So, I would like to move to a Shimano Ultegra 9spd short cage derailler, but I'm not sure about the capacity. The derailler has a 29 teeth capacity. On my bike I'm running a super compact double (46-30) and a 12-27 cassette, thus--if I'm calculating correctly--I need a RD with a 31 teeth capacity. So, the question is whether I can use an ultegra short cage (they look SOOO much better than long cage models) with this set up?
Hmmm, I believe the 29 tooth capacity you refer to for the rear derailleur means the maximum cog size on the wheel end of the drivetrain. In otherwords you'll be fine with a 12-27 & short cage. On my road bike, I use a short cage (with 50-34 on front) and it's good up to 27 teeth on the cogset I think. Long cage would be more appropriate for a triple.
On the touring bike I use a 44-32-22 though with a 12-30 rear, hey, I'm old and need the advantage.
capacity for rd = (big ring-little ring)+(big cog-little cog).
the rd cage has to take up all the slack chain, whether it comes from rings or cogs.
in addition shimano normally specifies a min and max cog size.