Using common wheel truing stands you have to find the biggest error by visual judgment. Usually you use calipers while turning the while to find a lateral rim run-out. If you have difficulties to find the problematic rim position with naked eye it might help you to move the calipers until one side begins to lightly scrape the rim. However this method is not possible if you turn the wheel slowly (you cannot hear the scraping), but by turning the wheel fast it is not easy to find the position.

That is why I started to use electric conductivity to measure the distance between caliper and rim. This post shows a first and very fast prototype to test the idea. I am not sure yet how I want to continue. I think it would be nice to have a very compact tool which can be mounted directly on the bike (fork or frame) or to build a stand for all available sizes of hubs. If everything works fine, I want to try moving the calipers using small stepper motors or servos. A micro controller could assist unexperienced mechanics.

As first test I used:

• fork
• wheel
• button cell
• transistor
• LED
• screw and screw nut
• a piece of wire

Two cables are connected to the circuit. One is connected to the screw (touching eventually the rim) and the other one is connected to the fork. The transistor makes the LED light up bright also if there is a high resistance between the two cables. I can turn the screw slowly until it touches the rim and the LED lights up.