A car with active driving functions that, in addition to the kind of ride comfort typically found in the luxury segment, sets new standards of driving dynamics and handling, plus a level of active safety that improves even further on the excellent standard already reached by the previous model - these were key development targets for the running gear of the new Audi A8.
The new A8 has an aluminium chassis with four-link front suspension and self-tracking trapezoidal-link rear suspension - both concepts that were already responsible for its predecessor's characteristic agility and high steering precision. The Audi suspension engineers have now developed its strengths with a specific goal in mind. The brakes and steering system with variable ratio and servotronic are a new design.
The adaptive air suspension - a 4-corner air suspension system with continuous damping control - is the first air suspension concept to enter the luxury class that resolves the classic conflict of aims between good handling and a level of ride comfort befitting a vehicle of this class. At the same time the lowering of the suspension at high speeds enhances the aerodynamic characteristics of the new Audi A8 and therefore also improves its economy.
The front suspension
The four-link front axle on the new A8 has been moved forward by 65 millimetres compared with the previous model, paving the way for a much more favourable axle load distribution with corresponding advantages in terms of handling.
Weight-optimised components help to reduce unsprung masses. In the case of the subframe, for example, a new material made it possible to use sheet-metal shells with a greatly reduced wall thickness. This resulted in a 20 percent reduction in weight in this area alone.
The entire kinematics and elastokinematics have also been redefined, for maximum dynamic behaviour and comfort.
The rear suspension
The trapezoidal-link rear suspension has also been advanced in terms of kinematics. A reduced bump toe-in across the entire spring travel enhances directional stability, steering precision and handling convenience. At the same time the comfort potential of this axle design on the new Audi A8 can be exploited even more effectively thanks to the air suspension system.
Lightweight materials were used specifically in the development of the axle components. As a result, the weight-saving for certain components is up to 40 percent compared with comparable steel constructions.
The A8 has rack-and-pinion steering with variable ratio and speed-dependent power assistance, servotronic, as standard equipment.
The variable ratio means above all an improvement in steering stability at high speeds without comfort having to be compromised at low speeds.
The advanced servotronic, a standard feature, enables the driver to manoeuvre into parking spaces with a minimum of steering effort and maximum accuracy, and also provides a clear response from the road at higher speeds. The system also offers a high level of sensitivity around the straight-ahead position and even more comfortable initial response behaviour.
The steering kinematics have been optimised in such a way that despite the longer wheelbase and driven front wheels, the car's turning circle is no larger than that of comparable saloon cars with rear-wheel drive.
The steering wheel in the new Audi A8 can be electrically adjusted vertically by 30 millimetres and axially by 50 millimetres. Intensive detail work in the area of the steering column and the point at which it is connected to the body makes sure that vibrations at the steering wheel are suppressed almost entirely and are practically no longer noticed by the driver.
Adaptive air suspension with continuously variable damping
Audi has adopted a new, independent approach in the luxury class by giving the new A8 air suspension as standard.
The air suspension's technology has already emphatically demonstrated its potential and versatility at Audi: since summer 2000, the allroad quattro has been fitted as standard with air suspension which combines the dynamism of a sports saloon with the distinctive off-road capability of a genuine off-road vehicle. The version used on the Audi A8 is nevertheless completely new, placing the emphasis clearly on dynamic driving.
The "adaptive air suspension", as this system is know, resolves the classic conflict of objectives between the handling qualities of a sports car and the suspension comfort of a luxury class vehicle.
In addition, the air suspension allows the speed-dependent lowering of the body - this change in ride height means a low centre of gravity and significantly increased directional stability as a result. At motorway speeds the lowering of the body also optimises the car's aerodynamic properties and thus reduces fuel consumption. At low speeds the adaptive air suspension establishes greater ground clearance if the driver so wishes. This is an additional bonus on uneven terrain.
Four different defined ground clearance stages are therefore available for the new Audi A8:
The components and their functions
This is what the adaptive air suspension, which entirely replaces the conventional steel suspension of the previous A8, consists of:
At all four wheels, the new A8 has air-suspension struts on which there are air-suspension bellows arranged concentrically around the continuously variable twin-tube shock absorbers. The air-suspension bellows consist of a special, multi-layered elastomer material with polyamide cord inserts to increase strength. This layer absorbs the forces produced in the air spring.
The advantage of this design is that the combination of individual layers enables the new A8 to achieve its excellent running characteristics and the sensitive response of the suspension in the event of minor impacts.
The suspension struts are pressurised by an electrically driven compressor in the engine compartment and a 5.8 litre pressure reservoir located at the rear of the vehicle with a maximum pressure of 16 bar. Plastic air lines connect the individual components; electromagnetic valves regulate the air flows.
The data from four sensors on the axles and three acceleration sensors on the body is evaluated in the adaptive air suspension's central control unit. This computer prompts the adjustment of the individual shock absorbers in a matter of milliseconds, based on the driving situation identified, thereby always assuring optimum driving dynamics and ride comfort.
Adaptive air suspension moreover offers the advantages of a traditional self-levelling suspension system. The vehicle's suspension height remains constant irrespective of the load it is carrying. This ensures that the Audi A8 is consistently presented to its best advantage.
Air suspension also has a system-specific quality: the occupants additionally benefit from a constantly agreeable vibrational behaviour, again irrespective of the car's load. This is because extra air is pumped into the springs at high gross weights, and discharged again when the load is removed. The firmness of the springs thus adapts to the load situation at all times.
Body movements effectively eliminated
The air suspension in conjunction with the electronically controlled, continuously adaptive damping system provides perfect driving comfort because the system responds to numerous different sensor signals: the acceleration of the body, the vertical movements of the four wheels and many other parameters too, such as the current steering angle and the position of the accelerator and brake pedals, are used to determine the optimum damping force for every driving situation.
Provided no higher damping forces are required - for instance when driving straight ahead on good roads - the damper settings remain comfortably soft. Specific adjustments to the damping force at individual wheels permanently eliminate body movements which could be detrimental to occupant comfort.
This configuration is beneficial both to occupant comfort and dynamic handling properties. When cornering, braking or moving away, the new Audi A8's adaptive damping automatically reduces rolling or pitching movements.
Dynamic response to order
The adaptive air suspension also allows the A8 driver to influence the suspension characteristic - and thus the operating dynamics - as individually preferred. The damping characteristics and ride height can be adjusted in just a single process via the "Car" menu of the MMI terminal.
The driver can choose between various driving modes.
Two other, less frequently required modes can be activated via a sub-menu of the MMI control system:
Distinctly dynamic, consistently versatile: adaptive air suspension - sport
In addition to the standard options, the "adaptive air suspension - sport" sports suspension is available for the A8. The control modes available have the same name as those offered with conventional adaptive air suspension, but have their own characteristics.
Here, the damper and suspension characteristics are noticeably firmer than in the dynamic mode of the A8's standard adaptive air suspension. The vehicle level is always 20 mm lower than the standard suspension.
Like the standard suspension, adaptive air suspension - sport has four driving modes.
In the comfort mode, the shock absorbers are actuated in such a way that damping forces are not increased as often in the lower speed range. This adds up to an impression of comfort that conventional sports suspension could never offer. In order to be able to exploit the full spring travel when driving at high motorway speeds, the body level is not lowered further even above 120 km/h.
In the dynamic mode, the spring and damping rates are higher than in the automatic mode; the vehicle's ride height does not alter. Finally, in the lift mode the body is raised by 25 millimetres and, at the same time, the spring/shock absorber characteristics are adjusted to the same level as in the automatic mode.