Active Transfer Torque System

The Active Transfer Torque System, or ATTS, is a Honda-developed front-axle differential that actively redistributes drive between the two front wheels to sharpen cornering. It was introduced in 1997 on the Japanese-market Prelude Type S and SiR, and it represented an early production attempt to address the inherent handling limitation of a powerful front-wheel-drive car: the tendency to understeer, or run wide, when power is applied through the same wheels that must also steer.

Unlike a conventional differential, which simply allows the two driven wheels to rotate at different speeds, ATTS can deliberately drive one wheel faster and with more torque than the other. It uses a planetary gear arrangement combined with hydraulically actuated clutch packs. When the system senses cornering, it engages the clutches to overspeed and over-drive the outer front wheel, which is the wheel travelling the greater distance and able to put down more power. The result is a yaw moment that helps rotate the car into the turn rather than letting it push wide.

For the driver of a quick front-wheel-drive coupe, the effect is a noticeably keener, more neutral turn-in and the ability to carry power through a bend without the nose washing out. By transferring up to around 80 per cent of the available torque to the outer wheel, ATTS counteracts the understeer that would otherwise dominate, giving the car a balance more usually associated with rear-driven machinery and allowing the chassis to be exploited more fully on a twisting road.

The system is best understood as a mechanical and hydraulic forebear of today's electronically governed torque vectoring. Where modern systems often achieve a similar end through brake intervention or compact electric actuators integrated with stability control, ATTS did it through dedicated gearing and clutches devoted solely to the front axle. Honda did not deploy it widely, and it remained something of a niche showcase, but the principle it demonstrated, that of actively biasing drive to the outer wheel to manage yaw, became central to a generation of performance differentials.

In practice the system added weight, complexity and cost to the front axle, and its hydraulic clutches represented additional components to maintain. Because it concentrated drive on the outer wheel, its benefits were most apparent under power in corners rather than in a straight line, and it did little for low-grip traction of the kind a limited-slip differential addresses. These factors, alongside the broader decline of the front-wheel-drive sports coupe, limited its longevity.

Conceptually ATTS sits within the family of torque-vectoring technologies and is related to the limited-slip differential, which tackles the simpler problem of wheelspin rather than active yaw control. As a special form of front differential it illustrates how the basic differential can be re-engineered to improve, rather than merely accommodate, the dynamics of a front-wheel-drive car.