Hmmm...no, I mean a medium cage derailleur has approximately 33T chain wrap capacity (slightly higher for SRAM I think--maybe 37T). And a full range mountain bike gear set is usually around 45T (for example, a 22/32/44 up front and an 11/34 rear would give you (44-22)+(34-11)=45T chain wrap). Therefore there would be some gears that would simply not work with a medium cage derailleur. Depending on your chain length, if it was long, then you'd eventually lose the ability to take up slack in small/small chainring/cog combos, or if your chain was too short, you'd jam up the drive train trying to shift into large/large combos.
Last edited by IntoTheWild; 10-15-11 at 06:52 PM.