layout of a Subaru engine has inherent advantages.

With cylinders directly opposite one another, piston movement is in a line all the way across. Power is distributed evenly on either side of the crankshaft.

In an in-line engine, all the cylinders are laid out on only one side of the crankshaft. Power drives against it from only one direction.

V-type engines have cylinders connected to the crankshaft from both sides of the V. However, instead of having the balance of direct opposition, piston movement in either bank tends to push the crankshaft off-center.

ENGINE TORQUE DISTRIBUTION – DIRECTING THE FLOW OF POWER

In an all-wheel-drive vehicle, engine power can be directed to all four wheels. Subaru Symmetrical AWD differs slightly from model to model in how it directs power to the wheels, depending on its transmission.

MODELS WITH FIVE-SPEED MANUAL TRANSMISSION – CONTINUOUS ALL-WHEEL DRIVE: A viscous-type locking center differential and limited-slip rear differential help distribute torque – normally configured at a 50/50 split front to rear. If wheel speed differs between front and rear axles, the center and/or rear differentials lock up to help distribute power to the wheels with the most traction.

MODELS WITH FOUR-SPEED AUTOMATIC TRANSMISSIONS – ACTIVE ALL-WHEEL DRIVE: An electronically controlled variable transfer clutch and limited-slip rear differential distribute power to where traction is needed. Sensors monitor parameters such as wheel slippage, throttle position, and braking to help determine torque distribution and direct it to the wheels with optimum traction.

MODELS WITH FIVE-SPEED AUTOMATIC TRANSMISSION – VARIABLE TORQUE DISTRIBUTION ALL-WHEEL DRIVE: As with Active All-Wheel Drive, an electronically controlled variable transfer clutch distributes power, but through a planetary-type center differential and a viscous-type limited-slip rear differential. Torque distribution is normally configured at a performance-oriented rear-wheel-biased 45/55 split front to rear. Sensors monitor the same parameters as for Active All-Wheel Drive.

WRX STI, WITH SIX-SPEED MANUAL TRANSMISSION – DRIVER CONTROLLED CENTER DIFFERENTIAL (DCCD) ALL-WHEEL DRIVE: The STI uses an electronically managed multi-plate transfer clutch and a mechanical limited-slip differential in conjunction with a planetary-gear-type center differential to control power distribution between the front and rear wheels. Featuring manual and three automatic modes, DCCD is normally configured at a 41/59 split front to rear. Sensors monitor parameters such as wheel slippage, steering angle, throttle position, and braking to help determine torque distribution and direct it to the wheels with optimum traction. DCCD also features a limited-slip helical front and Torsen® rear differential.

BALANCE FOR CONTROL

For safety sake, drivers don’t want any surprises from their vehicles when they’re on the road. Having a balanced vehicle that delivers power to the wheels with the greatest traction contributes to driving control. At the same time, this is the type of control that makes a Subaru fun to drive, which is demonstrated by the success that Subaru has had in rally and road-racing championships around the world.

Symmetrical AWD provides safe, predictable, fun-to-drive Subaru vehicles.