As with many motorsports design and engineering departments, the University of Glasgow’s Formula Student Team, UGRacing, splits design work involving the unsprung mass (everything not supported directly by the suspension) into an independent sub-team away from the sprung mass development teams. Over the past three years, UGRacing’s unsprung mass team has been on a mission to reduce the weight of the unsprung components to improve multiple areas of performance. UGRacing found that traction, steering responsiveness, and wheel acceleration were the three significant beneficiaries of low unsprung mass (USM).
One of the most significant components in the USM assembly is the wheel. In 2020, UGRacing decided to move to OZ Racings’ 10″ Centrelock wheels. These magnesium racing wheels are significantly lighter than most other Formula Student-style wheels, weighing only 1.6 kg per rim, resulting in an instant 600-gram reduction in each corner assembly. The centre locking geometry required a redesign of the wheel hubs.
The wheel hubs in the UGRacing UGR20 racing car attach to the rest of the unsprung mass assembly. A retaining nut keeps the hub inside the wheel bearings and provides preload to the bearings. As unsprung mass is one of the areas of Formula Student that is almost entirely unrestricted in terms of design, UGRacing encouraged concept sketching and planning at the initial stages of the car’s design to ensure optimal overall assembly configuration.
After the initial concept stage, the team created CAD prototypes to develop the most promising sketched ideas further. This method allowed the team to estimate the mass of components relatively quickly, one of the leading indicators of whether a concept is worth developing further to the detailed analysis stage. The figures below compare the previous generation wheel hub to the new CAD concept that the team chose to develop.
Initial concept drawings of hub.
The key objectives of the new wheel hub are as follows:
‘bolt on’ feature.
The four holes within the central ‘ axle ‘ are shown above. These threaded holes allow a constant velocity housing to attach via fasteners rather than integrated into the rear hub. The knock-on effect is a smaller wheel hub and wheel bearings, reducing the size of the upright supporting the wheel hub and the weight of the USM. It also reduces the necessity of having a different geometry for the front and rear wheel hubs. Implementing this allows the UGRacing team to have all four wheel hubs almost identical, reducing manufacturing complexity.
Straight axle.
UGRacing team previously implemented a step in the wheel hub, requiring two different sized wheel bearings. After reflecting on the 2019 design, UGRacing found this of no benefit. It improved the ease of assembly, as a step in the axle meant the two bearings could simultaneously press onto their final seat. However, removing the step indicated that all eight of our wheel bearings could be of the same type, reducing the mass, complexity, and USM assembly cost.
Centrelock feature.
With the OZ Racing wheels no longer requiring four bolts to mount the hub to the wheel face, UGRacing dramatically reduces the complexity of this design. UGRacing chose to have a central main thread for a centre locking nut to clamp the wheel onto the hub, and then three short locating studs to keep the wheel in place until the nut is torqued up.
The resulting hub design.
Due to the high tolerance nature of the wheel hubs and the uprights, UGRacing attempted multiple iterations before both designs progressed to their final stages. The team made this easier by using Fusion 360 for simulations. The speed and ease with which the team could simulate the loading of parts made iterating designs much more effortless. UGRacing simulated the hubs for maximum cornering, braking, and a bump force, adding a load case of engine torque to the rears and incorporating these in the wheel hub assembly in Fusion 360. All forces transfer through the wheel face, and the wheel bearings are considered fixed points, allowing the hub faces to rotate but not move axially.
The maximum stress on the hubs of 322.9 Mpa gives us a factor of safety of 1.56. Although UGRacing aimed to replicate the loading as practically as possible, a significant safety factor allows the team to account for differences between the actual world loads and the simulated. From experience, the team has found a safety factor of 1.2 to be the minimum appropriate for the Formula Student application.
Alongside all the new features implemented in the hub design and the knock-on effects these had on the uprights, brakes, and unsprung mass, the final weight reduction of the USM assembly was 10.75 kg. That accounts for roughly 5% of the weight of the 2019 assembly, and an increase in rotational acceleration of 67% meant our 2020 car could accelerate, brake and steer much faster and more accurately.
Castle Precision Engineering manufactured the wheel hubs, and various components were also aqua blasted by Castle to provide some corrosion resistance to the unsprung mass parts. The final wheel hub design is pictured above, with anodised Centrelock nut manufactured by Hague Fasteners.
