Subaru Enthusiasts :: Tech Stuff

All Wheel Drive - The Beauty and The Beast

We often refer to the All-Wheel Drive (AWD) capabilities of a Subaru, but don't specifically mention the type of AWD system used. Because of this, many Subaru owners assume that the same All-Wheel Drive system is used in all Subaru models. In fact, two different types of Subaru AWD systems are available across the model line. The system type is dependent upon which transmission the vehicle has, manual or automatic. Although the two transmission and All -Wheel Drive types are different, both systems automatically distribute power to all four wheels as needed to maintain traction. For example, if the front wheels start to slip, power is instantly directed to the rear wheels, and if the rear wheels start to slip, power is instantly directed to the front wheels. When a vehicle is braking or accelerating, it causes the weight to shift, thereby reducing traction. Subaru AWD transfers power from the wheels that slip to the wheels that grip.

Viscous Coupling Locking Center Differential AWD For 5-Speed Manual Transmissions

In vehicles with the 5-speed manual transmission, the All-Wheel Drive uses a viscous coupling in a center differential inside the transaxle case. It contains a series of opposing discs attached to the front and rear output shafts, surrounded by a silicone fluid. In normal operation, power is distributed equally between the front and rear wheels (50/50 power split). Loss of traction at either the front or rear wheels causes a rotational difference between the front and rear discs in the viscous unit, which then shears the silicone fluid.

The shearing action heats the fluid, causing it to thicken. As the fluid thickens, the discs lock together to transfer power from the slipping wheels to the wheels with the best traction. When traction is regained, all the discs turn at the same speed, restoring the 50/50 power split. The process is quick and unnoticeable to the driver and passengers.

The 5-speed All-Wheel Drive system is simple, compact and virtually invisible in operation. Its traction adds a significant margin of safety on all road surfaces.

What Is A Differential ?

A differential is the driving-axle gear assembly located in the center housing between the driving wheels on rear-wheel drive vehicles, or as part of the transaxle on front-wheel drive vehicles or between front and rear differentials on all-wheel drive vehicles. The differential transmits power to the wheels while allowing each wheel to rotate at different speeds, such as when turning a corner.

Active Power Split AWD For 4-Speed Automatic Transmissions

Subaru vehicles equipped with the 4-speed Electronic Automatic Transmission (4EAT) feature a different type of All-Wheel Drive than with the 5-speed manual transmission. Instead of a viscous coupling center differential, an automatic transmission-equipped Subaru features an electronically managed continuously variable multi-plate transfer clutch located in the transaxle's tailshaft. Power transfer is governed by slippage in the clutch plates, which use a special friction material that easily handles the loads generated during power transfer.

The Transmission Control Module (TCM) controls the All-Wheel Drive multi-plate clutch. Under normal driving conditions, the power split is biased toward the front wheels. The active All-Wheel Drive system can adjust the power split in an instant, depending on many input factors. If the front wheels begin to lose traction, the TCM increases hydraulic pressure on the clutch, reducing slippage of the plates to transfer power to the rear wheels. As the front wheels regain traction, the TCM reduces pressure on the clutch, increasing slippage of the plates and transferring power to the front. The TCM monitors inputs from speed sensors on the front and rear output shafts and from the throttle position and transmission shifter position sensors. These factors cause the TCM to select a software "map" that determines how aggressively the TCM adjusts the power split between the front and rear wheels.

Viscous Limited-Slip - Rear Differential

For even greater traction capability, select Subaru vehicles feature a viscous limited-slip rear differential. If one rear wheel starts to lose traction, the differential automatically transfers the power to the other wheel. A viscous limited-slip differential not only helps traction in very slippery on- and off-road conditions, but it's also a cornering aid. With some vehicles, tight cornering can cause the inside rear wheel to lift, which diminishes traction and can compromise safety. The viscous limited-slip differential automatically transfers power to the rear wheel with the best traction, providing improved cornering and superior stability.

Benefits Of Subaru - All-Wheel Drive

Watch our site for future for more information about the third Subaru All-Wheel Drive system.