Actuation Ratio
When a mechanical gear shifter is moved by the rider's hand, it will pull a fixed amount of inner gear wide through the outer. The shifter at the other end will move (actuate) a certain amount, which will be different.
Naturally there are many standards, along with exceptions to standards. In brief:
Shimano 6/7/8/9 - Rear shift ratio is 1.7 so for 1 mm of cable, the Rear Deraileur moves left/right by 1.7 mm
Shimano calls this 2:1 ratio for marketing reasons.
All these RDs are compatible and any 6 to 9 speed RD will work perfectly with either 6, 7, 8 or 9 speed shifter assuming it can physically span the width of the cassette. Regardless whether it’s a MTB, or road shifter, or RD. They are also compatible with Shimano 10 speed road shifters (except the Tiagra 4700 series)
| Name | Ratio |
|---|---|
| Shimano standard 6/7/8/9 speed and 10 speed Road excluding Tiagra | 1.7x |
| Shimano Dura Ace 6 to 8 speeds (vintage stuff) | 1.9x |
| SRAM 2:1 | 1.7x |
| Campagnolo old | 1.4x |
| Shimano 10 MTB | 1.2x |
| SRAM 1:1 | 1.1x |
| Campagnolo new | 1.5x |
| Shimano 11 road and Tiagra 10 speed 4700 | 1.4x |
| SRAM Exact Actuation | 1.3x |
| Shimano 11 MTB | 1.1x |
| SRAM X-Actuation 11-speeds | 1.12x |
| SRAM X-Actuation 12-speeds | 1.01x |
Note that Shimano Internally Geared Hubs (Nexus and Alfine) do not have equal pull distances, so adjacent clicks are NOT the same length. These shifters are incompatible with derailleur gears, and the IGHs do NOT work with normal shifters.
Further info: