Originally Posted by rfields
So if I've understood correctly, I have to change to a XT RD-M771 rear derailleur. Will I be able to utilize all 10 cogs on the cassette or???
If you'll be using a mountain derailleur, the primary thing is for it to move the right distance from cog to cog when the shifter moves one click. Shimano's 9-speed mountain derailleurs share the same movement pattern as their 10-speed road derailleurs, so any 9-speed Shimano mountain RD would be a basic match and let you use all the cogs. I think the traditional dual-servo designs (that's Shimano's lingo) will track the cassette cogs more closely than the Shadow variants and provide you with better results, so I suggested one of that type, the 771. But it's just one of the possibilities... there's also the XTR RD-M971 as the top-of-the-line option, or various models below XT too.
When shopping around for a viable RD, also note that there's backwards-sprung variants that default to the inboard
rear cog, instead of the outboard
rear cog. Most people do NOT want the backwards derailleur (aka the "low-normal" or "RapidRise" type), so be careful which you choose. Easy visual guideline: the backwards derailleurs have their cable-clamp bolt on the inboard link of the parallelogram rather than the outboard link.
Last edited by mechBgon; 05-20-12 at 03:19 PM.