What Are Audi's GFRP Springs?

2015 audi a7 ・ Photo by Audi

Audi has played a leading role in lightweight automobile construction, being one of the first companies to make cars mostly out of aluminum. They are now introducing lightweight suspension springs made out of glass fiber-reinforced polymer (GFRP) in one of their upper mid-sized models by the end of the year. While normal springs are made out of heavy (and corrosion prone) steel, the glass fiber-reinforced polymer springs are 40% lighter for a more precise drive.

The GFRP spring looks like a normal spring, but with slight differences. Instead of being painted black like most normal springs, the GFRP unit is light green. The fiber strand is thicker than the steel wire in a normal spring, and the GFRP spring has a slightly larger overall diameter and a lower number of coils. More importantly however, a steel spring weighs nearly 6 pounds while the glass fiber-reinforced polymer spring weighs in at 3.5 pounds. As a set of four, weight is reduced by 9.7 pounds, half of which is unsprung mass.

To fabricate the springs, first a thin core of long glass fibers only a few millimeters thick are twisted together and impregnated with an epoxy resin. A machine then wraps additional fibers around the thin core, alternating between angles of positive 45 degrees and negative 45 degrees to the longitudinal axis. The positive and negative angles create tension and compression plies that support each other and absorb the stresses acting on the spring. The last step is to cure the spring in an oven at temperatures over 212 degrees Fahrenheit.

The glass fiber-reinforced polymer springs can be precisely tuned, the material is impervious to chemicals such as wheel cleaners, and it does not corrode, even after stone chipping. Production of GFRP springs also requires signficantly less energy than the production of traditional steel springs.