F2008-03-008
Dynamic Steering and Quattro with Sport Differential - Two Perfect Partners for Highest Agility and Active Safety
In accordance with the vehicle´s road speed, the new dynamic steering can vary its ratio by almost 100 percent. When the car is being parked, the steering is extremely direct, whereas when driving in a straight line at high motorway speeds a more indirect steering ratio and reduced power assistance help to maintain the car´s outstanding directional stability.
The dynamic steering system can also correct a small to moderate angle of float as rapidly and effectively as a rally driver could ever react, by automatically turning the wheels slightly in the opposite direction. Brake applications are normally only needed if the angle is greater, and then in most cases only to exert a damping effect.
The dynamic steering system uses a superimposed zero-play transmission to vary the effective steering ratio according to road speed. It is integrated into the steering column and combined with an electric motor. The axial transmission is extremely compact, light in weight and torsionally rigid. It operates without any play and is therefore extremely precise, with low friction. It can transmit very high torques and operates at a high level of efficiency.
A perfect partner for the dynamic steering system is the active sport differential. An innovative regulating system distributes torque from the engine in continuously variable proportions between the rear wheels. This greatly enhances agility, driving pleasure and active driving safety. The controlled power flow enables the car to take corners even more directly and responsively, and to retain its directional stability considerably longer. Understeer becomes a thing of the past.
Both when the steering wheel is turned and when accelerating in a corner, vehicles with conventional drive to the axles tend to understeer for as long as the tyres have adequate grip. The laws of physics cause a moving body to resist any change of direction initially.
When a vehicle´s steering is turned away from a straight line, the tyres and suspension pivots absorb some of the initial movement before the direction changes. When accelerating in a corner, the front axle load is reduced and therefore less lateral force can be transmitted to the tyres: the vehicle understeers and is forced out on to a larger-radius curve.
With the active sport differential driving the rear wheels, the car quite literally takes every corner as if on rails. Depending on steering angle, lateral acceleration, yaw angle, road speed and other signals, the control unit calculates the most suitable distribution of torque to the wheels for 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 wheels also exerts a steering effect, so that the usual corrections by the driver at the steering wheel are no longer needed. Understeer, that is to say the tendency for the car to run wide at the front, is completely eliminated.
Session: Vehicle Dynamics