The rear axle of the new Volvo XC90 features a transverse leaf spring made of lightweight composite material.
The rear axle of the new Volvo XC90 features a transverse leaf spring made of lightweight composite material.

Benteler-SGL, a joint venture of Benteler Automobiltechnik GmbH and carbon fibre supplier SGL Group, is aiming to produce several hundred thousand of the leaf springs annually.

The polyurethane matrix resin used is Loctite MAX 2 from Henkel, which worked with Benteler-SGL to develop the resin transfer moulding (RTM) process for the leaf spring. Because of its low viscosity, the polyurethane resin rapidly fills the mould and quickly impregnates the fibre reinforcement, resulting in short injection times. With the curing rate also substantially faster than that of epoxy resins, cycle times are shorter overall. Selected additives also make the curing process faster and more flexible.

“Within the development network comprising ourselves, Benteler-SGL and Volvo Cars, the component manufactured with our polyurethane resin was subjected to extensive automotive-specific testing and validation,” reports Frank Kerstan, Global Program Manager Automotive Composites, Henkel Adhesive Technologies.

“Applying our expertise in process optimisation, we were able to demonstrate that this polyurethane technology could satisfy all the criteria of long service under environmental influences such as temperature and humidity.”

Rear axle concept

The suspension of the Volvo XC90 is based on a transverse fibre-reinforced composite leaf spring instead of the usual array of coil springs. This results in a very compact design as well as a weight saving of some 4.5 kg compared to the conventional concept.

Further benefits include a smoother ride and improved NVH (noise, vibration, harshness) behaviour. The trunk volume is also increased because there are no suspension turrets.

The new rear axle is destined to be incorporated into additional models in the years to come, leading to several hundred thousand vehicles being fitted with it every year.

“The example of our leaf spring for Volvo Cars demonstrates that fibre composite mass production is today already a reality," says Frank Fetscher, Head of Business Development at Benteler-SGL Automotive Composites.

"What’s more, with this flexible component design, we are able to cover an entire platform with a relatively small number of spring variants. Having the right partner in Henkel, we have been able to build on our expertise, enabling us to further optimise the level of automation applied in our composite manufacturing operation."