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Weighing up the difference: Audi's use of weight-saving GFRP instead of steel coil springs for the latest A6 ultra.

Audi 'springs' material surprises on A6 model

Making the most of new materials to reduce weight, improve efficiency, enhance aesthetics, and achieve environmental-centric performance is the aim of every automaker. Audi is among those demonstrating how it can be done by determined focus on details and imaginative applications, without resorting to high-cost exotic solutions.

Audi’s latest version of its A6 Avant, the 2.0-L TDI ultra (high efficiency) launched this month, is using glass-fiber-reinforced polymer (GFRP) springs to succeed conventional steel components. The A6 range is also getting an infotainment carrier that uses a continuous fiber-reinforced polyamide composite and a polyamide 6 over molding material, to cut weight by almost 50% compared to a conventional steel solution.

Sporting GFRP springs

Audi’s new coil springs reduce weight by some 40%. Developed in collaboration with the Italian company Sogefi, the coil springs are of slightly larger diameter than conventional steel. There are also fewer coils.

The springs comprise long glass fibers twisted together and impregnated with epoxy resin. Additional fibers are wrapped around the resultant core at alternating angles of plus and minus 45° to the longitudinal axis.

Audi explains that these tension and compression plies mutually support each other, and the springs can be precisely tuned according to behavior requirements, exactly as can steel springs.

Further advantages of using GFRP include lack of corrosion. The springs, light green in color, are said to be impervious to the deleterious effect of chemicals such as wheel cleaners.

The manufacturing process demands lower energy consumption.

Audi states that typically a steel spring weighs some 2.7 kg (6.0 lb) compared to the 1.6 kg (3.5 lb) of the composite alternative. Of the total 4.4 kg (9.7 lb) saved for the A6 ultra, about half concerns unsprung weight, so there is the double bonus of weight saved and ride improved, as the suspension reacts more quickly to road surface variations.

Prof. Dr. Ulrich Hackenberg, Audi’s boss of technical development, describes the springs as being “at a crucial location” in the car’s chassis system: “We are, therefore, making driving more precise and enhancing vibrational comfort.”

Christoph Bayerlein, Audi Development Suspension Systems Specialist, added: “The GFRP springs are very high strength, and we are confident that they can withstand stone chips that might otherwise penetrate a conventional steel spring coating and cause degradation over time. This is because the glass fibers are about 1.5 mm below the surface, so we do not expect any issue of fatigue behavior to adversely affect the composite springs if there is some surface damage.”

The frugal A6 Avant ultra was chosen to be the first model to use the lightweight GFRP springs as it would clearly demonstrate how the weight saving they brought could make a tangible difference, said Bayerlein: “Furthermore, we wanted to choose a model in the A6 range with a volume that also fits with the volume the supplier can deliver in this current phase. For the future, we want to increase our usage of composite springs.”

At present, 34 million steel springs are produced per year for use by Audi vehicles. Said Bayerlein: “Although the composite springs are initially a very small project for us, it is something we are very interested in applying to future models. In particular, there are no limitations with the springs; a crucial part of the original design brief was that they could be used on both front and rear strut configurations and are suitable for all future Audi projects.”

Audi’s “ultra” designation for some of its models describes particular focus on the latest technological advances spanning lightweight construction to engine development.

The venture is a joint one between Audi and Allevard Rejna, part of the Sogefi Group, which spends almost 3% of its total revenues on R&D. Sogefi states that the patented new coil spring can show a weight saving of up to 70% of that of a comparable steel spring. The GFRP springs have been designed to be assembled on cars and light commercial vehicles without affecting the suspension system architecture for a weight reduction of 4 to 6 kg (8.8 to 13.2 lb) per vehicle (depending on coil spring design and vehicle type) and a significant reduction of unsprung mass.

CO2 is also reduced during the production process of the GFRP springs compared to steel.

Sogefi CEO Guglielmo Fiocchi said: “Innovation in the car sector, starting from reducing weight and improving the efficiency of vehicles, now depends significantly on components companies.”

A6 composite carrier

The A6’s weight-saving infotainment system carrier holds an amplifier and optional TV tuner. It is fabricated with Tepex and Durethan from supplier Lanxess to deliver required high stiffness and fatigue resistance levels.

Lanxess’s Lightweight Design Specialist, Martin Klocke, said: “This application underlines the enormous weight-saving potential of hybrid technology and continuous fiber-reinforced polyamide composite for lightweight design of structural components. We are confident that this hybrid design also is suitable for other components in motor vehicles, such as carriers for pre-installed electric and electronic modules.”

Lanxess created the solution together with Audi’s Control Device Package Development and Fiber-Reinforced Plastic Technology departments. Other companies involved included KraussMaffei.

For the prototype system, KraussMaffei developed a fully automated manufacturing cell with a bespoke adjusted handling system and a heating unit placed directly above the mold platen. Using this as a basis, Reinert Kunstofftechnik created an optimized manufacturing cell for fully automated series production of the carrier.

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