Something I don't understand... If i run Dura Ace 10 speed, what options do I have for a rear casette? What I mean is, I currently run a 12-23 rear casette, but I would like to incorporate an 11 in the back, and sometimes, a 25. Do I buy an entirely new casette? Or just an 11 and somehow incorporate it into my existing cogset? Are there limited options for the mix-and-match you can do in the back, or any guidelines to follow so you don't have too wide (11-27?) of a range?



Thanks for any and all help.