The theory
The pull ratio for Shimano mountain bike 10 and 11 speed shifting components (shifters and derailleurs) are very similar (1.2 v's 1.1 respectively). Basically, the 11 speed Shimano MTB shifters pull 3.6mm of cable per click. 10 speed shifters pull 3.4mm of cable per click. Accordingly, to use this shifter as a 10 speed shifter (with one un-used click) with a 10 speed derailleur and cassette, we need to reduce movement at the derailleur arm by about 5.5%.
In practice
It works! Basically, nothing is really needed to get this setup working properly (even though in an ideal world you want a 5% reduction in cable pull at the rear derailleur). Here's how:
- install your 11 speed shifter and cable it up to your 10 speed rear derailleur;
- put the shifter in the "11" (the smallest cog on the cassette);
- adjust the bottom limit screw so the jockey wheel lines up with the smallest cog on the cassette;
- install the shift cable so there's no slack;
- use the shifter to shift up 9 gears;
- adjust the top limit screw on the rear derailleur so it is not possible to do the 10th click on the shifter (preventing you shifting up into a non-existant 11th gear);
- shift into the 5th smallest gear on the cassette and fine adjust the shifting so its crisp between gears 4, 5, 6 and 7 on the cassette.