The aluminium body
The innovative aluminium body, based on a further developed version of the Audi Space Frame ASF, provides the high-strength basis for the excellent performance and handling of the new Audi A8.
Compared with the previous A8 generation, the number of body components in particular has been significantly reduced by using more functional large castings and extruded sections. The new A8 in addition has a fully enclosed space frame. The most important result of this approach is that the static torsional rigidity, a critical measure of both vibrational comfort and dynamic potential, is around 60 percent higher than on the predecessor model.
The new A8 has the lightest bodyshell in the entire D segment, into the bargain - its weight is around 50 percent lower than that of an equivalent steel body. The new A8 3.7 quattro consequently weighs only 1,770 kilograms. An advantage that is doubly valuable in the luxury segment, benefiting both dynamic behaviour and operating economy.
Audi Space Frame ASF - minimum mass, maximum stability
Since the first generation of the Audi A8 made its first appearance in 1994, the principle of aluminium body construction has established itself as the synonym for a responsible form of mobility in the luxury car class. The significant reduction in body weight called for the consistent implementation of a revolutionary new approach: the Audi Space Frame ASF. This is a high-strength aluminium frame structure into which the large aluminium panels are integrated, thus assuming a supporting function.
The actual space frame consists of a combination of aluminium extruded sections and castings that are positively connected to each other. This results in a combination of minimum mass and maximum stability. As far as the new A8 is concerned this adds up to high rigidity despite a body structure weight of just 215 kilograms - the best figure in the luxury class.
With the evolution of the intelligent ASF concept Audi has proved once again that generously dimensioned and luxuriously equipped saloons in the premium class do not have to be heavier than cars at the top end of the mid-size class.
The development goals for the new A8 were extremely ambitious: without neglecting the maxim of lightweight construction - so the specifications read - the body had to offer increased comfort and satisfy much higher safety requirements.
For this reason new casting techniques and new alloys were used. Multifunctional large castings, long continuous profiles and a high proportion of straight extruded sections help to reduce the number of parts quite considerably. The proportion of large castings, for example, has increased from 22 to 34 percent of structural weight compared with the predecessor. Instead of 50 castings, there are now just 29 components in the new ASF structure.
The improves comfort considerably because a lower number of connections between components makes it possible to achieve higher body rigidity. At the same time the fact that there are fewer components has a positive effect on production processes and quality.
The connections are formed by riveting as well as various welding techniques including MIG and laser welding and, for the first time, the particularly efficient laser-hybrid welding process. Laser welding allows large-area panels to be connected to the body structure particularly efficiently owing to the fact that the resulting linear joins achieve superior strength and rigidity values to punctual ones. The body of the new Audi A8 includes a total of 20 metres of laser connecting seams.
The body structure
The central component of the front end is the large cast radiator tank that connects the two A-posts. The radiator tank consisted of seven individual parts on the previous A8. The advantage of this new construction technique is a weight reduction from 5.5 to just 3 kilograms.
The A-post itself also consists of two large cast shell halves that enclose the sill and the continuous roof frame, thus connecting them. These two extruded sections, together with the tunnel structure, also form the basis for the body's torsional rigidity.
The front longitudinal members have a two-piece, bolted design to facilitate repairs after a frontal cash.
The rear end of the A8 is a completely new development. This is due to the fact that the air suspension and stricter laws on safety in a rear-end crash call for a much more rigid construction. A large casting connects the longitudinal member with the sill and also supports the entire rear subframe. Its high rigidity also protects the fuel tank effectively in a rear-end collision.
The forward and rear structures are joined together by the roof frame, the sill, the seat cross-members, the B-post and the floor panels to form a continuous space frame. The B-post is another large multifunctional casting that, as well as serving as the mount for the door hinges and strikers, has to satisfy certain requirements in its role as the central component in a side collision. This component is also elementary for the excellent vibrational comfort of the A8 body: the quality of its join to the roof frame and sill plays a major part in ensuring the rigidity of the entire frame.
The side panel of the new A8 consists of a single section which extends from the A-post to the rear end of the body. Like the roof, the side panel is also laser-welded to the supporting structure.
Top of the class for dynamics and vibrational comfort
Apart from torsional rigidity, it is the natural frequency of torsion that determines the quality of a body. This parameter determines the vibrational comfort and the dynamics of a vehicle: the higher the natural frequency, the lower the amplitude - in other words, noticeable vibrations. Once again the new Audi A8 has made a quantum leap compared with its already outstanding predecessor: its natural frequency of torsion is no less than 38 percent higher.
This can be described quite literally as noticeable progress in all driving situations and speed ranges.
Hardly any idle vibrations or oscillations produced by the wheels now penetrate through to the steering wheel, for example. This is made possible by the systematic matching of steering wheel, steering column, module cross-member and connections to the body. The result is a high steering-wheel natural frequency that sets standards in the luxury class and at the same time provides the basis for the low vibration amplitude.
Another example is that of the dashboard. A new development goal resulted in the integration of the MMI display into this area so that it is easy to read at all times. Vibrations with a high amplitude are consequently extremely undesirable. The display is integrated into the dashboard so rigidly that all high-frequency vibrations that might make the information difficult to read are prevented.
Rigid structures for improved safety
The new ASF meets all current safety standards as well as plenty that have not yet come into force. This means that its crash strength is considerably better than that of its already very safe predecessor.
A progressive stiffness distribution in the front of the car ensures good structural behaviour in a front crash. This makes it possible to convert the maximum amount of energy along defined paths in the course of the impact. At the same time the integrity of the occupant cell is preserved as this is the only way - in conjunction with the restraint systems - to guarantee survival space.
The bumper cross-member of the new A8 is designed as an extruded section and can therefore absorb a large amount of energy. Together with the longitudinal member that is next in sequence, it can cope with the consequences of more minor accidents without causing damage to the remaining load-bearing structure.
The two-piece A-posts also help to protect the occupants in a frontal crash. Because they enclose the sills and roof frame, they ensure, together with the longitudinal members, that forces are transmitted effectively into the structure. Ribbing further increases the rigidity of the component. The roof frame, sills and floor are hardly deformed at all. A particularly large survival area is guaranteed; the doors are also made easier to open. Large castings in the footwell and in the region of the dashboard also ensure effective protection against intrusions.
The ASF structure also provides extremely effective protection against the consequences of a side collision. The rigid diecast B-post and the extruded sections integrated into the side of the roof frame, embedded in the one-piece side-panel frame, together form the basis for the structure's high stability.
The door structures are additionally reinforced by impact members consisting of highly rigid aluminium extrusions. Transverse extrusions in the floor, which serve as mountings for the seats, and the seat itself reinforced with cross-bracing ensure that survival space is preserved in the event of a pole impact.
The new Audi A8 already meets the requirements of the US directive FMVSS 301, which is expected to come into force in 2005. It thus guarantees protection against the consequences of a rear-end impact against a deformable barrier at 80 km/h. The fuel system and occupant cell are protected by a rigid structure of longitudinal and cross members which absorbs a large amount of energy by folding and buckling.
Safety in the interior
In addition to its high structural safety, the new A8 of course represents the benchmark for restraint systems and crash sensor technology: all components and their functions are precisely matched and together complete the saloon's outstanding safety standard.
For instance, the front occupants are now protected by newly developed front airbags with two-stage activation. Their concept is based on the objective of "as much protection as possible and as little force as necessary". The stages are applied adaptively depending on the severity of the accident. In other words, the necessary airbag stage is now released by a load-dependent release logic.
The driver is protected by an innovative ring-type full-size airbag, which is being used for the first time worldwide in the new A8. Its radial unfolding pattern means that it is also very effective at protecting drivers who are for instance sitting very close to the steering wheel.
The protective effect of the front airbags is supplemented by additional padding under the steering column and in the footwell. Crash-optimised pedals and the safety steering column increase the safety of the driver in the event of sudden forward displacement.
The safety equipment naturally also includes seat-integrated side airbags to protect the thorax and pelvis for all outer seats, as well as the Audi sideguard head-level airbag system which covers almost the entire side window area between the A and D-posts. Sideguard protects passengers not only in the event of direct side and post impacts, but also in oblique and secondary collisions.
A complex system of sensors is responsible for triggering the airbags. This uses sensors on the front of the vehicle under the headlights and on the B and C-posts to detect an accident. The system's logic means that the restraint systems can be triggered at an extremely early stage so that the loads to which the occupants are subjected are kept to a minimum.
In addition to triggering the restraint systems, the crash sensors initiate a series of other steps depending on how serious the accident is: they activate the hazard warning lights, shut down the engine and fuel pumps, release the central locking and switch on the interior lighting.
At the same time, the sensors are able to assess minor accidents accurately to prevent the restraint systems from being activated unnecessarily.
Active head restraints are another new feature at Audi. They incorporate a mechanical system which effectively reduces the risk of whiplash injuries by pivoting the head restraints forward in a fraction of a second in the event of a rear-end collision. The mechanical system is triggered by pressure on the lumbar support's plate. The head restraint unit is moved forwards by means of a lever mechanism, thus reducing the gap between it and the occupant's head.
Aerodynamics and aeroacoustics
The aerodynamics experts have also performed sterling work on the A8 body. The drag coefficient of only 0.27 is eight percent lower than that of the predecessor model and represents a new record in the luxury saloon segment. The A8 is also top of the class in terms of lift at the rear axle: on no other luxury saloon is this parameter, which affects dynamic handling and directional stability, so low.
These excellent figures are the result of numerous measures: for instance, the new A8 has a full aerodynamic underbody, which drastically reduces turbulence beneath the vehicle's body. Equally important: an improved high-performance cooling system with all-round sealing reduces inner air resistance. The lowering of the vehicle's ride height via the air suspension also plays a part - this reduces drag significantly at high speeds.
However, high speeds also make conditions more difficult with regard to aeroacoustics. Irritating wind noise though is certainly not acceptable in a luxury saloon of the calibre of the new A8. In order to achieve this goal, the Audi aeroacoustics engineers had to put together a comprehensive package of detailed solutions.
Seals flush with the outer panels, on the doors and on the joint gap of the door mirror, prevent high-frequency disturbing noises from occurring in the first place.
On the doors and windows of the new Audi A8 there are in principle three sealing lines to insulate the interior as effectively as possible.
Panes of glass five millimetres thick were chosen for the cabin windows. These provide outstanding acoustic insulation even as single-layer glass.
The windscreen wiper has also been carefully optimised: thanks to a special control device it can be moved into a particularly low rest position when not in use so that it does not produce any wind noise.
Even in action, the wiper on the A8 is also noted for its particularly low wind noise. This is made possible by the new flat wiper technology which ensures a lower noise level and a more pleasant frequency characteristic than a conventional wiper, without any adverse effect on performance, even at high speed.
This wiper technology - a first in the luxury class - benefits above all from the ingenious guiding of the air around the wiper arm and the wiper blade with integral spoiler. The wiper arm is noted for its particularly high rigidity and thus produces additional downforce on the wiper blade.
The Audi wind tunnel centre opened in 1999 made a major contribution towards these effective aerodynamics and aeroacoustics measures. Here, wind speeds in the region of 300 km/h can be realised; at the same time the process of ground simulation produces results that are far more realistic than in conventional wind tunnels. Wheels and tyres contribute around 25 percent to the vehicle's overall drag coefficient and must therefore be given due attention.
The development of a fully faired underbody such as on the new Audi A8 would not have been possible without realistic ground simulation: thanks to the detailed test and evaluation series, the exhaust system and rear axle could be integrated so cleverly that the underbody has better aerodynamics than a completely smooth underbody panel with the same area.