The sportiest premium sedan in the full-size class – the new Audi A8
Amazingly light: Weighing just 231 kilograms (241 kilograms for the A8 L)
[509.27 lb/531.31 lb], the ASF body of the A8 sets standards in the luxury class. With its standard quattro drive system, the A8 3.0 TFSI quattro has a curb weight of just 1,830 kilograms (4,034.46 lb), significantly less than its competitors with all-wheel drive.
The Audi Space Frame (ASF), which the brand debuted 20 years ago, is roughly 40 percent lighter than a corresponding steel body. Its layout follows bionic principles: Like the bones in a skeleton, all of the parts combine optimal function with low weight. Aluminum extruded sections and die-castings form a frame to which the aluminum sheets are joined by friction connections. Audi always uses the material in a tailored configuration. The ASF of the A8 combines 13 different grades of aluminum.
Castings are used anywhere high local forces are introduced and versatility and design freedom are particularly important. The multifunctional A-pillar node, for example, connects the longitudinal member, the windshield crossmember, the roof frame, the strut mount, and the omega bracket in front of the footwell. Most of the 25 castings in the A8 are manufactured using the vacuum casting process.
The extruded sections are also characterized by design flexibility, with each one optimized precisely for its intended purpose. The roof arch, for example, is hydroformed under very high water pressure. Its cross-section changes multiple times over its length, with smooth transitions.
The A8’s B-pillars are an exception in the ASF: They are made of hot-shaped steel. The blanks are heated in a furnace to over 1,000 degrees Celsius and shaped immediately thereafter at around 200 degrees Celsius in a water-cooled pressing die. This abrupt change in temperature creates an iron-carbon structure of maximum tensile strength. This measures 1,500 Newtons per square millimeter in the upper section of the pillars; somewhat lower in the lower section because this is where most of the energy is dissipated in the event of a side impact collision.
Thanks to their low wall thickness, the hot-shaped B-pillars are relatively lightweight, accounting for eight percent of the A8 body’s weight. The aluminum sheets and castings account for 35 percent, and the extruded sections 22 percent. The distribution is similar in the A8 L.
Joining the wrought components and individual parts is a high-tech process. 1,847 punch rivets, 632 self-tapping screws and 202 weld points help keep things tightly together. A structural adhesive is used at the bolted connections of the B-pillars to enhance strength. It also serves as a separating layer that prevents contact corrosion. All together, the bonded seams measure 44 meters (144.36 ft) long.
The invisible roof seam: The epitomy of Audi precision
Another Audi domain is laser welding. The particularly strong and rigid seams it produces makes it possible to attach large sheet metal parts to the structure. The 1.8-meter (5.91 ft) aluminum invisible seam between the roof and the side of the car is produced via this method. It epitomizes the precision of Audi. The maximum tolerance for the outer skin is just 0.1 millimeters (0.004 in). The development engineers have also trimmed every unnecessary gram of weight from the add-on components. The lower crossmember of the front end, for instance, is a matrix of fiber-reinforced polymer with three embedded aluminum panels.
The ASF body is extremely light, very stiff and offers a high degree of vibrational comfort. For example, the front axle subframe directs the impulses it receives from the wheels into a multi-element structure consisting of beams and sections. All A8 models include two vibration dampers mounted on the front suspension.
Considerable attention was also paid to the sound radiation of the large sheet metal panels and body cavities during the A8 update. Acoustic bulkheads, absorbers, insulation and layers of foam prevent them from vibrating. Fine-seamed seals block out airborne sound. The cabin has also been carefully insulated, such as in the area of the front carpeting. The quiet in the cabin – the typical Audi feeling – is now even more impressive.
With a coefficient of drag of just 0.26 for the 3.0 TDI quattro, the A8 slips through the wind quietly and easily. Low lift coefficients at the front and rear axles guarantee outstanding stability even at high speeds. The flow of air along the underbody and through the engine compartment also had a prominent place in the requirements specification. Advances with the radiator package and the efficiency gains with the air conditioning system reduce fuel consumption by several grams CO2 per kilometer.
The underbody panel, which exposes only the exhaust system and the rear axle, offers additional advantages beyond its aerodynamic effects. It protects the sheet metal and engine against salt and stone impacts, and also serves as noise insulation. A NACA vent directs the slip stream to the transmission oil cooler. At the rear of the car, a cover plate and the mufflers form an upwardly angled diffuser surface.
Quiet cabin: Excellent wind noise comfort
The Audi A8 L is also at the head of its class in wind noise comfort. To further improve exterior noise insulation, Audi offers two optional glazings. The first is double/acoustic glazing. Composite safety glazing, which also includes an acoustically insulating film in between, is available as an option in the A8 and standard in the S8. The second option is insulating acoustic glass (standard in the A8 L W12). It uses a metal vapor coating to reflect the infrared component of sunlight. Audi offers darkened side rear windows of double acoustic glass in the rear.
The body of the Audi A8 protects passengers in any type of collision. In a frontal collision, four load paths on each side of the vehicle absorb the forces. Two of these are located in the upper portions of the fenders, the others in the longitudinal members and the buffers in front of the subframe for the engine and the front axle.
A crossmember comprising a double box section bent into the shape of an omega provides additional protection for the passenger cell. Positioned at the level of the footwells, it directs the forces into the floor and the A-pillars. The longitudinal members in the floor panel come together in the shape of an arrow below the rear seat bench. Together with the center tunnel they constitute the body’s strong backbone. In the back, large members protect the passenger cell.
The adaptive restraint system stands guard in the cabin of the Audi A8. It protects passengers of different sizes particularly effectively by intelligently managing the collaboration between the airbags and the belt force limiters. In a rear-end collision, the integral head restraint system averts the danger of whiplash injuries. In a side-impact collision, four side airbags and the head airbag system protect the occupants. Automatic three-point seat belts with adaptive force limiters complete the restraint system.
The trunk of the new Audi A8 has a capacity of 520 liters (18.36 cubic ft). Loading is comfortable thanks to its low loading lip and the revised layout. There is plenty of space for four golf bags. The trunk lid swings upward when unlocked. Pull the lid down and the self-closing trunk does the rest. The optional trunk package includes a floor net, two hooks for bags, an umbrella holder and a 12 V power outlet. A load-through hatch with a removable ski bag and a power-retractable trailer hitch are also available (both not for the A8 hybrid).
Audi also offers gesture control for unlocking the trunk lid as an option in combination with the convenience key (standard with S8 and A8 L W12). The top-of-the-line solution is the power-closing trunk lid (standard with A8 L W12). There are two buttons on the inside of the trunk lid. One starts the closing process, the other locks the entire car.