SRAM X5 9 vs. 10 speed compatibility
 

I've got a '13 Specialized RH that has 9x3 SRAM X5 on it (drive train swap w/ my wife's bike... long story). I'm interested in converting it to 10 speed but I'm wondering if the existing derailleurwill work on a 10 speed cassette. Obviously I need a different shifter but hoping the existing derailleurwill work. Not near the bike right now to look and see specifically what model X5 it is.

It probably won't. I'm pretty sure 9spd and 10spd Sram derailleurs have different pull-ratio requirements because of different anchoring points.

Sorry.

Yep 9 to 10 speed means a new rear derailleur with Sram or Shimano.