December 10, 2003

Subaru brings rally car technology to the street
by Jim Kerr
Photos by Grant Yoxon

Some of the technology that makes Subaru all wheel drive vehicles handle so well comes from their many years of World Rally Car experience. Subaru has excelled in this form of racing and recently offered some of their hot technology to us in the form of the WRX STi. This rally-inspired street car features a 300 horsepower intercooled, turbocharged four cylinder engine, a six speed manual transmission and dramatic styling. Another important feature of this quick and fun to drive sports car is its DCCD driveline.

DCCD stands for Driver Controlled Centre Differential. The WRX STi used three differentials. The front axle is a cam type limited slip unit. The rear axle differential is a mechanical limited slip unit, and the third differential is located at the rear of the manual transmission and can be controlled by the driver. This centre differential can automatically vary the ratio of the power split between the front and the rear axles, or the split can be set by the driver with a switch on the console.

Click image to enlarge

With the DCCD selector switch set in Auto mode, the torque split between the front and rear is 65% front and 35% rear. This torque split ratio is a result of the gear ratio of the planetary gear set that makes up the centre differential. Because of the torque split in the differential and the balance of the car, the Subaru WRX STi tends to oversteer. According to Subaru, this car was "built for people who want to drive".

A manual selector switch on the console enables the driver to control the torque split. Set it at 100% and the 100 indicator on the dash display lights up. This represents fully locked which is actually a 50/50 torque split in the centre differential. Several other torque splits can be selected for different road conditions or car balance conditions the driver wishes, all the way down to 0% or completely unlocked.

Inside the DCCD housing, two cam plates with ramps between them and small balls in the ramps are located at the rear of the differential. The Subaru system energizes a computer-controlled magnetic clutch to vary the torque split. When the coil assembly is energized, one of the cam plates is slowed, causing the balls to roll up the ramps, forcing the planetary gear set forward. A multi-plate limited-slip clutch at the front end of the planetary gear set is forced together by the end thrust inside the unit, locking the parts of the planetary gear set together. With the planetary gear set locked, the torque split is 50% to the front and 50% to the rear.

Several inputs enable the computer to calculate the amount of torque split desired and apply the clutch. The main sensor is a lateral G sensor located in the centre of the car. Lateral acceleration or the cornering force of the car is balanced by varying the torque split. The throttle position is also monitored so the differential can be locked during acceleration. The stop light switch sends a signal to unlock the differential so the four channel ABS braking is optimized.

A parking brake switch signals the computer to unlock the differential, enabling drivers to do "handbrake turns" during rally-type driving. Finally, a rear differential temperature sensor is used so that if the rear differential overheats, the centre differential will unlock. During extended periods of driving aggressively with the centre differential locked, the load on the drivetrain can cause differentials to overheat, so the rear axle temperature is monitored as a safety factor.

Let's look at how the system responds automatically if the driver is aggressively navigating a hairpin corner. As the car enters the corner, the brakes are applied and the differential unlocks for optimum braking with the ABS system. As the driver turns into the corner, the differential provides a 65/35% front to rear torque split for optimum traction and continues to vary the torque split until the gripping point of the corner is reached. Then the differential locks up in response to throttle opening for straight line stability and acceleration.

DCCD is Subaru's most performance-oriented All-Wheel Drive system. It provides optimum traction, straight-line stability and enables the driver to adjust the car so it can match both the road conditions and the driver's needs. Rally inspired systems like this make the WRX STi a fun, responsive car to drive.

Jim Kerr is a master automotive mechanic and teaches automotive technology. He has been writing automotive articles for fifteen years for newspapers and magazines in Canada and the United States, and is a member of the Automotive Journalist's Association of Canada (AJAC).

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