Audi is expanding the reach of its torque-vectoring Sport Differential option, which is now available on A5 and S5 cabriolet models, as well as the recently introduced S4.
Torque-vectoring diffs have been around for a while, and the new system that has been introduced on the S4 will migrate to other performance models imminently, says Audi. If you want the clever rear diff on an S4 you’ll need to specify the £990 Drive Select system plus an extra £460 for the Sport Differential itself.
The Audi system uses electronically-controlled and hydraulically-actuated clutches to adjust torque to each rear wheel, and works with Audi’s front/rear torque-splitting quattro system to maximise directional stability and minimise understeer.
'With the new sport differential influencing drive to the rear wheels, the S4 exhibits exceptional traction and stability,' says Audi. 'Close to the car’s handling limits, it acts like ESP, but with the principle reversed: corrective movements are not initiated solely by altering the engine settings or applying the brakes, but also by controlled redistribution of tractive force. As a result the car’s progress is distinctly smoother and more free-flowing, since ESP comes into action much less frequently.'
For those of us without an engineering degree, Audi offers the following explanation of its system: 'Depending on steering angle, lateral acceleration, yaw angle, road speed and other signals, the car’s control unit calculates the most suitable distribution of torque to the wheels for
How the Sports Diff integrates
every driving situation. When the steering wheel is turned, for example, or the car accelerated in a corner, power is redirected in a controlled manner to the outer rear wheel. This has the effect of 'forcing' the car into the corner so that the angle of the front wheels is followed accurately. The difference in tractive force between the left and right rear wheels also exerts a steering effect, so that the usual steering corrections by the driver are no longer needed. As a result understeer, or the tendency for the car to run wide at the front, is to all intents and purposes eliminated.'