While recently adjusting my wife’s front derailleur, I noticed that the clip which the braze-on front derailleur is attached to holds the derailleur so that the cage is slightly out of parallel with regard to the chain. This makes her front derailleur adjustment more exacting than it would otherwise be, and we end up adjusting it more often than we’d like to. With a clamp-on derailleur I’d simply loosen the clamp and move the derailleur ’til the cage was perfectly parallel,but, because the seat tube is dimpled at this point and the back wheel sits in the dimple, I probably can’t remove the braze-on clip and fit an adapter clamp; is it possible to shim the braze-on clip, (it attaches to the seat tube with 2 small bolts), can it be removed and slightly bent, or is there any other solution to align the derailleur cage perfectly with the chain in this situation?
Usually on an aluminum frame, those are riveted on, not screwed. If you can actually remove it, then you could probably do something with it, like bend it slightly, or I suppose shimming one edge away from the seat tube (with a beer-can shim!) might work, too. Bending it when it is attached to the frame is a bad idea, as it will tend to push-in the seat tube, rather than bend the derailleur boss. The latter warninga lso applies to brazed-on and welded-on versions on steel, titanium and aluminum frames.
Normally, in cases where the boss is not removable, I have had success with filing one side of the boss’s bolt slot (with a jeweler’s flat file )until it allows the front derailleur to twist to where it needs to go. This definitely works if the tail of the derailleur needs to rotate outward.If it needs to go inward, sometimes there is another problem that you may have and have not noticed, namely that a part of the derailleur itself is hitting the seat tube and preventing it from rotating inward enough. If so, you need a different model of derailleur. Also, derailleurs do vary in how they sit in the boss, so just buying a different front derailleur without even messing with the boss might also fix the problem.