One of the best features of many new component groups is the ability to run a wide range of chainring and cassette setups. 1x, 2x, and even triple crank setups are possible within the same component group, and cassettes range from 28t to as big as 52t.
All this availability has made it more important to ensure you're using compatible parts, but sometimes it's difficult to know the correct derailleur to use with the cassette you're using. I hope this article helps understand why.
Shimano relies on the upper derailleur pulley tracking as close as possible to the cogs. This is why their rear derailleurs have a low tolerance for cassette sizes. Let's take Deore XT M8100 series 12 speed rear derailleurs for instance. There are three versions, each with a specific purpose:
GS- Short cage, designed for a single chainring
SGS - Medium Cage, designed for a single chainring
M8120 SGS - Long cage, designed for a double chainring
All of these derailleurs look very similar and can easily be mistaken for each other. Looking at the specs at bike.shimano.com, we see that there's very specific cassette compatibility:
GS- 10-45 12 speed cassette
SGS: 10-51 12 speed cassette
M8120 SGS: 10-45 cassette with a maximum chainring difference of 10 teeth
Of course, there's some cross compatibility that will work, but there's some that won't work at all. For instance, the M8120 SGS derailleur can easily be used on a single chainring setup and 10-45 cassette with no problems. If you attempt to use it on a 10-51 cassette it MIGHT shift if you turn the 'b adjust' screw all the way in. However, shifting will suffer in the smaller cogs because the upper pulley will track too far away from the cogs. Likewise, an M8100 SGS derailleur will shift on a 10-45 cassette but the gap between the pulley and the cogs will not stay constant, and shifting won't be as quick and smooth as it should. You should never use the short cage GS derailleur with a double crank - the capacity (the difference in cog sizes plus the difference in chainring sizes) is too small. When you size the chain according to Shimano's instructions, the chain will be too long when using the small chainring and smaller cogs because the derailleur cage isn't long enough to take up the slack.
All this is true because each rear derailleur is designed with a specific geometry that tracks the cogs at a precise angle. This keeps the pulley-cog distance constant, ensuring smooth and accurate shifting. If you mix up the compatible parts, shifting will begin to suffer. Sometimes it's an acceptable amount, sometimes it could slip and mis-shift. This is why Shimano 'requires' a specific derailleur for each setup.
SRAM approaches the issue a little differently. The position of the upper pulley on the cage allows the pulley to track different range cassettes. However, they rely on proper chain length to do this. Even though an Eagle rear derailleur will work on both a 10-50 and 10-52 cassette, it's best to size the chain to the specific cassette. If you use a chain that's sized to a 10-52 cassette with a 10-50 setup, the upper pulley could track too closely to the cog when the 'b-adjust' screw is adjusted properly. You could possibly have some grinding in the larger cogs. If you were to adjust the 'b-adjust' screw in to compensate, then the upper pulley won't track the rest of the cogs properly and you could end up with unreliable shifting. Likewise, if you use a chain sized for a 10-50 cassette on a 10-52, it might shift to all the gears, but the upper pulley will track farther away from the smaller cogs and shift accuracy will suffer. Beware though - running a chain that's too short could break the derailleur, especially if you have a full suspension bike.
In many of these cases, the drivetrain may shift acceptably. From a mechanic's point of view, it's our job to make it shift as smoothly and precisely as possible so we recommend using the correct setup to achieve that.
SRAM's AXS standard road derailleurs are compatible with 10-28, 10-33, and 10-36 cassettes with either single or double chainrings; the Xplr road derailleur is compatible with 10-44 or 10-36 cassettes and a single chainring. Like before though, proper chain length is critical for precise shifting.
I hope this helps clear up some confusion about compatibility issues that aren't straightforward. Thanks for reading!
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