Wheel Hubs, '22-'23 Queen's BAJA
I helped design the wheel hubs for the ’22-’23 Queen’s BAJA car in my freshman year of university. You can read about my experience below!
The hubs are fastened into the front and rear axles by inserting a steel thread through the middle of the hub and into the axle, and then fastening the hub with a hexagonal bolt and locking pin, as you can see in the photo below. The bolt and pin prevent any axial translation of the hub and wheel, so they stay where they are supposed to be.
The wheel hubs needed to meet the following design specifications. The loading values were passed down from the previous year’s design teams. It was okay to use these values given that there were not any significant changes in vehicle properties (overall vehicle weight, motor torque, max speeds) and it saved us some hand calculation time. - Each spoke must withstand a radial force of 6000N and a shear force of 4500N at their end points- The hub must be able to withstand an axle torque of 460Nm- The hubs must be able to withstand the above loading in low temperature conditions (-20 degrees Celsius) because of competitions at Winter BAJA in Quebec- The hubs must be machined out of AISI 4130 Steel- Have a mass of approximately 600 grams- Factor of Safety of 2
Iteration
As is typical with student design teams, designs are typically recycled year over year and iterated on. Since ’22-’23 was the first year that Queen’s planned to build a car again post-pandemic, the focus was directed towards getting a working product on the track over making elaborate and complex design progressions. Therefore, at the start of the year, the design goals for each part (wheel hubs included) set out by the technical leads were relatively conservative. For the hubs in particular, the largest challenge was that we no longer had access to the AerMet steel (E = 1723MPa) that was used for the 2020 hubs. As a substitute, AISI 4130 Steel (E = 460Mpa) was the only material that was available to us. Evidently, using a material that was approximately four times less rigid necessitated the 2020 design to be re-profiled.
I worked on this project with my friend Emanuel Piccininni, a (now) fourth year mech-eng student who has a lot of experience with the BAJA wheel package and vehicle dynamics sub team. He gave me a lot of great feedback on some of the iterations that I proposed to him. We began the process by looking at how the 2020 design would hold up if we simply changed the material from AerMet to 4130 Steel. Notably, when loading was applied to the 2020 model, the maximum stresses (pictured in green on the inner portion of the spoke hole cutouts) was super close to the yield strength of 4130. With a near 1:1 FoS, this design was not viable and had to be changed.
After a couple failed iterations, the fourth version of the design featured smaller spoke cutouts, filleted edges to reduce weight, a triangular-shaped pocket cutout shape and shallower curves that join the four spokes. This version passed the FEA loading test with flying colors.
This version had a FoS of 2, a mass of 625g, and was properly sized to connect with the axle and the wheel. The only adjustment that had to be made was to remove the back-side spoke cutout fillets because they could not be machined effectively. You can see the minor change that was made between the two versions below.
Final Design
The final ’22-’23 wheel hub can be seen below. Unfortunately, the team was running behind schedule and so the hubs were outsourced to be manufactured, so I have nothing to report on that front.
Takeaways
Manufacturing Matters – A super promising CAD model that passes all FEA tests and meets all design specifications is not necessarily a viable model! If a design cannot be manufactured, or manufactured accurately, then the design is no good. There were several times during the iterations of the hubs where I made design decisions that looked viable in theory but would never be able to be executed on a CNC machine properly. It is important to understand the limitations of your manufacturing method before you undertake a comprehensive part design.
You Need to Talk to People – One thing that this project really made clear to me is that no two people see a problem from the same perspective. People come from different backgrounds, have different skillsets and experience. By getting a lot of input from Manny and other VD members on problems like how to strengthen the hub spokes or reduce part mass the ‘pool of meaning’ is enriched, and decisions become more well-justified.
You Need to Talk to People – One thing that this project really made clear to me is that no two people see a problem from the same perspective. People come from different backgrounds, have different skillsets and experience. By getting a lot of input from Manny and other VD members on problems like how to strengthen the hub spokes or reduce part mass the ‘pool of meaning’ is enriched, and decisions become more well-justified.