Over the past several decades, car chassis have continuously become safer and more efficient. However, higher payloads, increased driving stability, and better dynamic driving properties also often mean more complex axles with increased weight. ZF has been working toward reversing this trend for a number of years. The newest version of a wheel-guiding transverse spring made of glass-fiber reinforced plastics (GRP), which ZF has integrated into a lightweight rear axle for passenger cars, bears evidence of our efforts. The ZF design weighs about 13 percent less than a multilink rear suspension – the optimal solution from driving dynamics point of view. This weight reduction with comparable performance is made possible primarily by the holistic lightweight design approach rigorously implemented at ZF. ZF has been weight-optimizing individual chassis components for years: lightweight shock absorbers and control arms are available in volume production applications and contribute to weight reduction in passenger cars.
Alternative materials, new designs, optimally adjusted tolerances, and less joining material during production are all strategic approaches to automotive lightweight construction. The number of kilos really begins to drop when multiple lightweight design strategies can be combined. For this to be feasible in car chassis, design engineers take a completely new look at the overall axle system to consider lightweight design aspects. This is precisely what ZF did with its concept for a lightweight rear axle for passenger cars. The central component is a wheel-guiding transverse spring. This spring is made of GRP, a material with significantly less mass than steel, the material predominantly used in passenger car axles. In addition, during the development process, ZF engineers integrated a high number of classic axle functions – such as suspension, stabilization, and wheel guidance – into this new component. Part of the GRP spring also assumes the functions of a chassis control arm, thus reducing the total number of individual components used for the axle. In addition, the transverse spring can be designed for various stroke and roll stiffness, a requirement for use in passenger car models with varying chassis characteristics.
Highly integrated and lightweight without sacrificing driving dynamics
Using this highly integrated component as a starting point, ZF designed a McPherson-type rear axle, to which a trailing link, a toe link, and a wheel-guiding damper are added as additional components. This reduces the complexity of the rear axle, which now consists of fewer components and can be assembled more efficiently. The weight of the axle is reduced by approximately 6 kilograms, which corresponds to 13 percent compared to a multilink rear suspension. Furthermore, from a driving dynamics point of view, the ZF lightweight axle displays the same high level of agility that characterizes the multilink rear suspension for passenger cars.
In order to produce the transverse spring on an industrial scale, possible process technologies for volume production were analyzed at the ZF Composites Tech Center. The manufacturing process currently preferred makes it possible to produce springs with various stiffness, for instance.
Lightweight design in volume production
While the concept of the lightweight axle with wheel-guiding transverse spring waits for its volume production application, ZF has already brought a variety of other weight-optimized components into volume production.
For example, a lightweight damper weighs 25 percent less, because its aluminum reservoir tubes are only reinforced where the load profile requires it. Other areas are designed to be very thin.
SMiCA (Sheet Metal integrated Control Arm) proves that ZF is already offering mature lightweight solutions. The control arm requires no rivets or screw connections and is therefore up to 23 percent lighter than conventional sheet metal control arms. Moreover, it requires less installation space and allows for more flexibility during chassis design.
A brake pedal made of fiber-reinforced composite (FRC) – developed and produced by ZF – weighs approximately 50 percent less than a standard steel brake pedal. Fiber-reinforced composites are continuous, fiber-reinforced thermoplastics that boast extremely high material strength and rigidity, and are fully recyclable.