Carbon-fibre reinforced sheet moulding compound in brakes: design and simulation of a lightweight motorcycle brake calliper

Brake callipers are generally produced by casting, forging or milling; typically in aluminium or spherical graphite iron. With the current shift towards electric propulsion systems, the load spectrum of brake callipers can be reduced due to regenerative braking. Therefore, alternative materials such as carbon-fibre reinforced polymers could be used for brake callipers. Within this paper, a carbon-fibre reinforced Sheet Moulding Compound (CF-SMC) is applied in a novel motorbike front calliper design. Besides ensuring function and stiffness of the system, the main challenge is to provide engineering processes for the structural validation of such a thick-walled CF-SMC component. CF-SMC is commonly used for thin-walled bracings, reinforcements, bodywork and aerodynamic devices. These parts e.g. support a car´s chassis, for example as strut bars, or guide the airflow, as e.g. diffusers, wing elements or endplates. Thick-walled components, however, are not yet established in CF-SMC technology. The complex stress situation of a brake calliper and the exposure to brake fluid, combined with high temperatures, pose a completely new challenge. Hence, the design unites a beneficial flow of forces with efficient manufacturing. Additionally, tests are performed to ensure the chemical resistance of the thermoset matrix to DOT4 brake fluid. Hand in hand with that, material flow simulations, curing simulations and structural FEM simulations determine the optimal way to arrange the ribs, the layers of pre-impregnated carbon-fibre in the mould and the manufacturing parameters. The iterative design process results in a two-piece fixed brake calliper with an expected weight saving of about 20 %, compared to a high-volume sports bike brake calliper. It contains metallic inserts which handle various tasks such as guiding the brake pistons or enabling the transfer of tangential forces via fitting bolts. Currently, the physical prototypes are still in the manufacturing phase and not tested yet. Strength and stiffness are only evaluated by simulative measures, showing also a limited stiffness of the parts compared to an aluminium reference calliper. An AK Master test is planned, to compare the stiffness performance to the simulations and evaluate the material behaviour at high temperatures. Since this CF-SMC has not been used for thick-walled components, its application in a brake calliper is a bold idea. But, considering the increasing ability for regen braking in battery electric and hybrid vehicles, the brake system is facing reduced loads during customer-typical driving cycles. This load reduction could allow using previously uncommon materials. Nonetheless, the challenges related to thermal capability, chemical resistance and structural ability, are still large. The presented work shows novel approaches regarding design and simulation techniques of thick-walled CF-SMC in order to explore the composites boundaries. It shall be seen as an incentive to further exploit the potential of this group of high-performance lightweight materials.