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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Three factors to look for when acquiring a rear derailleur:
The first is indexing. Any Shimano indexing rear derailleur will work. That is to say, any modern Shimano rear derailleur.
The second is large cog size. Count the teeth on your biggest rear cog. Road bike rear derailleurs, like the 105 you mentioned are rated for 27 tooth cogs or smaller. If the rear cog is too big, the upper derailleur pulley might rub on it.
The third is chain wrap. Subtract the number of teeth on your smallest rear cog from the largest. Do the same for the front chainrings. Now add those two numbers together. That's the amount of chain wrap that your rear derailleur should be able to handle. A short cage 105 will handle 29 teeth. A long cage 105 will do 37 teeth.
The catalogues often list rear derailleurs as 7-speed, 8-speed etc. Since the rear derailleur guides the chain from the middle, in real life that's usually a non-issue.
Since your bike is a 12-speed you should also look at how it bolts onto the frame. All but the cheapest modern derailleurs are designed to bolt onto an arm that hangs down from the bottom of the frame. If your frame doesn't have that you can either reuse a separate bolt-on arm, if your bike has one or you will have to acquire a Tourney or Altus derailleur that comes with it's own hanger arm attached.