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Condensed Answer:

10-speed Shimano and SRAM MTB derailleurs have a different rear shift ratio than 9-speed derailleurs and cannot be effectively combined with a 9-speed shifter and cassette.

A 10-speed Shimano road derailleur has the same rear shift ratio as 9-speed Shimano derailleurs and can therefore be combined with a 9-speed drivetrain.

A 10-speed Campagnolo derailleur can be coupled with a 9-speed Campagnolo cassette.

If the user relies on friction shifters, a 10-speed derailleur of any kind will be compatible with a 9-speed cassette as long as the mech has the capacity to cover the cassette’s range.

Rear Shift Ratio

The rear shift ratio (RSR) describes how much the derailleur moves per 1mm of cable pulled or released by the shifter during up and downshifts.

10-speed Shimano MTB derailleurs have a 1:1.2 rear shift ratio. This means that the derailleur moves 1.2mm per 1mm of cable pull or release.

Conversely, 9-speed Shimano MTB derailleurs have a 1:1.7 rear shift ratio. Thus, the 9-speed derailleur is engineered to move 1.7mm per 1mm of cable movement initiated by the shifter.

When you combine a 10-speed Shimano MTB derailleur with a 9-speed shifter, the derailleur won’t move sufficiently for crisp shifting. As a result, the shift either won’t take place or will be incomplete, and the chain will keep rubbing against the cogs.

A 10-speed MTB derailleur has a smaller rear shift ratio because 10-speed cassettes are narrower than 8 and 9-speed models.

The cog pitch (center to center distance between two adjacent cogs) is reduced so that the overall width of 8,9,10 and 11-speed cassettes is similar.

The ultimate goal is to keep 8,9,10 and 11-speed cassettes compatible with the same freehub body (the part of the rear hub on which the cassette slides).

In order for a 10-speed derailleur to operate properly with a 9-speed cassette and 9-speed indexed shifter, it must have the same rear shift ratio as a 9-speed derailleur.

Those conditions occur in the following cases:

• 10-speed Shimano road derailleurs:

10-speed Shimano road derailleurs have а 1.7 rear shift ratio and are therefore compatible with 9-speed drivetrains.

• 10-speed Campagnolo derailleurs:

10-speed Campagnolo derailleurs have а 1.5 rear shift ratio – the same as 9-speed Campagnolo derailleurs. This makes them compatible with 9-speed Campagnolo drivetrains.

Friction Shifters = Ultimate Compatibility

Indexed shifters (the type found on most modern bikes) move а predetermined amount of cable with each click. This makes them “rigid” in their performance.

If you combine an indexed shifter with a derailleur that has a different rear shift ratio than what’s required for the cassette in question, you will experience non-satisfactory shifting performance.

For example, 9-speed Shimano MTB shifters have a cable pull of about 2.5mm. Or in other words, the shifter moves 2.5mm of cable with each click, and the derailleur moves 2.5mm x 1.7 (Rear Shift Ratio) = 4.25mm.

A 10-speed Shimano MTB derailleur has a 1.2mm rear shift ratio. When combined with a 9-speed indexed shifter, it moves 3mm (2.5mm x 1.2) – 1.25mm less than needed for a proper shift.

1.25mm may not sound like much but make a difference especially on cassettes with many cogs and a narrow cog pitch.

Friction shifters, on the other hand, are not “rigid”. They move as much as the cyclist wants them to. Thus, friction shifters compensate for a mismatched rear shift ratio and allow the mixture of otherwise non-compatible bike parts.

The downside of friction shifters is that they’re slow and more difficult to work with, especially if you’re a beginner cyclist. Hence why they are absent from most commercial bikes. That said, many touring cyclists like the simplicity and reliability of friction shifters and continue to rely on them.

Reference Data

The table below contains the rear shift ratios of 9 and 10-speed derailleurs.