Originally Posted by
phantomcow2
Why would the RD need to be changed? This doesn't make sense. Think about it: does the derailleur move in set increments, corresponding to some specific spacing? No, the cable pulls it, and the amount the cable pulls the RD is dictated by the shifter, whatever brand it is.
You need to think about it, not me.
You're overlooking a very important part of the sytem - the RD actuation ratio. Every brand uses a different cable pull amount, even if the RD shifts to the same cog spacing. For example, SRAM shifters pull 3.1mm of cable and Shimano only pulls 2.3mm, but the both shift the same 3.95mm cog spacing. That's because the RD acutation ratio is different. Campy 10 pulls an average 2.8mm to shift to 4.15mm spacing.
You absolutely must match the shifter to the RD or use a J-tek shiftmate to correct the cable pull.