With each decade, 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 to reverse 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 these efforts. The ZF design delivers a comparable performance, yet weighs about 13 percent less than a multilink rear suspension – the optimal solution from a driving dynamics point of view. What makes this possible above all is the integrated lightweight design approach that ZF also systematically applies in other product areas. For example, ZF has designed a significantly lighter axle drive for the driveline. Innovative airbag housings as well as special pedals add to a sleeker interior feel. The pedals are just one way the technology company has been slimming down volume-produced cars for some time – along with its lightweight shock absorbers, suspension control arms, and brake systems.
Alternative materials, new designs, optimally adjusted tolerances, and less jointing material during production are examples of and keywords for strategic approaches to automotive lightweight construction. The number of kilos really begins to drop when multiple lightweight design strategies are combined at the same time. For this to be feasible in car chassis, design engineers have to take a completely new look at the overall axle system in consideration of 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 which is 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 levels of 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 easily. The weight of the axle is reduced by approximately 6 kilograms, which corresponds to 13 percent compared to a multilink rear suspension. From a driving dynamics point of view, however, 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, for instance, to produce springs with various levels of stiffness.
New lightness for driveline and interior
ZF demonstrates what additional potential is available in power transmissions with a new lightweight design axle drive: Its optimized aluminum housing which is divided lengthways features varied wall thicknesses that are even more precisely geared to the real stresses that occur. Together with lightweight design features on the bevel gearset and a lighter differential case, these improvements add up and save about 5 kilograms, which means that the ZF axle drive is roughly 20 percent lighter than a current series solution. The high quality strength, endurance, and acoustics remain the same.
There's another innovation concealed in the instrument panel: The housing of the new passenger airbag module from ZF TRW is made of a special high strength airbag fabric instead of the usual fiber-reinforced plastics or metals such as steel or aluminum. This makes the entire assembly up to 30 percent lighter. And along with the material substitution of the passenger airbag housing, the engineers re-designed the module and sub-system concept in a way that, when the cushion deploys, the airbag has the same high robustness and restraint performance as the latest conventional modules.
Slimming Down in Volume Production
Volume production with the above lightweight concepts has not yet started, but other weight-optimized components from ZF are already implemented in manufacturing very successfully.
For example, a lightweight damper weighs 25 percent less, because its aluminum reservoir tubes are only reinforced where the load profile requires. Other areas are designed very thinly.
Recently, ZF also started volume production of a lightweight design control arm named SMiCA (Sheet Metal integrated Control Arm). The control arm with integrated ball joint 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.
When it comes to disc brakes, ZF TRW has several lightweighting solutions. For sliding calipers, there are well-proven aluminum housing designs as well as Thin-Wide Bridge (TWB) calipers that, compared to conventional designs, enable both a housing mass reduction of around 10 percent and an increase in fitted disc diameter of around 10 percent. FBC calipers, like TWB, provide mass reduction and increased fitted disc diameter by joining a steel or nodular iron bridge to an aluminum cylinder housing. For fixed, opposed piston calipers, ZF TRW offers conventional designs, this means two-piece housings in nodular iron or aluminum, which are bolted together or monobloc designs with one-piece aluminum.
The ZF TRW brake booster in "Girvac" design offers similar advantages. Its central characteristic is a special tie-rod design that not only improves performance, but also allows a much thinner and lighter housing than usual with a typical weight reduction in the 20 percent range. The innovative components can optionally be fitted with a Fixed Seal Master Cylinder for further weight savings.
Also without cutting back on robustness, a brake pedal made of fiber-reinforced composite (FRC) weighs approximately 50 percent less than a standard steel brake pedal. FRCs are continuous, fiber-reinforced thermoplastics that boast extremely high material strength and rigidity. They are also fully recyclable.