Using 10 speed rear derailleur with 11 speed shifter and cassette 

Facts and figures

Component 10 speed11 speed
Cog spacing3.953.9
Shifter cable pull3.43.6
Pull ratio1.21.1

Calculations 

Each click of a 10 speed shifter moves the 10 speed rear derailleur 3.95mm using 3.4mm of cable pull. An 11 speed shifter pulls 3.6mm per click and to use a 10 speed rear derailleur will need a modification to the pull ratio so it moves 3.9mm (the 11 speed cog spacing). So we need to move the cable mounting point 3.6/3.4×3.95/3.9=1.07. In other words, we need to put a spacer under the 10 speed cable mounting point that will make the lever arm 7% longer.  Arm length is about 31mm, so a 7% increasing spacer would be about 2.1mm thick.

Conclusions 

An extra 7% longer is not a huge amount, so at least on paper it would seem possible to adapt a 10 speed derailleur for use with 11 speed shifters.

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