Mountain bike hubs: how do they survive downhill world cups?

The UCI mountain bike world cup wrapped up in Mont Saint Anne this past weekend in a nail biting, bike smashing, classic Canadian finale. It is a high-speed track with many rock gardens and sharp edges that took their toll on riders and equipment throughout the weekend. In the men’s race the young Canadian Jackson Goldstone came out on top with this FPV drone capturing his skills and pace.

This next video captures the sort of forces which the hubs, spokes and rims are subjected to at this level of competition. The skills of the riders allow them to negotiate the course at high speeds placing huge demands on their equipment.

Failure of the equipment, specifically the wheels, can be catastrophic and was characteristic of much of the weekend. These two riders in the men’s junior finals highlight the difficulties and race ending consequences of equipment failure.

We recently undertook a project at the Sports Engineering Research Group to measure the strength and stiffness of mountain bike hubs to help a local manufacturer to understand the performance of their project. KOM ltd recently entered the mountain bike market with their first product the XENO hub. This hub has been designed to maximise the diameter of the main axle, increasing its stiffness to minimise deflection.

To make comparative measurements of rear hub stiffness, we developed a test rig to enable a range of rear hubs to be loaded and the resulting deflections measured. Using an industry standard Instron testing system the hubs were mounted in our custom fixture and a vertical load applied centrally with a force representative of that transmitted through the wheel under normal use. The fixture supported the hub in a manner that simulated the hub clamped in the dropouts of a bike frame, allowing the system to flex while maintaining the clamping action of the bolt through axle.

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Test Method

A 4kN vertical load was gradually applied, shown by the blue arrow in figure 1, causing a vertical deflection of the hub which was measured at the point of force application. This deflection is illustrated in figure 1b as the change in position of the circular upper fulcrum. In some cases, the applied load resulted in an angular deflection of the hub which we measured across the spoke flanges, illustrated by the green arc in figure 1b. The support fixture was designed to permit this rotation. The maximum deflection of each cycle was measured with a linear dial gauge at a known distance from the fulcrum then the angular deflection was calculated.

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Each new boost through axle rear hub, 148mm x 12mm as detailed in Table 1, was clamped into the fixture with a consistent torque, 8 Nm, then placed into the compressive test rig and aligned centrally with a reference laser. The load was ramped up from 0.1kN to 4kN and back down to 0.1kN and was repeated 6 times in total. The vertical deflection was measured throughout, and the angular deflection measured at maximum load. The mean and standard deviation of the maximum linear and rotational deflection over the six compressions was then calculated.

To measure the inherent stiffness of the fixture and measurement system, a reference measurement was taken. A solid bar with the same dimensions as a hub (length, flange diameter and bore diameter) was machined and subjected to the same test method.  The deflection of the hub was then calculated by subtracting the fixture deflection.

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Results

The stiffest hubs produce the smallest deflections under load. The maximum deflection results are tabulated below with the standard deviations indicating very consistent results over the 6 compressions.

The linear deflection of the hubs is represented graphically in Figure 4, comparing the range of performance across the selected hubs. The DT Swiss hub exhibited the greatest deflection with the KOM hub displaying the minimum deflection.

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Similarly, Figure 5 graphically represents the angular deflection of the hubs at 4 kN, the KOM hub deflects a tenth of the magnitude of the other hubs.

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As the linear deflection of the hubs was recorded throughout the application of load, we are able to plot the effect of that load on the hubs. Figure 4 graphically compares the linear stiffness of each hub model during the second compression cycle by subtracting the deformation of the reference. The steeper the curve, the stiffer the performance of the hub, and each model deforms near linearly throughout the range of load.

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Conclusion

To differentiate the performance of a range of hubs under simulated riding loads we developed a test method to measure their linear and angular deflection.  A custom fixture was created to allow the hubs to be held in a Instron test system which allowed the hubs to flex in an expected manner, as if supported within a frame. The load on each hub was cycled six times between 0.1 to 4kN measuring linear deflection continually and the angular at maximum load.

Stiffer hubs will deflect less under load and the results showed a range of performance of the hubs, from 1.02mm down to 0.36mm linearly, with the KOM hub deflecting the least. The largest differences were seen in the angular deflections with the KOM hub deflecting 0.02° and the others between 0.22° and 0.24°.

The Sports Engineering Research Group work with many regional SMEs to conduct R&D projects helping to understand develop their products. This particular project was funded through Sheffield Innovation Program which ended in June 2023. However, it is hoped that subsequent funding to work with similar regional companies will be forthcoming.

To learn more about nicks work, if you would like more information or to conduct similar research check our his SHU profile or contact n.hamilton@shu.ac.uk.