Composite leaf springs are not new to the automotive industry. In fact, the leaf spring itself dates back to the horse-drawn carriage. By design, leaf springs absorb vertical vibrations caused by irregularities in the road. Variations in the spring deflection allow potential energy to be stored as strain energy and then released more gradually over time. Composites are well suited for leaf-spring applications due to their high strength-to-weight ratio, fatigue resistance and natural frequency. Internal damping in the composite material leads to better vibration energy absorption within the material, resulting in reduced transmission of vibration noise to neighboring structures.
The biggest benefit, however, is mass reduction: Composite leaf springs are up to five times more durable than a steel spring, so when General Motors (GM, Detroit, Mich.) switched to a glass-reinforced epoxy composite transverse leaf spring (supplied by Liteflex LLC, Englewood, OH, US) on the 1981 Chevrolet Corvette C4, a mono-leaf composite spring, weighing 8 lb/3.7 kg, replaced a ten-leaf steel system that weighed 41 lb/18.6 kg. This reportedly enabled GM to shave 15 kg/33 lb of unsprung weight from the Corvette, yet maintain the same spring rates. The leaf spring was transverse-mounted; that is, it ran across the car’s width at each axle. This eliminated the coil springs that sit up high in a spring pocket on the frame. Thus, the car can sit lower to the ground, which improves car handling.
Today, GM continues to employ transverse GFRP composite leaf springs on the front and back of its Corvette models. The 2014 Chevrolet Corvette Coupe includes a double-wishbone suspension, which, at GM, goes by the name short/long arm (SLA). SLA refers to the fact that the upper control arm is shorter than the lower one. A transverse composite leaf spring presses against the lower arm and spans the width of the car. In fact, the spring is always loaded against the subframe. This design directs shock loads into the frame side, eliminating the standalone rear antiroll bar that must be incorporated into models with standard suspension packages. The spring’s camber curve also is said to improve tire contact with the road during cornering.
Composites also have the potential to replace steel and save weight in longitudinal leaf springs (see “Building a stronger longitudinal leaf spring,” under "Editor's Picks," at top right). These run parallel to the length of the vehicle, providing suspension as an integrated part of the wheel guidance system. “Longitudinal leaf springs have a higher safety factor,” claims Frank Fetscher, head of business development, Benteler-SGL (Ried, Austria), a joint venture of Benteler Automotive and the SGL Group – The Carbon Company (Wiesbaden, Germany, see “SGL Automotive Carbon Fibers opens new plant in Washington,” under "Editor's Picks"). “They can have a linear spring rate or a progressive spring rate — multistage springs — and must perform better with respect to torsion and side stiffness than transversal springs.”
