Audi A8 features new dynamic all-wheel steering, AI active suspension

Audi A8 features new dynamic all-wheel steering, AI active suspension

17 July 2017

Audi’s new A8 is a technology showcase for the brand, from powertrains (earlier post) and materials (earlier post) to autonomous driving and advanced driver assistance systems (earlier post).

The A8 also features two new technologies for the suspension. New dynamic all-wheel steering combines direct, sporty steering with unshakable stability. The steering ratio for the front wheels varies as a function of speed; the rear wheels are turned in or against the direction of steering depending on the speed range. The car’s handling becomes even more dynamic and precise with the sport differential. This actively distributes the drive torque between the rear wheels, complementing the quattro permanent all-wheel drive that is now standard in the new A8.

The second new technology, Audi AI active suspension, is a fully active suspension system. (Earlier post.) Depending on the driver’s wishes and the driving situation, it is capable of raising or lowering each wheel separately with electric actuators.

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This flexibility imparts the driving characteristic with enormous latitude. In combination with pre sense 360°, the car is quickly raised if there is an impending lateral collision, reducing the potential consequences of the accident for all occupants. This highly innovative suspension system obtains the energy it requires from the 48-volt electrical system. In conjunction with the advanced air suspension for the A8, the innovative suspension concept delivers a new driving experience.

The suspension. The front and rear axle, both of them mounted on subframes, take the form of high-precision five-link constructions made extensively from lightweight aluminum. Both the progressive steering, which becomes more direct the further it is turned, and the adaptive air suspension with controlled damping are standard features.

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The driver can define the air suspension settings using the three Audi drive select driving profiles comfort, auto and dynamic, and also in a separate lift mode. From a speed of 120 km/h (74.6 mph), the body is automatically lowered by 20 millimeters (0.8 in) for improved aerodynamics. For this to take place, the car must have been running in the auto or dynamic mode continuously for at least 30 seconds.

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The Audi AI active suspension will be available from market launch as an option for the V6 TFSI and the two V8 versions; it is a standard feature on the W12. Audi AI active suspension is a fully active, electromechanically operated suspension system. For each wheel there is one electric motor supplied by the 48-volt primary electrical system. In each case there is also a transmission, a rotary tube housing a titanium roll bar and a lever that exerts up to 1,100 newton-meters (811.3 lb-ft) on the suspension via a coupling rod.

A new feature in active suspension is that the load can be increased or reduced individually on all four wheels, for active control of the body in every driving situation.

This introduces a new dimension of flexibility: If the driver selects the dynamic mode in the Audi drive select system, the new A8 becomes an agile sports car —it turns in firmly, and its roll angles are roughly halved compared with the normal status. By contrast, in comfort mode, the body floats smoothly even over rough bumps in the road.

As well as improving comfort and handling, active suspension in combination with crossing assist improves passive safety in the event of a lateral collision. In an impending lateral collision at more than 25 km/h (15.5 mph), the suspension actuators raise the body on the side exposed to the danger by up to 80 millimeters (3.1 in) within half a second. The aim is to present an even more resistant section of the body to the other party by way of an impact zone. The side sills and floor structure accommodate a large portion of the impact forces.

Deformation of the cabin and the loads acting on the occupants, above all to the chest and abdomen areas, can be reduced by up to 50% compared with a lateral collision with no raising of the suspension. The pre sense 360° system is a requirement for the new function.

The electronic suspension platform (ESP). The second generation of the ECP features in the A8. It brings together all signals and data relating to the suspension in a single control unit and offers a new function with the integral driving dynamics controller. It influences the dynamic all-wheel steering, sport differential and active suspension— a new level of integration in the luxury class.

The sensors also detect the driving status, such as understeering or oversteering, and the road surface friction. The control unit uses the calculated values to adjust the above components as well as the damper control and air suspension. The customer experiences this in the form of optimum cornering behavior, improved transverse dynamics and ideal ride comfort. The combined algorithms allow faster, more in-depth coordination of the individual systems.

Dynamic all-wheel steering. Optional dynamic all-wheel steering is also controlled by the ESP. The new steering system approaches the steer angle and directional stability as separate issues.

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The optional dynamic all-wheel steering combines two distinct high-tech steering systems. The dynamic steering on the front axle incorporates what is known as strain wave gearing in which the ratio changes as a function of speed. The rear wheels are turned by a spindle drive and two track rods—at low speeds by up to five degrees against the direction of steering, to facilitate maneuvering and reduce the turning circle by about one meter to 11.4 meters (37.4 ft) on the A8 or 11.8 meters (38.7 ft) on the A8 L.

At medium and high speeds, the wheels are turned by up to two degrees in the direction of steering, for increased stability. Dynamic all-wheel steering matches the steering ratio optimally to the driving situation, adjusting the ratio across a range of 9.5 to 17. At the limits of driving dynamics or on surfaces that are slippery under one side of the car, dynamic all-wheel steering again stabilizes the new A8 by manipulating the steering at the front and rear axles.

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