Active Yaw Control

Active Yaw Control, abbreviated AYC, is Mitsubishi's torque-vectoring system, most famously fitted to the high-performance Lancer Evolution rally-bred saloons. Its purpose is to sharpen a car's response when cornering by actively varying how much torque is sent to each of the rear wheels, encouraging the car to rotate into a bend and counteracting the natural tendency of a fast four-wheel-drive saloon to push wide, or understeer. It was a landmark system, among the first production applications of genuine torque vectoring, and it gave the Evolution a distinctive agility that helped define the car's reputation.

The name refers to yaw, the rotation of a vehicle about its vertical axis, the very motion that turns a car through a corner. AYC works by managing this yaw deliberately. Built into the rear differential is a hydraulically actuated clutch arrangement, controlled by an electronic unit that reads steering angle, throttle, lateral acceleration, yaw rate and individual wheel speeds. Based on these inputs the controller can send more torque to the outer rear wheel and less to the inner one, or vice versa, in real time as the car corners.

The effect of driving the outer rear wheel harder than the inner one is to generate a turning moment, a yaw moment, that helps swivel the car into the bend in the direction the driver wants to go. In practical terms this means the nose tucks in more keenly, the car follows the intended line with less steering effort and the chronic understeer that afflicts powerful all-wheel-drive cars is markedly reduced. The driver feels a car that turns in crisply and can put its power down out of a corner without washing wide.

AYC evolved over successive Evolution generations, beginning as a system that primarily redistributed torque side to side and later developing into the more sophisticated Super AYC, which used a planetary gear differential capable of transferring a greater proportion of torque and acting more decisively. It was typically integrated with the car's other chassis electronics, including the active centre differential and the braking-based stability and traction systems, to manage the distribution of drive front to rear as well as across the rear axle.

For all its brilliance, AYC was a complex and specialised piece of hardware. The hydraulic actuation and dedicated differential added cost, weight and maintenance demands, and the system was very much a halo feature rather than something destined for ordinary cars. Its calibration was finely tuned to the rally-derived character of the Evolution and was part of what made that car feel special.

Historically, AYC is regarded as an influential precursor to the torque-vectoring systems now common across performance cars, whether realised through clutch-based differentials, twin-clutch rear drive units or selective braking. It is closely related to the broader concept of torque vectoring, to the notion of yaw it so directly manipulates, to the electronic stability control that manages yaw by braking, and to the limited-slip differential whose traction benefits it extends with active intelligence.