Join Date: Feb 2004
Location: St Peters, Missouri
Bikes: Rans Rockst (Retro rocket) Rans Enduro Sport (Retro racket) Catrike 559, Merin Bear Valley (beater bike).
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Seatpost clamp size, up-pull vs. down-pull, top swung vs. bottom swing, cage arc diameter, cage width. They pretty much all have to match. Older mountain bikes tended to have larger chainrings and front derailleur cages had larger arc diameters to match. The bio-pace chainrings make that issue a little trickier to solve. Nine-speed front derailleurs have a slightly narrower cage width and probably won't work quite as well with your wider chain.
All that said, if you really need a new front derailleur, I'm thinking that a Tourney TZ31 might meet all of your needs. I've got to tell you though that actually wearing out a front derailleur is pretty rare.