Why not just bond a strain gauge to the spoke. Assuming you can find Youngs modulus and diameter, you should easily find the strain. Strain gauges cost $5-$10, and a bit of effort to set up the bridge circuit. The advantage will be negligible change in mass, so if the wheel is running at high speeds, centripetal acceleration wont have the effect that an extensometer may have.
What are you trying to measure strain for?