Aerodynamic Wheel Jackets

Overview

Wheel jackets form an integral part of our Solar Car's aerodynamic package. They are close-fitting covers that hug the wheels at less than 10mm clearance, containing the turbulent and energetic air kicked up by rotating wheels, reducing internal flow and thus aerodynamic drag. Every top challenger-class team runs wheel jackets, and we estimate its effect on reducing aerodynamic drag to be in the magnitude of ~10%, although this number is very contentious.

For context, the clean-simulated weighted (straight-line and sailing) aerodynamic drag coefficient area of SC4 is under 0.1CdA (exact number is a team secret sorry!), although like every team, we expect there to be at least a 20-30% increase in real-world drag due to manufacturing, surface, and internal flow effects

Constraints

I inherited the responsibility for the wheel jacket system late into the project iteration, by which a geometric design had already been locked down, and the tooling plug was already machined. I saw through mould production, laminate design, manufacturing, fitment, and geometric modification under the following less-than-ideal constraints:

Within the constraints, I needed to work with a 3mm core thickness, whereas other teams typically ran 10mm of outwards-building core. Thus, I optimised the laminate for stiffness within that envelope, using a global ±45° layer to carry shear loads through the mounting, directional fibres with foam core ribs adding stiffness triangulating the outer mounts, de-coring and epoxy backfilling central mounting regions for local stiffness, and, after on-road high-speed buffeting, adding stiffening L-brackets tying the jacket into the suspension upright to expand the supported region.

Manufacturing

The wheel jackets were made by a vacuum-consolidated wet layup into a fibreglass mould. This was our team’s first production fibreglass mould, taken off the machined surface-prepped foam plug.

Wheel Jacket mould production and final mould being release coated

The laminate design was optimised for global stiffness. A global +/- 45 layer carried shear loads through mounting, while directional fibers with foam core ribs provided stiffness reinforcement in directions of interest. Local stiffness was achieved through de-coring and epoxy backfilling at the central mounting points. On-road testing also saw the addition of stiffening L-brackets to stabilise the top of the wheel jacket with the suspension upright.

Fitment

Installation of the wheel jackets involved mounting and fitment onto the suspension uprights, interfacing around brakes and brake lines while maintaining precise positioning for adequate wheel clearance.

This involved heavy use of templating in the field, as well as plenty of hand sanding; all of this was done in an outback mobile workshop with very limited resources. Due to the lack of adjustability in the mount design, the fitment must be perfect.

Result

This was our team's first attempt at Wheel Jackets, built with minimal resources and experience, working around many mistakes we made early in the process. We produced and integrated low-clearance wheel covers that held for our full race distance. A future improvement on this system has clear pointers: distributed mounting and better mould design that allows for the right thickness, with proper interfacing with the brake system.