This thomson oversize seat clamp is for saddles with carbon rails as they tend to be oval and larger 7 x 9 mm and 7 x 10 mm being the common over sized rail dimension there are also downhill and bmx saddles with round metal rails at 8 and 9 mm so if you have rails larger than 7 mm do you need this seat clamp maybe maybe not thomsons standard rail clamps can accommodate rails up to 7 x 10mm but riders using their setback post with over sized rails may have issues getting the saddle nose