Rear Derailleur
 

How can I tell if my rear derailleur is a short or medium cage? Thanks

Is there a brand and model number?

Compare the cage length to other bikes with known cage lengths.

It is a shimano 105 5600

Need a more descriptive part number. There is a short cage and a medium cage.

http://www.ebikestop.com/derailleur_...s%29%29502.php

IIRC, if you have 2 chain rings, chances are you likely have a short cage RD, 3 chain rings would require a long cage.

It's either a short cage, SS, or a long cage, GS. There are no Shimano medium cage road derailleurs. The SS is sold with double cranksets, the GS is sold with triple cranksets and may be used with double cranksets. Shimano does make some medium cage mountain derailleurs, but the 105 is a road RD. Take a look at the examples shown at http://www.shimano.com.

Road SS (short cage):
http://bikeforums.net/attachment.php...hmentid=150555
vs.

Road GS (long cage):
http://bikeforums.net/attachment.php...hmentid=150556

Once you've seen the two compared, I imagine it will be pretty easy to figure out which any given one is.

Or you can look at it like this, there are three cage lengths, SS, GS & SGS, i.e. short medium and long. If you're dealing with road group derailleurs then it's usually SS and GS, if mountain then it's generally GS vs SGS. Just relative to what your usual application is. Mountain bikers often call the GS short cage just as road bikers may call it long cage.