Long cage vs. short cage
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Derailleur cage lengths
- Maximum amount of chain slack that can be taken up (important to take up the difference in chain from the smallest to largest sprocket and chainring combination). The longer the cage, the more chain slack it can take up
- Maximum rear sprocket size that the derailleur will accomodate. Typically, longer cages are designed to match larger sprockets.
So, which one will work on a particular bike depends on the cassette and chainring sizes on the bike. According to the Campagnolo Rear Derailleur Instructions, they recommend the following derailleur usage:
Short cage: All double and compact cranksets + all cassettes except 13/29
Medium cage: All double and compact cranks + all cassettes; or all triple cranks + all cassettes except 13/29
Long cage: All triple cranksets + all cassettes
Well, the main difference is the size and shape of the cage, obviously. But the cage size and shape also determines two things:triton said:
- Maximum amount of chain slack that can be taken up (important to take up the difference in chain from the smallest to largest sprocket and chainring combination). The longer the cage, the more chain slack it can take up
- Maximum rear sprocket size that the derailleur will accomodate. Typically, longer cages are designed to match larger sprockets.
So, which one will work on a particular bike depends on the cassette and chainring sizes on the bike. According to the Campagnolo Rear Derailleur Instructions, they recommend the following derailleur usage:
Short cage: All double and compact cranksets + all cassettes except 13/29
Medium cage: All double and compact cranks + all cassettes; or all triple cranks + all cassettes except 13/29
Long cage: All triple cranksets + all cassettes
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Thanks for Mark for a succinct and clearly-written factually correct answer. I mean that in a complimentary way. 
more.....
There is no difference in the cage "shape" only length. In reality, all of the derailleurs will handle a 29T largest cog, since the upper parallelograms are identical. Only the wrap capacity differs, due to the longer cages on the medium and long cage RDs.
Campy may have made changes to their RD cage lengths after the introduction of compact cranks, but the wrap capacities used to be 27, 36 and 39T respectively. The 27T wrap would not handle the extra 2T difference of a compact crank with a 13-26, but perhaps Campy fudged and the short cage really has a 29T wrap.
I know that the medium cage only has a 36T wrap, since I'm using it with a 38T wrap drivetrain and the smallest 2 cogs are unuseable with my 28 little ring. I've tried a 1inch shorter chain and it's too short to wrap the 53/25.
http://www.campagnolo.com/groupsets.php?gid=3&cid=1
http://www.branfordbike.com/hub/hub07.html#item8
There is no difference in the cage "shape" only length. In reality, all of the derailleurs will handle a 29T largest cog, since the upper parallelograms are identical. Only the wrap capacity differs, due to the longer cages on the medium and long cage RDs.
Campy may have made changes to their RD cage lengths after the introduction of compact cranks, but the wrap capacities used to be 27, 36 and 39T respectively. The 27T wrap would not handle the extra 2T difference of a compact crank with a 13-26, but perhaps Campy fudged and the short cage really has a 29T wrap.
I know that the medium cage only has a 36T wrap, since I'm using it with a 38T wrap drivetrain and the smallest 2 cogs are unuseable with my 28 little ring. I've tried a 1inch shorter chain and it's too short to wrap the 53/25.
http://www.campagnolo.com/groupsets.php?gid=3&cid=1
http://www.branfordbike.com/hub/hub07.html#item8
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