Rans Enduro Sport, Hase Kettweisel Tandem, Merin Bear Valley beater bike
Well, you have to match whatever the frame of the bike is. One measurement is sometimes worth 1,000 guesses. If you don't have a caliper, wrap a piece of paper around the seat tube, mark where they meet, measure in millimeters and divide by 3.14. As a general rule, I'd say that you are most likely to find a 28.6 seat tube on a steel framed bike.