Dynamic steering varies the steering ratio by up to 100 percent based on the driving speed, steering angle and selected mode in the Audi drive select handling system. The central component is superimposition gearing in the steering column, which is driven by an electric motor. Known as strain wave gearing, its construction is compact, lightweight and torsionally rigid. It is free of play, precise and exhibits low friction. The gearing can transmit tremendous torques extremely rapidly and achieves a high level of efficiency.
The strain wave gearing performs its task with just three key components. An electric motor turns an elliptical internal rotor, which deforms a thin-walled sun wheel via a ball bearing, which is connected to the steering input shaft. At the vertical axes of the ellipse it meshes with a hollow wheel that has a sprocket and acts on the steering output shaft. When the internal rotor turns, the large axis of the ellipse shifts, which brings it into the tooth engagement zone. Because the sun wheel has fewer teeth than the hollow wheel, the two exhibit a relative motion to one another – they are superimposed. The large gear ratio of the fast-running electric motor makes it possible to build up this ratio quickly and precisely.
At low driving speeds – in city traffic and in maneuvering – the dynamic steering operates very directly; all it takes is two full turns of the steering wheel to travel from end stop to end stop. The power steering boost is also high, making parking maneuvers very easy. On country roads, the directness of the steering response and steering power assist are reduced progressively. At fast expressway speeds, indirect gear ratios and low power assist are used to smooth out unsteady steering movements to enable impressive straight tracking.
Dynamic steering works closely with the electronic stabilization control program, ESC to achieve sporty handling and driving safety. If necessary, it counter-steers slightly; its slight interventions, most of which are unnoticeable to the driver, reduce understeer and oversteer due to load alterations in the vast majority of situations. When braking on road surfaces with split friction coefficients, the system helps by means of stabilizing steering interventions.
Dynamic steering takes less time for its corrections than the brake system needs to build up pressure at the wheels. In many situations, it handles the primary work – brake interventions either become unnecessary or they just serve a dampening function that reduces driving speed. The advantages in terms of driving safety and sportiness are especially noticeable at high speeds and on slippery surfaces such as snow.