Originally Posted by
DougG
The bike I've got my eye on is a Synapse Carbon 5 that is available with the same 105 setup that I'm now using, or with a compact 54/30 and 12-25 cassette. From reading the posts here, I'm now thinking of going with the double, but maybe installing an almost-new 105 12-27 cassette that I happen to have. I hope this can be done without derailleur or chain-length issues.
In any case, I've almost never been in the 30/27 gear combo on the steepest hills that I'm likely to encounter, so the 34/27 low gear should be sufficient. Thanks for the excellent discussion -- this section always has good info without some of the stuff that goes on elsewhere.
Are you certain the compact crankset is a 50/30? Most are 50/34 or 50/36. You most likely will be able to use the 12-27 cassette without going to a long cage derailleur, chain length would be the only concern if it had been shortened.