Join Date: Aug 2005
Location: Pittsburgh, PA
Bikes: '''96 Litespeed Catalyst, '05 Litespeed Firenze, '06 Litespeed Tuscany, '12 Surly Pacer, All are 3x8,9 or 10. It is hilly around here!
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For the vast majority of bikes, the seat tube OD is one of three sizes: 1-1/8" (28.6 mm), 1-1/4" (31.8 mm) or 1-3/8" (34.9 aka 35 mm). Even eyeballing a ruler should tell you which you have.
If there was an old clamp-on fd, look inside the clamp as the diameter is usually engraved there.