Originally Posted by rltrock
I assume that I can swap out the 12-23 for the 12-27 without having to change anything else on my bike. I have a 9 speed ultegra derailer for a triple. I hope I do not have to change anything other than the freewheel. Do I have to lengthen the chain?
You may or may not have to lengthen the chain, but it's easy to find out. Remove the chain from the rear derailleur pulleys. Wrap the chain around the big front cog and the big rear cog. You should have enough chain to overlap by two links. Too short a chain can make things messy when you shift into the "big-big" combination. If you always avoid the big-big combination, you'll be okay. But everyone has shifted into big-big by accident before.
Since you alreay have a triple, I suspect your Ultegra rear derailleur can handle the 12-27. I believe all Shimano derailleurs can handle a 27 cog, the only issue is whether or not the derailleur cage can handle the slack caused by the extremes of gears (30/12 vs 53/27).
According to the
Shimano web site, the Ultegral GS rear derailleur can handle up to a 37 tooth difference, so you're fine. The SS can handle only 29, so you'd have a problem with that version. But the SS isn't used with triples.