To understand the factors influencing the compatibility between a derailleur, a shifter, and a cassette, it’s necessary to become familiar with the following terms.
- Indexed Shifters
The movement of modern shifters is segregated into audible clicks. Each click indicates a gear transition from one cog/chainring to another. To trigger a shift, the shifter pulls or releases a gear cable that tells the derailleur where to move to derail the chain and place it onto the desired gear.
- Cable Pull
During each shift, the shifter pulls or releases a certain amount of gear cable. That amount is known as “cable pull” and depends on the number of speeds, the bike type (road or MTB), and the component manufacturer.
- Rear Shift Ratio
Derailleurs have a property known as rear shift ratio. The rear shift ratio indicates how much the derailleur moves per 1 millimeter pulled or released by the shifter. For example, if the derailleur has a 1.1 rear shift ratio, it moves 1.1mm per 1mm of cable pull.
Like the cable pull, the rear shift ratio changes according to the number of speeds, the bike type, and the component manufacturer.
- Cog Pitch
The center-to-center distance from one rear cog to another is called cog pitch. The cog pitch is speed and manufacturer-specific.
Smooth index shifting requires the values of the cable pull, the rear shift ratio, and the cog pitch to be in unison with one another. That way the derailleur always moves to the necessary location for a shift to occur.
If one of the three is off, the shifting experience will be poor or a complete failure.
Combining a SRAM GX Derailleur and a Shimano Cassette
For a SRAM GX derailleur to be compatible with a Shimano cassette, one of the following conditions has to be met.
- Matching Cog Pitches
SRAM GX derailleurs are designed for 11 and 12-speeds by default and thus are usually coupled with an 11 or a 12-speed cassette.
If the total width and the cog pitches of Shimano and SRAM cassettes match, then a Shimano cassette can be integrated into a similar drivetrain.
The total width of SRAM 11-speed cassettes is 40.2 mm. The cog pitch is 3.9 mm. The respective values for Shimano are 40.9mm and 3.55. In this case, the values are too dissimilar for a smooth combination.
The total width of SRAM 12-speed cassettes is 40.3mm. The cog pitch is 3.65mm. Meanwhile, the total width of Shimano 12-speed cassettes is 40.9mm and the cog pitch is 3.55mm.
In this case, the differences aren’t substantial enough to cause an outright incompatibility. Thus, the data supports the anecdotal evidence from people who have combined those parts with a decent amount of success.
That said, most people report smoother shifting when using the recommended cassette. This isn’t surprising because even small discrepancies make a difference in the world of indexed drivetrains.
- The rear shift ratio of SRAM GX has to be close to that of a Shimano derailleur
Another option is to integrate the GX derailleur into a drivetrain using Shimano shifters and cassette. In order for that combination to work, the GX derailleur needs a rear shift ratio close to that of a Shimano derailleur.
SRAM GX derailleurs are part of the X-Actuation series. The 11-speed versions have a 1.12 ratio whereas that of 12-speeds is 1.01.
The rear shift ratio of Shimano 11 and 12-speed derailleurs is respectively 1.2 and 1.4. (The numbers are higher for Shimano derailleurs designed for fewer speeds).
Conclusion: The dissimilar rear shift ratios make it impractical to integrate a SRAM GX derailleur into an indexed Shimano drivetrain.