Front derailleur clamp diameter
 

i want to buy a new compact front derailleur for my lemond sarthe but i'm not sure what clamp diameter i need. is it 31.8 or 35. thanks.

measure the diameter of the seat tube in the area where the derailler will be mounted. Better yet look at the current derailleur and use the same size. My guess is that you have a 31.8 clamp size.

Your current derailleur should have the diameter stamped on the inside of the clamp or the braze-on adapter clamp. BTW, 31.8 mm is 1-1/4" and 35 mm (actually 34.9) is 1-3/8" so measuring with a ruler or calipers should be fairly simple.