Vehicle dynamics & chassis: A new leaf
ZF is developing composite leaf springs. Weight and cost savings are expected, together with better chassis dynamics
- Published in Focus.
Vehicle dynamics: ZF is developing glass fibre-reinforced composite leaf springs
ZF’s proposed suspension is designed around a single transverse leaf spring which, together with two longitudinal and two transverse links, locates the hub carriers and replaces the two coil springs and the anti-roll bar featured in a conventional multilink or MacPherson strut suspension. The ratio of opposite wheel travel spring rates at the contact patch and the parallel spring rate is 1.3-2.0.
The springs are made using hot compression moulding from thermosetting plastic reinforced by glass fibres.
The process gives better process control of fibre content and orientation than filament winding, but the springs are still difficult to design, develop and test because loading is complex.
“There’s the interaction between kinematics and elastokinematics, and then there’s the springing function and the anti-roll bar function so you have to use special design tools to get it right,” says Fruhmann. “Fatigue life is a big challenge, especially under environmental conditions.”
This is where torsion beams are attractive to OEMs: although the beam assembly is highly stressed, it’s a big, robust steel fabrication. Replacing that with a slender composite bar won’t be straightforward.
But fatigue performance is being assessed, especially under extremes of temperature and humidity, for example, and with stress raisers from surface damage such as chips caused by stone strikes.
“We’re running lots of tests with different environmental conditions and different levels of pre-damage so that we can find out how scratches and cracks propagate within the spring and how many kilometres they’ll last for,” says Fruhmann.
There are also complete suspension system rig tests, started in October, using data gathered on rough roads and handling tracks. Prototypes are also running in test vehicles.
There are no contracts in place yet but ZF is talking to its OEM customer base about developing the technology further – the supplier thinks that it could be ready for series production within five years.
OEMs are likely to welcome the potential weight and cost savings but will be naturally cautious when considering replacing something as simple and well-proven as the torsion beam axle with a composite spring.
Stringent testing and validation will be required, but Fruhmann says that new methods will be needed first: “You can’t use the same test procedures that you use for steel parts and we have to do this together with the OEMs – neither can do it alone,” she says.
Many expected Opel to adopt multilink with the latest Astra but it decided to optimise its torsion beam instead. With the growing pressure on OEMs to cut weight, and given the growing volumes in the C-segment, ZF’s plastic leaf spring could have a big future once its durability has been proven.
