I don't think the rs20 takes an 11 speed cassette. They make an rs21 now which does. How many spacers are behind your current 10 speed cassette on the rs20? Just a very thin (1mm) one...or that plus a thicker one (~1.85mm). If two spacers currently, the 11 might work. If just a thin one, probably not.
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'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur
Last edited by dtrain; 04-14-15 at 01:43 PM.