Re: Heres the bike I chose..
Yea diameter is the measurement across the circle. The circumference is the measurement around the circle. I hoped you had a caliper to measure the diameter otherwise it is really hard to guessimate by measureing with a tape.
If you do not have a caliper and did have a measuring tape they use to taylor a suit( flexible cloth type) you could get a close estimate I suppose.
Looking at the picture seems the stock motor mount fits the down tube so thats 1 /18 id guess just like the seat tube.
Still curious on rear wheel hub though.