Sebastian Ochoa - MY25 Hubs

My role this year is to design the wheel Hubs for the MY25 car using Siemens NX. As we're attempting four wheel drive for the first time, the hubs are a completely new design. To design this, I've had to conduct Static, thermal, and fatigue analysis. The slides from my First design review can be found here.

The MY25 Wheel Hub In the main assembly.

As always, we start off with a wheel package space fill to determine where we can add geometry.

This year, we're pursing in hub gearboxes to enable four wheel drive, so my hub must fully enclose and seal the gearbox off from the outside.

Once I had an idea of what the hub would look like, I moved onto drawing free body diagrams to derive the bearing and lug reaction forces from the contact patch forces.

I conducted a sweep across our traction ellipse in order to determine my driving load case, which ended up being maximum steady state cornering.

I then move on to determining my constraints. I knew that I would end up using Aluminum 7075-T6 as it has the best strength to weight ratio of any easily machinable material. However, I know that the gearbox will reach temperatures up to 100 degrees Celsius, so I have to apply a thermal knockdown as Al 7075 weakens with long exposure to high temperatures.

I then moved on to creating shear and moment diagrams using the derived forces. These allow me to calculate the stress throughout the hub.

I then moved on to analyzing the stress concentrations along the hub. Due to features such as pressfits and shoulders, there's a lot of stress concentrations, most of which are ~1.6.

Once I know the forces throughout the hub, I can calculate the stress along the hub using the Von Mises Stress Criterion and compare it to the tensile yield of Al 7075 to derive our factors of safety. The maximum stress ends up being at the outboard bearing, where we have a press fit creating a large amount of stress. The ideal factor of safety on our team is 1.5, however in order to preserve manufacturability of the hub, I'm forced to have a higher factor of safety.

As the hub is constantly rotating when driving, it sees large cyclic loading. As such, I analyzed its fatigue strength using the Stress-Life Method. I derived the cycle count from our expected testing mileage, and calculated the first principle stress at various points on the hub to determine a stress reversal rate, which ends up being around 0.5. From there, I can use an S-N curve found on MMPDS for a notched Al 7075 rod with a similar stress concentration to the hub, and compared the fatigue strength to the stress found in the hub. I found we had a factor of safety of around 1.8 for most of the hub, however the wheel tabs failed in fatigue, so I had to add more material there to reduce the stress.

My next step was to CAD. I created the model using Siemens NX.

These wheel tabs attach directly to our rims, and capture the head of the bolt to make changing wheels easier.

Later on, I changed my tab geometry to minimize the hub's camber deflection by increasing the tab's bending stiffness. I also switched from an AN bolt to a NAS bolt due to higher loads.