I am replacing a FD on my Trek 2100. The seat tube outer diameter is 38.1 mm but when I look at FD sizes I see options for 28.6, 31.8 and 34.9. I guess I'm measuring the tube incorrectly.
Is there any way to tell for sure? I guess 34.9 mm is only 3.2 mm shy of my measurement and that could be a measuring error on my part - 3.2 mm is fairly small and the tape measure I was using isn't very compliant for measuring a small round tube.