Dear CRs:
I have a "theory" regarding the disadvantages of clamp-on front derailleurs, as commented on by Chuck Schmidt (below):
I have seen seat tubes with small dents caused by clamp-on front derailleurs and I have seen lots of seat tubes with rust underneath the clamp when it chips the paint.
Chuck Schmidt South Pasadena, California
I suspect that the cause of vintage front derailleur problems (scraped paint, dented or rusted seat tube) is actually caused by a minor misfit between the front derailleur (inner) clamp diameter and the seat tube's outer diameter .
Consider that Campy made the NR seatpost in something like ten sizes, ranging from 25.0 mm up to 27.4mm. I recall that most Italian and French frames (in the early & mid-70s) used seatposts (seat tube inner diameters) of around 26.2 mm up to 26.8 mm, while British frames and the Schwinn Paramount used a 27.2 mm post (with a correspondingly larger outer ST diameter).
Since most seat tube thicknesses were relatively constant (around 0.9 mm for Columbus SL.) the seat tube's outer diameter would then vary proportionally to the seat post size.
So, if Campy made the front derailleur clamp to "best fit" Italian frames, then the FD clamp would be a bit undersized for British and American frames which used 531 tubes, with a 27.2 mm ST inner diameter (seatpost size). With the "undersized" front derailleur, it would then be easier to scrape off the seat tube paint or dent the seat tube.
After I dented the seat tube on my first (British) racing bike, I started filing down the NR front derailleur's inside clamp surface, where the fixing bolt passed thru. I filed it down enough to remove the semi-sharp edge on the far side and also remove some inside clamp metal to get a smoother fit.
In later years, the seat post diameters seemed to stabilize around 27.2 mm. All of the newer front derailleurs are more carefully made and seem to fit better as well.
I do not like a braze on front derailleur at all. The derailleur cuts into the brazeon's paint very easily which causes it to peel off or flake off if the derailleur is adjusted or changed. Retrofitting a newer derailleur into an older brazed on casting has given me grief more than once.
regards,
Andrew Gillis (overcast but good riding in Long Beach, CA)