The spoke material may be tested for its fatigue properties using an apparatus of the type shown below. The spoke material is held in a collett and rotated about its long axis.


A force is applied to the free end of the spoke, causing it to bend by a controlled amount. The stress in any small element of the spoke is then periodic as shown in (b). One revolution of the spoke takes the stress amplitude through one cycle. This test is destructive, the spoke being rotated under constant load conditions until it fails.

From: McMahon and Graham, :"The Bicycle and the Walkman," Merion (1992)

The test is repeated for other samples under the same or different load conditions and the number of cycles to fatigue fracture, N, is recorded as a function of the applied stress, S. The data is then plotted on a semi-logarithmic plot of S v. Ln(N), a so called S-N plot.

An important design issue is that there is a wide spread in N for a given S, the failure being statistical in nature and dependent upon the surface condition of the sample.