Manufacturing of the Carbon Fibre Rims
Shell Eco-marathon is a competition to create the most efficient car. In order to be as efficient as possible, there is one factor which is very decisive: the weight. Therefore, we are continuously thinking about how to reduce the weight of our car.
In the end of last year, we decided to build new rims manufactured out of carbon fibre composites to achieve our goal of getting the car weight from 94 down to 90 kg. The material selection of using carbon fibre reinforced polymers (CFRP) was heavily influenced by its material strength. as shown in figure 1 below. CFRP attains a specific strength of approximately 380 kNm/kg, which means high mechanical strength at low density.
The central plate is a sandwich-structured composite consisting of PVC foam and CFR. This composite material provides the required stiffness due to a high second moment of area caused by the 20mm thickness of the foam. The cross section of the central plate can be seen in figure 3 on the left.
The old aluminium rims weigh 2.5kg each. Our target weight for the new rims is 1.5kg while keeping the stability of the old ones. Saving at least one kilogram per rim means an overall reduction of 4 kg which would be a large step towards our weight objective.
Earlier this year, we started our mission with creating a CAD model and simulating the occurring loads. After a long time of experimenting and optimizing, the mechanical team decided upon the final geometry. The team then continued creating the moulds which are needed when manufacturing carbon fibre composites.
The rims are divided into two main parts: an outer ring and a central plate. We are using prepreg technology (0°/90° biaxial prepreg from easycomposites.com) for the outer ring as shown in figure 2 and cut rectangular patches which we are laying around the inner side of the mould. Based on our simulations, we are laying 7 plies with a total thickness of 3mm. After finishing the lay-up, the ring is cured under vacuum at a temperature of 120° in the oven.
In contrast to the prepreg lay-up of the ring, we are using dry fibres cut into equal parts as shown in figure 4 below and a epoxy-hardener mixture. We decided upon the wet lay-up in order to ensure that the CFRP sticks perfectly on both sides of the foam. After applying vacuum, the central plate cures due to the epoxy-hardener mixture at room temperature.
Finally, in order to connect the central plate with the outer ring, we developed a mechanism that places the axle in the middle of the ring. Due to this mechanism, we managed to get the axle perfectly centred so that the tyre will evenly roll and not “wobble” as shown in figure 5 above.
Some days before our unveiling, we finished the first prototype and are very satisfied with the result. Our goal for the upcoming weeks will be the manufacturing of four more rims with identical properties. This will be a challenge working with carbon fibres. The milling of aluminium or steel will impact the deformation of the CFRP. Also the mould material during the curing process will cause thermal expansion. However, we are confident that we will achieve similarly good results.