voshchev

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?

ericm979

Probably. It's only a few teeth past the spec of 29t wrap. I have used a 50/34 and 12-27 with a standard shimano road deraileur. You need to be careful about setting the chain length.

Retro Grouch 

I agree. Since you don't really use the big/big and little/little combinations, you'll never really use the full 31t capacity. I'm sure that you'll be fine.

Now here's where it gets interesting. Use the big/big chain sizing method. That'll give you the shortest chain that will safely cover the big/big combination. That's important. Even though you may never consciously shift into that combination, if you ever accidentally do it with a too short chain, the result can be real expensive. A slightly slack chain in the little/little isn't nearly so troublesome.

merlinextraligh

I've had no trouble with a Shimano short cage derailleur on 50/34 12/27 set up. Your 46/30 12/27 setup is the same amount of chain wrap. I think you'll be fine.