Strategic orientation – lightweight design is an Audi core competence
Lightweight design enjoys a special status among all the technologies that Audi Technical Development is constantly and intensively advancing. It is one of Audi's most important core competences. As the inventor of the self-supporting aluminum body, Audi is the worldwide leader in lightweight design, and this advantage is both a motivation and an obligation to continue to innovate.
“One of our most enduring aims for the future is to reverse the weight spiral,” says Michael Dick, Member of the Board of Management of AUDI AG responsible for Technical Development. “Lightweight design is the foundation of our entire approach to improving efficiency.”
Lightweight design is a strategic responsibility at Audi. It makes a significant contribution to sportiness and efficiency, thus it helps to conserve resources and reduce operating costs. The electric drives of the future will add additional weight to the car and will initially only offer a limited range, making systematic lightweight design all the more important.
When it comes to the body, the development engineers at Audi get a lot of ideas from the outside – and from the very best, as Heinrich Timm, Head of the Aluminum and Lightweight Design Center Neckarsulm points out. The aerospace industry and motor sports provide important inspiration.
Motor sports: Systematic lightweight design for 25 years
Audi has been driving progress in this area on the track, where every gram counts, for over 20 years. The rally and circuit race cars in the years around 1990 included many plastic parts in and on the body. Some of them were already using cardan shafts of carbon fiber composites at that time, for example. Today the Audi R15 TDI and the A4 DTM serve as high-tech labs for working with carbon and how to combine it with metal.
The best examples of lightweight design are provided by nature, however. In nature, only the amount of material required in the respective context is used. In the field of bionics, solutions to technical solutions are specifically sought in biology. “Many of the extruded sections we use in the ASF design, such as the sills of the TT, follow bionic principles in their topology. They are hollow, but heavily ribbed on the inside, reminiscent of the bones in the skeleton of a human or a bird,” says Timm.
The architecture of Audi’s ASF bodies differs widely between the individual model series. The superstructure of the R8 high-performance sports car makes extensive use of extruded sections, which make up 70 percent of the wrought components, in other words the starting components. In the TT Coupé, metal panels account for 45 percent and thus the largest fraction of the aluminum, whereas large, multifunctional castings play a decisive role in the structure of the A8, with 29 of them accounting for 34 percent of the weight.
Integrated approach: the reversal of the weight spiral
Heinz Hollerweger, Head of Total Vehicle Development, emphasizes that Audi considers lightweight design to be not simply a collection of individual components, but rather a complete, highly integrated project. “An aluminum tailgate permits a lighter gas strut. Axle components made of aluminum transmit lower forces to the body than do steel suspension links, thus the superstructure can be lighter, which in turn allows for more compact brakes, a smaller engine and a correspondingly streamlined exhaust system. This reverses the weight spiral while actually improving driving dynamics.”
The engine itself also holds significant untapped potential. A reduction of the conrod masses results in a reduced load on the crankshaft, which in turn allows for a lighter crankshaft design. The resulting reduction of the rotating masses with their moments of inertia has a very strong, positive effect on acceleration and fuel consumption far beyond the simple reduction in weight.
Lightweight design is the top priority for every Audi vehicle development project, and this applies to every step of the process through the construction of the prototype. Each individual component is assessed with respect to its weight and the effect on the total vehicle. Regular weight balance calculations help drive further continuous optimizations. Technology scouting and intensive benchmarking are also among Audi’s standard tools.
Lightweight design – more dynamics and passive safety
Lightweight design is a benefit in all respects, including driving dynamics and passive safety. The lighter a car is, the less kinetic energy it develops and the less of this energy needs to be converted into deformation in the event of a crash. Protection for other road users is also improved since a lightweight car places less of a load with which it collides.
Acceleration behavior plays a major role in the field of driving dynamics. A car weighing 1,200 kilograms reaches the 100 km/h mark from a standing start twelve meters sooner than a rival weighing 1,400 kilograms. The reduction of so-called rotating masses with their moments of inertia has a particularly strong effect on acceleration. Reducing the weight of a car's flywheel by one kilogram has the same effect as a 16 kilogram reduction in the weight of the translational (i.e. rectilinear) masses, such as the body.
A lower vehicle mass also has a positive effect on braking – and that in a number of regards. Overall stopping distance is shorter, brake pressure develops more quickly, pedal feel is tauter, and the discs do not get as hot, which reduces the risk of fading during a long mountain descent, for example. A lightweight car gets by with smaller and lighter brakes.
This reduction of the unsprung masses at the wheels brings numerous advantages. For example, a less stiff suspension setup can be used, thus improving vibrational comfort. A ten kilogram reduction in the unsprung masses reduces the load on the suspension strut by four percent. One would have to reduce the weight of the sprung masses by nearly 50 kilograms to achieve the same effect.
The lightweight aluminum bodies from Audi also have a very positive ecological effect. They spare the environment large quantities of CO2 emissions – through their low weight and the overall energy balance. At the end of the vehicle’s life, the material can be collected, treated, melted down and reused over and over again without any loss in quality. Although more energy is required to manufacture the primary aluminum than for steel, the effect of recycling makes the overall balance positive compared with steel.
Framework of aluminum – the ASF body
The basic structure of the Audi Space Frame (ASF) resembles the framework of a timbered framework. Its skeleton comprises extruded sections and pressure diecast parts of aluminum. The aluminum panels – the skin of the roof, the floor or the side panels – are integrated into this frame by means of a frictional connection so as to be semi-supporting. The components of the Audi Space Frame have very different shapes and cross-sections depending on their function.
The great strength of extruded profiles lies in their design flexibility. The side sills of the TT Coupé and the TT Roadster, for example, appear identical from the outside, but on the inside they have been topologically optimized according to bionic principles. This means that the geometry of a part at a given load is optimized to save as much weight as possible The differences in their ribbing determines their rigidity, which is higher in the Roadster than in the Coupé to compensate for the loss of the roof. The sections used in the TT are made of advanced alloys developed by Audi for greater strength and a further reduction in weight.
The profile and cross-section of each extruded section has been optimized for its respective use. In the Audi R8, for example, the roof arch is shaped by means of hydroforming – the section is shaped by a liquid forced into it at high pressure. This high-tech production process enables complex shapes that eliminate the need for a number of parts and ensures maximum precision. It also keeps the A-pillar narrow, thus reducing the obstruction of vision when looking forward at an angle.
Versatile and strong: aluminum pressure cast nodes
The extremely durable vacuum diecast components are used wherever high forces are induced locally and where there is a need for versatility and design freedom. A classic example is the A-pillar node in the TT, which reinforces the lower section of the A-pillar. This multifunctional component connects the longitudinal member, the sill, the pillar, the windshield cross-member, the roof frame and the strut mount with one another.
Like all aluminum castings, the cast nodes are characterized by precise geometry and optimal utilization of space. Such complex shapes are only possible with the use of intelligent design and computation programs. The expertise that Audi has gained over the many years is brought fully to bear here. Vacuum diecasting means maximum precision in fabrication. Casting under reduced pressure also improves component quality.
The latest version of the ASF principle is a hybrid construction of aluminum and steel such as that used by Audi in the TT Coupé and Roadster. The front end, the floor and the superstructure of the compact sports car are fabricated of aluminum, with deep drawn steel being used for the doors and the trunk lid. The rear section of the floor assembly, the tail panel and the bulkhead of the Roadster are made of high-strength steel. The distribution of the materials provides for an optimal distribution of axle loads and thus for dynamic handling.
The material mix is dominated by aluminum, which accounts for 68 percent of the Coupé’s weight and 58 percent of the Roadster’s. The body of the TT Coupé weighs 206 kilograms, which breaks down to 140 kilograms of aluminum and 66 kilograms of steel. It would be 48 percent or nearly 100 kilograms heavier if made entirely of steel. The aluminum fraction comprises 63 kilograms of panels, 45 kilograms of castings and 32 kilograms of extruded sections.
170 hp and 5.3 l/100 km: dynamic and efficient
The low weight of the body is a key factor for the highly dynamic road behavior and exemplary efficiency. Empty, an Audi TT 1.8 TFSI weighs only 1,240 kilograms. The TT 2.0 TDI quattro, which weighs 1,370 kilograms and is equipped with a 125 kW (170 hp) engine consumes on average only 5.3 liters of diesel fuel per 100 kilometers. The values are similarly low for the Roadster, which weighs only 45 kilograms more than the Coupé.
The ASF body of the Audi TT is in all regards the ideal solution for a sports car. Compared to the preceding model, static torsional rigidity increased by roughly 50 percent in the Coupé and 100 percent in the Roadster. This is the foundation for precise, dynamic handling. The rigid ASF construction is also responsible for the high vibrational comfort inside the car.
The TT makes no compromises when it comes to crash safety. The longitudinal members in the front end comprise extruded sections and highly durable castings at the transition to the passenger cell. In the back, large-volume members protect the passenger cell. High-strength aluminum profiles in the doors provide protection in the event of a side-impact collision. Transverse extruded profiles reinforce the floor of the passenger cell.
A strong roof frame provides protection in the event of a rollover. The Roadster also has high-strength tubes in the windshield frame and two rollbars on board.
Magnesium – an extremely lightweight material
Magnesium is a particularly lightweight metallic structural material. A solid block of magnesium with an edge length of 10 centimeters weighs only 1.8 kilograms, a third less than aluminum. The material offers good strength and rigidity relative to its weight, enabling even further weight reductions.
Audi began using a magnesium five-speed transmission casing in a volume production model back in 1996 for the A4. Today Audi uses magnesium materials in a number of areas, such as the variable intake manifold of the S5 and S6, the steering wheel skeletons for all models, parts of the steering column and in the dashboard of the A8. The six-speed transmission casings for the A3 and TT family are also manufactured from this material in large numbers.
In the R8 high-performance sports car, Audi even uses the extremely lightweight material within the aluminum space frame structure. The engine frame is made of pressure diecast magnesium and provides added rigidity in the upper section of the rear end. Aluminum bolts connect it to the Audi Space Frame.
Thanks to advanced alloys that can withstand higher loads than conventional ones, Audi will soon begin using magnesium for parts of the engine itself, such as the top section of the oil pan, the sealing flange in the V6 gasoline engines, or for the cover of the camshaft case. The next generation A8 will also be getting a new magnesium component – a transmission crossmember that also serves to stiffen the center tunnel.
The high-end material – components of CFP
Carbon fiber-reinforced plastics (CFP) are an excellent material in automotive construction. Their strengths have long been on display in motor sports. They attain outstanding tensile strength values ranging from 500 to 1,350 Newtons per square millimeter depending on the exact type and are very good at absorbing energy. They are also extremely light, having a specific weight of only around 1.5 kilograms per cubic decimeter, i.e. for a cube having an edge length of 10 centimeters.
A single carbon fiber is only five to eight micrometers thick, or roughly one tenth the thickness of a human hair. 1,000 to 50,000 of them are generally bundled together to form rovings (strands), which are the base material for the fabric. The layer structure is what determines the material properties. Because CFP materials are only high strength in the direction of the fibers, the individual layers are laid down in different directions. They are embedded in a matrix, usually epoxy resin. When the resin cures, the part is finished.
Audi's involvement in motor sports has enabled it to develop wide-ranging expertise in the use of carbon fiber-reinforced plastics. The Audi 200 TransAm, which dominated the North American ovals in 1988, had a CFP cardan shaft. Today the Audi R15 TDI sports prototype, the R8 LMS and the A4 DTM car are largely constructed of CFP components. Whereas the R15 TDI and the A4 DTM have a CFP body, the R8 LMS combines its ASF body with selected CFP components.
The Audi R8: CFP parts at the customer’s request
As far as production models go, Audi offers the sideblade, the front spoiler, the rear diffuser, the engine compartment lining and a number of interior parts made of CFP as options for the R8. In the upcoming R8 Spyder, rear side panels and a top compartment cover made of the high-tech material will be standard equipment.
The Lamborghini Gallardo LP 560-4 Spider, which drew on Audi’s expertise in lightweight design for many components, has a similar solution on board. Its large CFP engine cover comprises only two parts – a frame and an outer skin. It weighs only 16 kilograms, roughly five kilograms less than a cover of aluminum – a saving of nearly 25 percent.
Construction of the parts is still largely performed by hand, so that further developments in the area of fabrication technology are required before CFP components can be used in large-volume production. The greatest challenge when designing parts is the arrangement of the individual layers to achieve the Audi-typical level of quality.
The RTM (resin transfer molding) process is currently the optimum with respect to fiber content by volume and surface quality. In RTM, dry fibers in a mold are impregnated under pressure with epoxy resin; the part is then processed further once the resin has cured.
Carbon fibers are only one way to reinforce plastics. Glass or aramid fibers can also be used. Embedding them in a matrix of polyamide produces a solid structural part called an organic sheet. Audi will use such components reinforced with aluminum inserts in the fourth generation A8. Weighing only 5.4 kilograms, they are 2.3 kilograms lighter than a comparable steel solution.
Kilo for kilo – lightweight design as an integrated project
At Audi, lightweight design is an integrated approach that includes all aspects of the vehicle. By its very nature, the body harbors particularly large amounts of potential, but the drivetrain, the chassis, the electrical system and the passenger compartment can all make a significant contribution to weight reduction, frequently on the kilogram scale and very often in the hundreds of grams. And every gram counts.
With many models, Audi makes all of the chassis components or at least the majority of them out of aluminum. This reduction of the unsprung masses has an equal and positive effect on both sportiness and comfort. Audi even makes this great effort with the compact A3, in which the subframe, the control arms and the pivot bearing of the front suspension together weigh only 14.4 kilograms. They would be 5.9 kilograms heavier if made of steel.
The aluminum brake cover plates weigh only 149 grams each, or less than half as much as corresponding parts of steel. The large carbon fiber-ceramic brake discs that Audi offers in its top models are each 10.5 kilograms lighter than their steel counterparts, and are also clearly superior to them in terms of performance and durability.
Possible weight savings in the kilogram range can also be had in the wheels. Using a new hybrid technology in which the outer rim and the center are manufactured separately and then welded together, the weight of even large wheels can be kept below 10 kilograms. A corresponding cast wheel today weighs more than 12 kilograms.
Plastic honeycomb instead of wood – lightweight design in the passenger compartment
Lightweight components call also help to save a significant amount of weight in the passenger compartment. The cargo floor plate of the Audi Q5 is made of honeycombed polyurethane rather than multiple bonded layers of wood for a saving of 2.5 kilograms.
Audi has implemented another innovation in the TT Coupé, the A3 Cabriolet and the A5 Cabriolet. The backs of the folding rear seat backrests are made of a high-grade plastic rather than steel. The part is blow molded at high temperature and pressure. Weighing just 2.5 kilograms, it is only half as heavy as a steel backrest, but is every bit as safe and comfortable.
The A5 Cabriolet has a plate of pressure diecast magnesium in the end of its top that weighs 1.5 kilograms less than a comparable aluminum part. The use of an optimized plastic film in the windshield enables the glass to be made thinner and thus 2.4 kilograms lighter while still offering the same acoustic performance. A magnesium steering wheel skeleton with an integrated vibration compensator brings an additional 0.4 kilograms of weight savings.
There is a long tradition of lightweight design at Audi. From 1982 onward, the brand's classic car, the quattro, had a plastic trunk lid weighing only 8.0 kilograms. The entire outer skin of the legendary Sport quattro was made of a variety of completely new fiber composite materials such as carbon and aramid fibers. The hood alone was 10.4 kilograms lighter as a result.
In the years thereafter, plastic clutch pedals, induction manifolds and coolant hoses found their way into series production. In 1995, Audi became the first manufacturer to use a variable intake manifold made entirely of plastic. For some years now, Audi has used aluminum screws in certain areas to join the engine and the transmission, which saves 0.6 kilograms. Magnesium has been used for the gearbox housing since 1996, for a weight advantage of more than 4.5 kilograms over aluminum.
The exhaust system is another important area for intelligent lightweight design. In today’s Audi A4 1.8 TFSI, the entire exhaust system including the catalytic convertor weighs only 21.8 kilograms. Just a few years ago, the same system in an A6 with a comparable engine weighed 33 kilograms. The difference lies in the use of higher-grade steels. They are more corrosion resistant than their predecessors, can use thinner walls and can be of a tubular rather than a monocoque design.
Audi will soon begin using tailored strips and tailored tubes in its exhaust systems. The new high-tech tubes have different wall thickness at different locations depending on the loads on the individual zones. This represents a potential saving of more than 10 percent. Other projects include a hybrid roof frame of steel and carbon fiber composite and battery cables of aluminum rather than copper. Audi will continue to extend its Vorsprung durch Technik in lightweight connection one step at a time.
Lightweight and suitable for high-volume production: the hybrid brake disc
Running gear featuring lightweight design, particularly for the unsprung masses, is of great interest for two reasons: First, each gram of weight saved helps to reduce CO2 emissions, and second, it improves driving dynamics and ride comfort – both are hallmarks of Audi. One particularly appealing idea is to replace the cast iron brake discs commonly used today with a cast iron/light alloy composite.
Audi has already implemented this concept in its top-of-the-line sports cars, the R8, RS 6 and the TTRS. In these models, the friction rings are made of cast iron and the brake caps of aluminum. Drilled studs connect the two components. As an additional benefit, the studs prevent the transfer of peak temperatures to the brake cap.
New connection: hybrid brake discs for large-volume production
Because it is costly and complex to manufacture, this solution is not currently suitable for large-volume production. Audi has therefore developed a new technology: a pin connector integrated into the cast iron friction ring. The special shape of the pins allows them to dissipate the heat and ensures that rain water and saltwater flows away quickly. The friction ring is placed in the mold when the aluminum brake cap is gravity diecast so that aluminum is cast around the pins.
This innovation offers Audi major advantages with respect to weight reduction, saving around 30 percent or up to 5.5 kg per component. The new solution is currently in the prototype stage.