Vehicle dynamics & chassis: A new leaf

ZF is developing composite leaf springs. Weight and cost savings are expected, together with better chassis dynamics

OEMs want to reduce weight from every single component in the vehicle because it reduces fuel consumption and therefore CO2 emissions. And lighter vehicles can be more fun to drive, especially if you can save unsprung mass from the suspension.

The general trend is to substitute aluminium for steel. But ZF wants to use plastics.

The supplier is developing glass fibre-reinforced composite leaf springs which offer a 10% weight saving compared to the equivalent torsion beam axle, as well as better dynamic behaviour.

Composite leaf springs aren’t a new idea. Liteflex in the US has supplied them for Chevrolet’s Corvette sportscar since 1980, and IFC Composite in Germany makes them for Daimler’s Sprinter van. The difference is that ZF wants to integrate a wheel guidance function – they’re structural parts.

The first prototype was shown at the 2009 IAA show. The target market is the C-segment. “At the start of the project there was the task to find a suspension equal in cost to a twist beam rear suspension but showing a functional improvement,” says project leader Gabriele Fruhmann. “We said we’d start with the compact car class because there’s a transition here over to multilink designs.”

While models such as the Ford Focus, Volkswagen Golf and Kia cee’d use multilink, the Opel Astra, Renault Mégane and Citroën C4 all still have simpler, cheaper torsion beam axles.

The torsion beam has an advantage over multilink in that it has fewer parts – the anti-roll function is performed by the beam itself – and can be lower in friction, but a weakness is the attachment method. They attach to the underbody with two large rubber bushes. These are designed to offer the best compromise between lateral stiffness for handling and longitudinal compliance for ride comfort but there are weaknesses.

Under cornering, side loads tend to cause the axle assembly to twist about the two bushes, causing the wheels to toe-out: “Our suspension is stiffer and exhibits toe-in characteristics,” says Fruhmann.

tags: Dec 2010 ZF Chassis