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Two factors determine whether an 11-speed derailleur is compatible with a 12-speed cassette and shifter:

• The derailleur rear shift ratio
• The derailleur’s total capacity

Understanding Derailleur Rear Shift Ratio

Apart from electronic derailleurs, all other models are controlled by a gear cable pulled and released by a shifter.

When the shifter pulls the cable, the derailleur moves up the cassette; when the shifter releases the cable the derailleur shifts down towards the smallest cog.

The rear shift ratio of a derailleur indicates how much the derailleur moves per 1mm of cable pulled or released by the shifter.

Index Shifting

Modern shifters are indexed. This means that each shift represents a click that pulls or releases a pre-determined amount of cable. The purpose of index shifting is to have complete control over how much the derailleur moves.

To ensure this outcome, derailleurs have a pre-determined travel per each shift/click. The travel depends on the bike type (road or MTB) and the number of gears that the cassette has.

In this case, we need to know the rear shift ratio of 11 and 12-speed derailleurs. If the pull ratio is identical, or extremely close, then one of the requirements is met. The table below shows that data:

Conclusion: The rear shift ratio of 11 and 12-speed derailleurs matches when both are made by the same brand.

Thus, one of the requirements is met. That doesn’t mean, however, that one can mix 11-speed derailleurs and 12-speed drivetrains directly with 100% success.

Total Capacity

If a derailleur has a short cage, it will fail to climb a cassette with a massive first gear (e.g., 50t cog).

Consequently, one should make sure that the unit in question is adequate for the cassette that is going to be used.

Some Shifters and Derailleurs Are Labeled As 11 and 12-speed Models

Another indication that 11 and 12-speed components are interchangeable to a degree are the shifters and derailleurs labeled as 11/12-speed.

The classic example would be the Shimano XTR SL-M9100 shifter which has a screw underneath that could be used to add or remove one shift and essentially switch the unit between 11 and 12 speeds.

The derailleur from the same group set Shimano XTR M9100 was also labeled as an 11/12-speed unit.

It’s also worth mentioning that SunRace has a derailleur that is labeled as 10/11/12-speed and is compatible with Shimano 10 & 11 speed MTB and SRAM 12-speed shifters. The model is RD-MZ80.

Possible Problems

Two derailleurs may have identical pull ratios and total capacities, but they could still perform differently due to their overall shapes and the size of the jockey wheels.

For example, 11-speed derailleurs are known to have smaller jockey wheels and a shorter offset. Those properties make it harder to climb large cassettes without a fairly long cage.

For that reason, one cannot conclude with 100% certainty that an 11-speed derailleur will perform perfectly on a 12-speed system.

That said, if the above requirements are met, the chances of decent shifting are high.

Additional Changes

When integrating a new derailleur into an old drivetrain, the following changes may be needed:

• Adjustment of the limit screws (the limit screws prevent the derailleur from moving the chain into the spokes or the frame)
• Adjustment of the B-screw (the B-screw determines the space between the upper jockey wheels and the cassette which is usually 5mm)
• Chain length (the chain length is dependent on the derailleur’s cage. For example, a longer cage requires a longer chain to operate properly.)

Tips For Increasing The Chances of Decent Compatibility

• Stick to the same brand for every part of the drivetrain (derailleur, cassette, chainring, chain, shifter). The chain and the cassette are particularly important because both have ramps and indentations meant to match one another. A mixture not only decreases the chances of acceptable performance but wears down the cassette and chain faster too.

• Set the B-screw to an acceptable setting and adjust the chain length as needed