The Audi R8: Body
New facet to lightweight construction quality
The outward body shape of a high-performance sports car is of course the aspect that initially matters most: its design must win the hearts of car enthusiasts. But in the technical domain, too, the metallic structure has to meet particular requirements: the body of a sports car must be light in weight, to maximise its dynamic performance. It also needs to be highly rigid as a condition of ultra-precise handling. And then, of course, good aerodynamics are called for not simply in order to achieve an impressive top speed, but equally for stability and driving safety. And last but not least, passive safety is as important in a sports car as in any other model.
The perfect solution to all these requirements is the Audi Space Frame (ASF). Audi developed this trailblazing aluminium technology in the early 1990s for the first-generation A8 and has since perfected it over many stages. In ASF technology, the body’s supporting structure is made of extruded aluminium sections and die-castings. Aluminium panels are incorporated into this skeleton such that they form a positive connection and perform a load-bearing role. Each individual component of the ASF space fame is optimised for its specific task by the use of widely differing shapes and cross-sections, thus combining maximum stability with minimal weight.
Audi is a world leader in aluminium lightweight construction of cars. The latest expertise in calculating and optimising every component was used in the development of the high-performance sports car. The structural components of the R8 are moreover made from innovative aluminium alloys; they exhibit superior strength and as such offer scope for further weight reduction.
Extremely rigid but low in weight
The entire bodyshell of the Audi R8 weighs just 210 kilograms. In terms of lightweight design quality, this is an absolute top figure compared with competitor sports cars: it is based on the ratio of body weight to torsional rigidity, as a function of the vehicle’s size. It reveals the R8 to be particularly light and rigid, an outcome that can be felt in the ultra-precise driving feel.
The body comprises 70 percent extruded sections, 22 percent metal panels and eight percent vacuum-cast nodes. The profile and cross-section of each individual extruded section have been optimised for the specific application. The curved roof is a special case, because its shape is produced by hydroforming. This means that the profile is pressurised from inside by a liquid, pressing it into the desired shape. This allows a complex shape to be produced, avoiding the need for several different body components. This, too, contributes towards maximum precision as part of Audi’s quality philosophy. The elaborate structure moreover keeps the A-post narrow, thus minimising the degree to which the view to the front is obstructed.
The cast nodes, too, are highly complex components. As well as connecting the profiles, they perform other tasks: the node on the A-post, for example, connects the A-post to the floor structure, serves as a mount for the shock absorber bracket and is even the point to which the windscreen wipers are attached. It has only been possible to realise such complex shapes with the aid of design and calculation programs.
A supporting component made from diecast magnesium is used for the first time in the R8’s space frame. This is the engine frame, which reinforces the upper section of the rear structure. Magnesium has proven to be the ideal material for this component in terms of weight and rigidity.
The production shop: precision on a small scale
In keeping with the exclusive standards of the Audi R8, its body is assembled largely by hand. Highly qualified specialists produce weld seams measuring a total of 99 metres to connect the castings and profiles. The metal panels in the structure are connected by a total of 782 punch rivets and 308 automatically set, self-tapping screws. Instead of a hole first needing to be drilled for these specially developed flowdrill screws, they are set into the solid material under high force. The result is a particularly strong connection. A total of 38 welding machines, five sets of riveting tongs and just five robots are used in the body shop the latter for processes where a particular level of force is required.
Working methods in the body production shop are characterised by superlative precision. The dimensional accuracy of every single component is examined to within one-tenth of a millimetre by a fully automatic measuring system. This scanner operates without making any contact; its 95 laser sensors check a total of 220 points on the structure in the space of just five seconds. Before that, all 52 connecting points for the running gear and steering are drilled and cut on the finished body structure in a single pass. This assures maximum precision in the axle geometry.
Computer tomograph: quality with micron precision
Another example of Audi’s unstinting quest for quality is the new computer tomograph that thoroughly examines everything from minute components to whole vehicle bodies. Computer tomographs are better known in human medicine, where they provide a hitherto unprecedented view inside the body thanks to their fine-resolution representation and three-dimensional pictures.
Audi has now installed the only computer tomograph (CT) of its kind in the world in Neckarsulm, generating X-rays that are capable of detecting flaws of micron magnitude. That is about one-hundredth the breadth of a human hair. The CT examines primarily connecting points in aluminium lightweight construction to verify their high quality, and scans weld seams or punched joins slice by slice.
The whole process takes place non-destructively: the system is large enough to accommodate the entire aluminium space frame of the R8 in one piece. It is equally possible to check minute electronic components just three millimetres in size.
While the object is rotated in the X-ray, the computer tomograph compiles X-ray projection images for between 100 and 1,000 different angles. 3D reconstructions of the object being examined can be computed from these images: the observer can then “fly through” the weld seam or electronic component in order to obtain an impression of its physical properties from every perspective.
Aerodynamics: playing with the wind
With regard to aerodynamics, there are of course particularly close parallels between the roadgoing sports car, the Audi R8, and the car which earned its reputation on the racing circuit at Le Mans. In both cases low drag is as important to a high top speed as it is to modest fuel consumption. Equally, in both cases downforce promotes good handling and optimum driving safety, including at high speeds. Because on most roadgoing cars and even on many sports cars, the air that flows through and around it at high speeds can in some cases produce considerable lift. This reduces the weight applied to the wheels and thus impairs directional stability.
Racing cars, on the other hand, produce downforce: the faster they drive, the greater the pressure their wheels exert on the road surface. Although this increases drag, it provides stability when braking from high speeds and allows higher cornering speeds. To this end, however, racing cars have giant wing structures and the front ramp angles are not exactly compatible with everyday driving as a result of the very low front aprons features that are unacceptable on an elegant road version of a sports car.
As on the sports racing car for Le Mans, downforce was more important than low drag in the development of the production R8.
A glance at the competitors reveals how successfully Audi’s aerodynamics specialists accomplished their task: with a drag coefficient of 0.345, the R8 has the lowest drag of any sports car producing downforce. The drag consequently does not cancel out the engine’s propulsive power until a top speed of 301 km/h.
Diffuser principle from motor racing
The downforce is achieved by means of the extending rear spoiler and the diffuser underbody. The rear spoiler intervenes in the airflow from a speed of 100 km/h, and below 35 km/h the spoiler is retracted back in until it lies flush with the body. It can of course also be extended and locked at the push of a button, for example for use on the racetrack. The rear spoiler’s core is filled with a special lattice structure. When in the extended position it does not impact the aerodynamic effect, but when retracted it serves as an additional means of dissipating heat from the exhaust zone.
Even more important in terms of its effect is the fully clad underbody with moulded-in diffusers at the front, ahead of the rear wheel arches and above all at the rear. These generate a low-pressure zone between the vehicle and the road surface, helping to keep the car firmly on the road in effect by suction. The diffuser underbody brings the car full circle, back to the racing car that shares the same aerodynamic principle. The calculation and testing methods, for instance in the wind tunnel with a moving floor, are again the same as for the racing car.
The fully clad underbody also covers the engine and transmission, the only small openings being those for the lubricating system’s dry sump and for engine compartment ventilation. Here too, considerable fine-tuning involving computer models of the airflow through the engine compartment was needed to ensure for instance that the air emerging from the car’s upper surface did not adversely affect its aerodynamics.
Although seemingly only a marginal issue on sports cars, aeroacoustics plays a very important part in determining long-distance comfort and everyday suitability. Audi was able to call on its wealth of experience as a manufacturer of premium saloon cars in making the R8 the sports car with the lowest level of wind noise.
The aim is ultimately to keep the driver and passenger in top shape over long distances, not unduly distracted by the fascinating sound of the V8 or the high fidelity of the Bang & Olufsen sound system.
Passive safety: sturdy cage
Thanks to its high rigidity and a structure resembling the safety cages encountered in motor sport, the ASF construction principle provides the basis for an excellent level of passive safety. The two longitudinal member planes in the forward structure absorb the forces arising in a collision, reduce them through targeted deformation and channel them into the tunnel and sill structure of the centre structure. The body is likewise very well equipped to withstand the consequences of a side impact thanks to its precisely calculated profiles and nodes down the sides and on the underbody. The fuel tank is located well away from impact zones in the centre of the vehicle, ahead of the engine.
The safety equipment is perfectly matched to this arrangement: the two front airbags have two-stage activation, unfurling their life-saving effect in conjunction with the belt tensioners and the belt force limiters. The seat backs conceal the combined head and thorax side airbags which protect the entire upper body of the driver and passenger if need be. The backguard system incorporated into the head restraints reduces the risk of whiplash injuries in the event of a rear-end collision.
The R8’s designers have of course also taken precautions to protect other road users: thanks to the favourable contours of the nose and its extensively optimised design, pedestrian protection is of a high standard. The sports car’s front end has a specially matched layer of backing foam six centimetres thick.
It should be emphasised that the overwhelming proportion of road accidents are comparatively minor affairs. The R8 is equally well equipped to take these in its stride: the body structure at the front and rear is bolted together in such a way that any crash damage sustained at an impact speed of up to 15 km/h and that covers most bodywork damage can be repaired without the need for welding work.
And with the exception of the roof, all metal panels on the outer skin are bolted to the structure and can likewise easily be changed. If a major repair should nevertheless be necessary, Audi’s workshops are perfectly equipped to rectify the damage thanks to their many years of experience in working with ASF bodies.
The equipment, data and prices stated here refer to the model range offered for sale in Germany. Subject to amendment; errors and omissions excepted.