Active Traction Control, or A-TRAC, is a Toyota braking system available in several trucks and SUV models, including the 4Runner and FJ Cruiser. Its purpose is to help a vehicle keep moving in the event of a loss of traction. The feature is most useful in encounters with diverse road types or conditions, where one wheel on either axle may slip more than its counterpart at any given moment.
On an axle with an open differential, when one wheel loses traction and spins freely, all of the engine's torque is directed to that wheel (along the path of least resistance), robbing power from the other wheel when it is most necessary. Traction control mechanisms, such as limited slip differentials and A-TRAC-type systems, aim to remedy this condition. A limited slip differential, or LSD, independently allocates torque between the left and right wheels of an axle when it detects a loss of traction.
The A-TRAC system is designed to mimic the function of an LSD, without burdening the powertrain with the added weight and wear of additional mechanical parts. Whereas an LSD responds to traction loss by modulating power along the axle, the A-TRAC applies brake power to a single wheel in order to slow its rotation and produce the same effect. By reducing the disparity in rotational resistance between the two wheels of each axle, A-TRAC essentially tricks the open differential into sending power to the wheels that have maintained traction.
A-TRAC and LSDs both circumvent the biggest limitation of a traditional open differential. However, the A-TRAC model poses several advantages. The simplicity of its design----and the lack of additional moving parts----makes the A-TRAC easier and less expensive to maintain than some LSDs. A-TRAC reduces some of the steering "binding" and tracking difficulties that occur in drivetrains with LSDs engaged at low speeds. Whereas the clutch-type limited slip differential must be rebuilt periodically, the A-TRAC is designed to last the life of the vehicle. A-TRAC is also more user-friendly and allows drivers to easily negotiate off-road conditions without extensive knowledge of differential locking.
Active Traction has its own weaknesses. Unlike an LSD, the A-TRAC system cannot send a high percentage of engine torque to the side of the axle with traction. Because the effect is achieved by robbing energy from the free-spinning wheel in the form of heat from the brakes, it is a mechanically crude model and can only make the power ratio equitable enough to get the other wheels rolling. Furthermore, this design can accelerate brake pad wear if frequently used.
