Originally Posted by
smontanaro
you supply the ruler.
I've seen a YouTube video where a similar style tool is made. In the demonstration segment, the guy used a ruler to measure the distance to the rim. Given the radius of most bike wheels, I'm sure that will be accurate enough. If you need to hold the ruler in place, a bit of tape or a rubber band should suffice.
I'll try and remember to report back when I've used mine. I have a Motobecane with a slightly bent hanger.
From what I see, the bolt supplied with that tool will clamp it on the hanger tight so that you won't be able to rotate it. What you need is a shoulder bolt that screws down tight to the dropout while allowing the tool shaft to rotate (and without too much slop). I reckon you could tighten the bolt, measure, loosen the bolt, rotate the shaft, tighten the bolt, measure ... i.e. a big hassle.
For a bit more money, you could buy something
like this that should be more functional.
Good luck to you in any case. Feedback about tools from real users is valuable to all of us.