Lightweight Composite Leaf Spring Suspensions for Plug-in Hybrid and Battery Electric Vehicles

The demand for increasing battery density vehicles to reach longer distance ranges has driven vehicles to very challenging trade-offs in terms of available suspension design package, high spring rates, and lightweight. The utilization of glass fiber reinforced plastics, in leaf spring constructions as well as new axle concepts offer further lightweight potential and, in addition, better integration of suspension functions and less installation space required for electrification

Suspensions springs play a major role in every aspect. The entire vehicle sprung mass rests on these springs, while their compliance cushions the transmission of shocks to the passenger compartment.

For the driver and passengers ,the tuning of spring force and spring rate on the front and rear axles ensures a safe and comfortable driving experience.

Regardless of efficiency, lightweight design in chassis for electrified vehicle platforms (Battery Electric Vehicles (BEVs) and Plug-in Hybrid Vehicles (PHEV))  continues to be of great importance. At the same time, the increasing vehicle weights require higher spring forces and rates and thus, a higher spring mass. Economical, lightweight design succeeds either through the best possible material utilization of the classic steel spring material or through alternative materials such as long fiber reinforced plastics in spring constructions.

GFRP Springs

Glass Fiber Reinforced Plastics (GFRPs) offer enormous light weight design potential due to their advantageous ratio of specific tensile strength and specific modulus of elasticity. Particularly when used as a material for leaf springs, these characteristics of an ideal lightweight material can be utilized in the best possible way. This is always possible when the spring is primarily loaded with tension, pressure or bending. This is because a unidirectional layer structure can be used, in which the load is essentially absorbed by the high-modulus fibers.

GFRP coil springs potential light weight design is substantially reduced as the spring cross-section is primarily stressed by torsion, which requires an oriented layer structure with different fibers orientation. The limiting factor is the transverse tensile strength of the material, significantly reduced when compared to the fiber-dominated strength.

In addition to lower light weight potential, such springs have reduced sag loss resistance and lower durability at high temperatures.

Leaf springs are the ideal application for unidirectional GFRP

Most commonly applied on light trucks and commercial vehicles, the longitudinal leaf springs allow high vertical suspension rates required for big vehicle payloads. Multi-leaf springs can accommodate different suspension rates according to the increasing vehicle loading condition, keeping reasonable suspension frequencies in very broad weight situations.

Tension Leaf Springs (a Mubea Patent)

GFRP Tension Leaf Springs can accommodate in a single leaf design and calculated spring shape several rates as result of the tensioning of the spring longitudinal fibers during vertical suspension travel. This spring design shows also a very smooth transition between different rates resulting in a more consistent ride comfort thru all loading conditions.

"Most commonly applied on light trucks and commercial vehicles, the longitudinal leaf springs allow high vertical suspension rates required for big vehicle payloads"

GFRP springs can be up to 50% lighter than a similar steel spring and reach up to 50kg saving in full size pick-up multi leaf spring applications, making it the best weight-cost alternative for electrified vehicles.

Transversal Leaf Spring Suspensions

An ideal application for unidirectional GFRP. Mainly if the leaf spring takes on further functions in addition to storing the elastic energy for the vertical motion of the vehicle. The integration of the roll control is possible by a double elastic inner bushing. Apart from the macroscopicdesign of the spring, the position and the stiffness of the inner bushings are decisive to reach the required roll-stroke-rate ratio, the larger the support in transversal direction of the vehicle, the higher the roll-vertical ratio.

A further stage in the transversal leaf spring design is to integrate other suspension functions to control the degrees of freedom of a conventional multi-leaf suspension, in addition to vertical-roll control base attributes. A transversal leaf spring suspension with integrated structural dampers replaces the entire multi-link axle system.

The elasto-kinematics can be significantly influenced by the spring dimensioning, the positioning and stiffness of the inner bushings, the design and angles of the spring connection to wheel carrier and damper.

This concept of the function-integral transverse leaf spring offers weight advantages, as all control arms and links can be replaced and the subframe and body structure can be simplified. In addition, potential for cost savings and space gain in the longitudinal direction of the vehicle is possible.

In an electrified test vehicle with rear-wheel drive, this function-integral concept was implemented and validated after a comprehensive Computer-aided Engineering (CAE) and Multi-body Simulation (MBS) analysis.

The kinematics and compliance results as well as the driving assessment conform to the ride and handling behavior under lateral and longitudinal accelerations which is subjectively and objectively in the range of the reference vehicle with a multi-link axle.

Comparable longitudinal accelerations could be achieved even without soft mounted subframe to body, through defined inner and outer rubber bushings compliances.

Toe change targets were achieved through optimization of axial stiffness of outer bushings, furthermore the axle concept was successfully tested using road load durability procedure, reaching 100% of the test with no degradation to its components.

Conclusion

In heavy vehicles such as PHEVs or BEVs. The highlight weight design potential of anisotropic materials such as GFRP can be an optimal solution for weight-cost-space reduction, while achieving similar vehicle behavior as multi-link suspensions and substantially improving ride comfort characteristics on longitudinal leaf springs light trucks and light commercial vehicles.