FRP Coil Springs
For Audi, ultra-lightweight vehicle design is more than engineering competency alone. It’s an attitude of mind that exists throughout the company. The brand’s engineers fight determinedly to avoid every scrap of unnecessary weight – not only in the body but also in every component and assembly that goes to make up the complete automobile.
Ultra-lightweight design is especially interesting in the chassis and suspension area. Every reduction in unsprung mass improves ride comfort and handling. Joachim Schmitt’s pre-development project is in this technological area. It is devoted to a component which is nothing special to look at, but has to withstand severe loads. A component which one might think could scarcely be improved any more: the coil spring. The project takes a different view, however, and aims to replace the steel spring with a spring made from fiberglass reinforced plastic (FRP).
The FRP coil spring, which Schmitt has developed jointly with the patent holder SOGEFI, is very different from a conventional steel spring. It’s bright green in color, the so-called wire is thicker, the spring’s overall diameter is greater and the number of turns is lower. Above all, this type of spring is much lighter in weight. Project engineer Schmitt has the facts: “Each of the steel springs in the A4 model’s front suspension weighs 2.66 kilograms. The FRP spring weighs only 1.53 kilograms, more than 40 percent less, but performs just as well.”
A helix made from fiberglass reinforced plastic is capable of absorbing torsional loads extremely well if it is designed with this purpose in mind. The core of the spring consists of long glass fibers twisted together and impregnated with epoxy resin. A machine wraps additional fibers around this core, which is only a few millimeters in diameter, at alternating angles of plus and minus 45 degrees to the longitudinal axis. These layers support each other and act in either compression or tension. Torsional loads across the component are converted in the fibers into tensile and compressive loads.
The next production stage is for the ‘wire’, while it is still wet and soft, to be wound onto a metal alloy core with a low melting point; this core is the negative form of the finished spring. The FRP material is then hardened in an oven at more than 100 degrees, so that the core melts. This is the process currently used to manufacture the prototypes. Later it will be very much faster and more efficient: a pilot batch will be made first, followed by the start of high-volume production: a million springs annually.
The new FRP suspension springs will make their debut before the end of 2012 in the electrically propelled Audi R8 e-tron model. In 2013 they will be introduced step by step in greater volume for Audi midsize and large luxury models. Long-term road trials are taking place at the moment, accompanied by marathon test cycles on special rigs, each of which imposes loads on the spring equal to about 300,000 kilometers of driving by the average customer.
“In the early days many of my colleagues were not convinced that the project would be a success,” Joachim Schmitt recalls. “But from the very start FRP proved to be an excellent choice of material. It is absolutely immune to corrosion, even if struck by stones flung up from the road surface, and effectively resistant to chemicals such as wheel cleaning products. Manufacturing FRP springs consumes less energy than for steel springs, and they reduce the weight of a car such as the current Audi A4 by about four kilograms – a notable step forward.”
Schmitt and his colleagues are already able to tune their FRP springs to suit the chassis engineers’ wishes, taking the car, its axle loads and its ride comfort and handling characteristics into account. All they have to do is vary the length of the wire, its diameter and the number of turns. In a few years’ time the engineers expect improved production methods to increase the load capacity of the FRP material, so that the space needed for the springs is no larger than for today’s steel springs.
Which just leaves the unusual bright green color – but Audi’s development engineers have tackled this as well. Joachim Schmitt: “If we add a certain amount of graphite to the mixture for the FRP springs, they are dyed black right through. And for enthusiastic drivers we’re already working on red and blue springs!”