ZF explores composites for lightweight truck-chassis design

Image: ZF composite 4-point link.jpg

ZF’s “study” of a four-point link made from glass-fiber-reinforced plastic reduces weight by approximately 11 kg (24 lb), or 25%, compared to the 46-kg (101-lb) standard cast component. The program’s target was to reduce weight by 30%, “so we have some more potential” through design optimization, said chassis development boss Holger Bublies. (Image by Ryan Gehm)

ZF is doing its part to help reduce emissions and improve the fuel efficiency of commercial vehicles, namely through its development of advanced transmission systems such as the new modular TraXon automatic transmission for trucks, which includes a hybrid module, as well as with other electrified driveline technologies. But as CEO Dr. Stefan Sommer noted at the IAA Commercial Vehicles Show in Hanover this fall, lightweight design is another major focus of the supplier’s development activities.

Along with optimized design, including the integration of functions for individual components, new materials for heavy-truck applications, particularly in the chassis, are leading ZF’s lightweight charge. One example on display at the IAA show was a “study” of a four-point link made from glass-fiber-reinforced plastic (GFRP) that reduces weight by approximately 11 kg (24 lb), or 25%, compared to the 46-kg (101-lb) standard cast component.

(See http://www.sae.org/mags/aei/7840 for coverage of the supplier’s use of composites in passenger-car chassis.)

“We have one four-point link in serial production for MAN. It is a cast part, and it’s nearly the same dimension, the same function. Our job was to look for more weight savings,” Holger Bublies, Head of Development for ZF’s Commercial Vehicle Chassis Modules business unit, explained to SAE Magazines. Given the weight-savings potential of composite materials, the investigation started there.

“We set out to answer, ‘Is it possible to use this material for this component?’ It is really a harsh function,” he said. The four-point link merges functions for longitudinal and lateral axle guidance as well as for active roll stabilization.

“With this function integration, you can have a solution without a separate stabilizer and stabilizer links; you can save about 50-60 kilos on one axle,” Bublies said.

Another target of the investigation was to examine production methods in an effort to reduce costs. For this prototype fiber-composite part, the process is more manual, but ZF is working on resin transfer molding (RTM) for serial production.

“In the truck business, you need to earn money with your truck. Weight savings is a big point, but cost is an even bigger point,” he said. This mentality explains the usage of glass-fiber reinforcement for the prototype part: “Carbon is a factor of 8 or 10 more expensive,” Bublies shared.

What is an acceptable increase in cost for a composite part to be competitive with an incumbent part? According to information Bublies has gathered, a cost increase of about €5-10/kg of weight reduction could tip the scales in favor of the lighter-weight material. The ZF project currently is still “a little bit higher” than that €10/kg bogey.

“At this really early stage, [the part and process] are not optimized,” he said. “Now after the test results, we have to validate our simulation models. We have to learn many things in this project because to design such a part with this material is a totally new thing. We think it’s possible to decrease weight another some kilos, and we are also working on production.”

Bublies’ hope is that eventually the GFRP four-point link will become a solution offered in truck OEMs’ “super efficiency” models.

“Of course today OEMs have lightweight vehicles, and this could be one more option for them,” he said. “Yes, we are a bit more expensive, but not as much as we thought at the beginning of the program.”

ZF currently has one workshop producing the composite part, and testing continues on the internal ZF program. (The supplier is not working with MAN on the project, Bublies said, but it has shared with the OEM some of the results thus far.)

“The whole technology—composites—is a big item in the ZF Group,” he said. “We have this central development center in Friedrichshafen, and they have their own experts that do this. So we are the business unit; we are the experts on the product, and we have at the central development center some experts for the material, for production, and then we have a joint team working on this program.”

He is not certain when the part will make it to series production, stating “not next year; maybe in five years or so.”

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