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On the surface, bicycles are tremendously simple two-wheel vehicles powered by the rider’s muscles. But when you get deeper into the cycling world, you realize that there’s more to the story.

On many occasions, you will find yourself solving problems that you couldn’t foresee before.

One of the common issues that you will experience is the incompatibility between parts.

Surprisingly to some, it’s not always possible to mix bicycle components even if they technically assist towards the same function.

This post focuses on a similar inquiry:

Will a 9-speed derailleur work on a 10-speed cassette?

(condensed answer)

A 9-speed Shimano derailleur will work on a 10-speed cassette if the shifter is also made by Shimano and designed for road bikes.

It’s also possible to combine a new 9-speed derailleur by Campagnolo with a 10-speed shifter and cassette made by Campagnolo.

However, a 9-speed MTB derailleur won’t be able to operate properly with a 10-speed MTB index shifter and subsequently with a 10-speed cassette.

If you want to use a 9-speed MTB derailleur with a 10-speed cassette, you can do so via a 10-speed road shifter or a 10-speed friction shifter.

It’s All About The Cable Pull

The position of a derailleur is controlled through a cable inserting into the shifter.

When the shifter is triggered, it either increases or decreases the tension on the cable. In consequence, the derailleur moves outward or inward. The chain follows the path laid out for it and wraps around a different cog of the cassette.

The same principle applies to all bicycles that use a derailleur-based gear system.

Index Shifting Causes Incompatibility

Modern shifters are indexed. The length of each pull or discharge that happens upon clicking the shifter is pre-determined. The rider has no control over it.

The main purpose of index gearing is to simplify the shifting process. If the system is set properly, all you have to do is click the shifter and keep pedaling.

This creates a problem when mixing components from group sets designed for different speeds.

The sprockets (cogs) of 10-speed cassettes are closer to each other than those of cassettes with fewer speeds.

As a result, it may be impossible to combine a 10-speed cassette and a derailleur designed for a fewer number of gears because the pull/release of the shifter may fail to move the derailleur as much as the cassette requires for effective shifting.

A 9-Speed Derailleur Can Work With a 10-Speed Cassette Only On Road Bikes

Luckily, the rear shift ratio of Shimano’s road bike groupsets is the same for 8, 9, and 10-speed transmissions. Hence why it’s possible to combine a 9-speed derailleur with a 10-speed cassette.

However, there are two currently known exceptions to the rule – the Tiagra 4700 and ST-RX400.

The Tiagra 4700 shifters are “downgraded” 11-speed shifters and have an 11-speed pull ratio.

As a result, a 10-speed Tiagra 4700 brake-shifter does not work properly with a 10-speed rear derailleur that isn’t a part of the Tiagra 4700 family.

Therefore, 10-speed Tiagra 4700 shifters cannot be combined with a 9-speed derailleur either.

According to Shimano’s compatibility chart the brake-shifters ST-RX400 are compatible with the long cage derailleur RD-4700-GS from the Tiagra 4700 series.

Based on that information, it would be logical to assume that this particular model wouldn’t work too well with non-Tiagra-4700 derailleurs.

And if the shifter cannot work with a non-Tiagra-4700 derailleur, it won’t be a good match for a 9-speed derailleur either.

A 9-Speed MTB Derailleur Cannot Work With a 10-Speed MTB Cassette

Mountain bike groupsets with 10 and more speeds have a different pull ratio than those with fewer speeds.

For that reason, a 9-speed MTB derailleur and a 10-speed MTB cassette aren’t a good match. The needed 10-speed shifter just won’t move the chain to the “right places”.

Conclusion: A 9-speed rear derailleur can work with a 10-speed cassette, but only if the following conditions are met:

• The shifters have to be designed for road biking and made by Shimano or Campagnolo. The system can include components only from one of the brands. Mixing cannot be done.
• The brake-shifters and the rear derailleurs shouldn’t be from the Tiagra 4700 and RX400 series.

To learn more details, keep reading.

Understanding The Term “Rear Shift Ratio”

The term rear shift ratio a.k.a actuation describes the lateral movement of the derailleur in relation to the cable length pulled or released by the shifter with each click.

The shift ratio is expressed with a single number which signifies how much the rear derailleur moves for every millimeter of cable pulled or released by the shifter.

For example, the rear shift ratio of Shimano 9-speed road derailleur is 1.7.

This means that for every millimeter of cable movement triggered by the shifter, the derailleur moves 1.7mm.

Since the rear shift ratio of a Shimano 10-speed derailleur is also 1.7 (except for Tiagra 4700 series which have a 1.4 ratio), a 9-speed road or MTB derailleur can effectively work with a 10-speed cassette and a 10-speed road shifter.

However, Shimano’s MTB 9-speed and 10-speed derailleurs have a different rear shift ratio.

The 9-speed models have a ratio of 1.7 whereas the 10-speed ones come at 1.2.

Therefore, you cannot effectively use a 10-speed MTB shifter with a 9-speed MTB derailleur.

But a 10-speed road Shimano shifter that isn’t a part of the Tiagra 4700 series would be compatible with a 9-speed MTB Shimano derailleur because 9-speed Shimano derailleurs have a 1.7 rear shift ratio.

The situation is different for SRAM components.

The rear shift ratio of a 9-speed SRAM derailleur is 1.1 whereas the 10-speed SRAM models have a rear shift ratio of 1.3.

The 9 and 10-speed SRAM shifters are designed to reflect that and have a different cable pull.

As a result, you will not be able to use a 10-speed SRAM shifter with 9-speed SRAM derailleur on neither mountain nor road bikes.

A 10-speed Shimano shifter isn’t compatible with a 9-speed SRAM rear derailleur either because the Shimano shifter has a different cable pull.

There are SRAM rear derailleurs compatible with Shimano’s stuff, but they aren’t designed for 9-speeds.

Illustrating the MTB Incompatibility

A shifter can be seen as the “mastermind” giving orders whereas the derailleur is the order follower.

A 9-speed MTB Shimano derailleur has a rear shift ratio of 1.7.

This means that it is designed to move 1.7mm for every millimeter of cable movement initiated by the shifter.

But to combine a 9-speed derailleur with a 10-cassette, you will also need a 10-speed shifter or else you won’t be able to use all cogs.

The 10-speed MTB Shimano derailleur has a rear shift ratio of 1.2, however. [It moves 1.2mm for a millimeter of cable movement.]

Since the 9-speed derailleur moves 0.5mm more than a 10-speed MTB Shimano derailleur for every millimeter of cable movement, it will cause a phenomenon known as over-shifting when combined with a 10-speed shifter.

The effect will be further amplified by the longer cable pull of a 10-speed shifter – a 10-speed MTB Shimano shifter has a cable pull of 3.4mm whereas that of a 9-speed MTB Shimano shifter is 2.5mm.

Тhe Formula For Efficient Shifting

There’s a formula that determines whether a combination of a shifter + rear derailleur will result in good shifting.

Before talking about the formula, it’s necessary to clarify a term known as “cog pitch”.

Cog Pitch – the center to center distance between adjacent cogs.

The cog pitch varies according to the number of sprockets. More cogs equal a shorter cog pitch.

The formula for efficient shifting is as follows:

Cog Pitch = Cable Pull x Rear Shift Ratio

According to the formula, the multiplication between the derailleur’s shift ratio and the cable pull of the shifter should equal the cog pitch of the cassette or freewheel.

Let’s see what numbers this formula would give us when combining a 9-speed Shimano rear derailleur with a 10-speed Shimano road shifter and cassette.

1. The cable pull of a 10-speed Shimano road shifter is 2.3mm.

2. The rear shift ratio of a Shimano 9-speed derailleur is 1.7.

The formula gives us the following:

Cog Pitch = 2.3 x 1.7= 3.91

And since the cog pitch of a 10-speed Shimano road cassette is indeed about 3.91mm, one can conclude that a 9-speed Shimano rear derailleur will work with a 10-speed Shimano road shifter.

Now, let’s see how the equation works for MTB components:

1. The cable pull of a 10-speed Shimano MTB shifter is 3.4mm

2. The rear shift ratio of 9-speed Shimano MTB derailleur is 1.7.

The formula gives us the following:

Cog Pitch = 3.4×1.7 = 5.78

The actual cog pitch of a Shimano 10-speed MTB cassette is around 4.08mm.

Therefore, in this case, a 9-speed derailleur will cause noticeable over-shifting.

A Friction Shifter Makes Everything Compatible

Before the index shifting revolution, cyclists had to deal with friction shifters.

Friction shifters have no index points. It’s up to the cyclist to decide how much to push or pull the cable.

Friction shifting appears to be a major inconvenience when compared to modern index shifting because you have to essentially search for the gears yourself.

Having said that, friction shifting has undeniable positive sides and its fans.

One of friction shifters’ strongest points is that they allow you to mix MTB and road parts.

For that reason, many touring cyclists love bar-end shifters that come with a switch between friction and index mode.

If a 9-speed MTB derailleur is combined with a 10-speed friction shifter, the combo will work just fine if both components have the capacity to cover the entire cassette.

If the friction shifter is designed for fewer speeds, it may not operate properly due to insufficient pulling ability. Or in other words, the shifter may be unable to move the derailleur onto the biggest cog of the cassette.

The downside of this approach is that you won’t benefit from the comfort offered by index shifting. Not to mention that a friction sifter is practically obsolete on a mountain bike that’s actually used on trails. Technical mountain bike riding and friction shifting do not mix well.

Also, high speed drivetrains make friction shifting more difficult because the space between the gear cogs is smaller and requires extra precision.

What About Campagnolo Components?

The old Campagnolo 9-speed rear derailleurs produced before 2001 aren’t compatible with a 10-speed shifter because they have a rear shift ratio of 1.4. As a result, they won’t be an appropriate match for a 10-speed cassette.

The new Campagnolo 9-speed rear derailleurs, however, have a 1.5 rear shift ratio which matches that of the 10-speed Campagnolo derailleurs.

Note: Campagnolo’s cassettes tend to have a different cog pitch than other manufactures. It’s not recommended to combine a non-Campagnolo cassette with Campagnolo mech.

Reference Rear Shift Ratio Table