Audi is in the news this week because it plans to start using suspension springs made of glass fiber-reinforced polymer (GFRP) in one of its cars before the end of the year. Audi says it developed the spring in collaboration with an Italian supplier.
As you might suspect, lower unsprung weight was the reason Audi made the move. Audi says a steel spring for an upper mid-size model weighs nearly 2.7 kg (6.0 lb), while the plastic equivalent weighs 1.6 kg (3.5 lb). Thus the four springs going into the new Audi will cut its weight by roughly 4.4 kg (9.7 lb), half of which pertains to the unsprung mass.
Audi says the core of the springs 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 ± 45° degrees to the longitudinal axis. These tension and compression plies mutually support one another. In the last production step, the blank is cured in an oven at temperatures of over 100°C.
Audi says the GFRP springs can be precisely tuned to their respective task. The plastic doesn't corrode, even after stone chipping, nor is it sensitive to chemicals such as wheel cleaners. And production requires far less energy than the production of steel springs.
Of course, Audi isn't the only car maker planning to use plastic springs. Here is what Ford told us about its plastic springs back in January at the Detroit Auto Show:
And two years ago, ZF was showing MacPherson struts made from plastic: