This year we are changing from aluminium 3-piece wheel shells to 3-piece carbon fibre wheel shells. The 3-piece design is chosen due to the ease of manufacture and tyre change accessibility. The manufacture process is made easier with the use of carbon wheel shells made with two separate moulds and one aluminium wheel centre. A 2-piece rim (i.e. one carbon shell and one aluminium centre) would require a complex mould and an even more complex layup sequence of the carbon fibre.
Since the wheel package is all unsprung, as well as a rotating mass, the weight reduction was crucial. Improving the stiffness offered by aluminium was another important design goal. The profile of the wheel was chosen so that it complied with the chosen tyre specifications (Hoosier R25B), whilst also giving the best load path from the wheel shells to the wheel centre. The profile that proved to deflect the least according to FEA results had the largest possible radius, which ultimately decreases suspension compliance and helps maintain the set suspension geometries while driving under load.
After finding the desired profile of the two carbon fibre shells, the layup sequence was determined. With the help of ANSYS composite packages, we were able to deduce the optimal thickness and fibre orientations. During analysis, the importance of symmetrical layups for improved load distribution became obvious. More layers were applied around the radii to further reduce deflection due to the out-of-axis loading nature of wheels.
An overall reduction in wheel shell mass is estimated at around 650g. This figure is close to half the total mass of their aluminium predecessors, thanks to the use of carbon fibre and an understanding of how its beneficial properties can be utilised to our advantage.
Cameron MacFarlane, Suspension Engineer