ZF is looking into new material options for chassis designs. At the Frankfurt Motor Show, the company revealed studies that use fiber composite materials to bring down the weight of the rear axles and dampers.
Modern chassis have to support driving safety and provide a dynamic driving feel, all the while enhancing comfort. These demanding requirements have taken their toll; chassis have put on weight over the years as the number of parts has grown and active components have been integrated.
ZF has developed a rear-axle concept reconciling this conflict through the use of new materials and functional integration. This compact rear axle features a wheel-guided transverse leaf spring made from glass fiber-reinforced plastic (GRP).
“This technology is ideally suited for vehicles with high demands on competitiveness,” said Dr.-Ing. Gerhard Gumpoltsberger of the project’s advanced engineering team in Friedrichshafen. “Economy is assured by functional integration, alternative materials, and a slimmed-down manufacture and assembly process. The axle concept has a low number of structural elements and offers a weight benefit even when compared to rear twist beam designs.”
“Despite this, the axle characteristics are close to a multilink axle,” Gumpoltsberger continued. The GRP material of the spring is very stiff in longitudinal direction, but active in vertical direction. By supporting the spring with two pivot points, wheel dynamics are decoupled, and there is no torsion of the spring. Proper wheel location is ensured by two metal struts.
“One of the key goals of this study is to optimize the design for mass production,” said Gumpoltsberger, as this is the real challenge of integrating fiber composite materials into suspensions.
The second ultralight chassis component prototype shown at Frankfurt is an upside-down monotube McPherson damper with integrated wheel carrier made from carbon fiber-reinforced polymer (CFRP). It combines two benefits: The upside-down damper design facilitates low weight damper rods as the damper tube supports the lateral forces. By combining this with a wheel carrier made from CFRP, the total damper and wheel carrier module weighs only half that of a current lightweight aluminum design.
The CFRP structure is a result of finite element analysis: By starting with the load paths of the wheel carrier, the load bearing braces are fully aligned with the thrust vectors.
“Currently, the CFRP structure of our prototype is only loaded to around 30% of its capacity. Our target is to select a material that is loaded up to 75%,” said Dipl.-Ing. Hendrik Marquar of the ZF Sachs development team.
“In addition to its weight advantage, the prototype design includes a plastic bellows spring that increases NVH as it damps vibration much better than steel,” Marquar added. “To support crash diagnosis, the fibers of the wheel carrier can be woven with a signal fiber that emits an electric signal under overload just like a strain gauge. Active safety systems could also use this information as the signal fibers instantaneously detect which loads are effective in what direction.”