cassette? i live in central florida. almost pancake flat. my bike came w a compact crank w an ultegra 12-23. i switched out to a dura ace 12-21 straight block cassette. totally great gearing for me, especially as the bike replaced an old 6 sp w a 13-18 freewheel. is there another source for a 12-21 other that a dura ace? kind of expensive and hard to come by. can dura ace cogs be replaced individually? is there an option other than the titanium innermost cogs that come on the dura ace? thanks.