Compatibility [04] Rear derailleurs

This post explains bicycle rear derailleur compatibility, i.e. what derailleurs can work with which number of rear sprockets (cassettes) and which rear shifters. For explanation of other rear derailleur functioning and limitations, such as chain wrap capacity, read this article: Rear derailleur.

Before you start, to avoid any misunderstanding:
please take the 5 minutes needed to read the compatibility articles use instructions.

Table Of Contents (T.O.C.):

TL/DR

0. Basic terms and what to pay attention to
1. Shimano
2. SRAM
3. Campagnolo
4. Rotor’s hydraulic derailleurs
5. Table of RD movement ratios for various standards
6. Problems preventing you from EVER tuning your derailleurs properly – video
7. BikeGremlin’s comment – explanation

TL/DR

Rear derailleurs (RD) are stupid: they just move left-right depending on how much you tighten/release the shifter cable. However, if you pull the shifter cable by 1 mm, some derailleurs will move for about 1 mm, while others will move for about 1.7 mm (with plenty of models in between).
This is called “RD movement ratio.“

Your shifter dictates how much cable is pulled/released by each click.
The trick is:

• Your shifter should match the number of sprockets (speeds) you have.
• Your derailleur movement ratio should match your shifter’s cable pull/release amount.

Otherwise, you can’t tune your derailleur to shift properly with each shifter click.

In this article I’ve explained how RDs work, and listed the movement ratios and which derailleurs are compatible with which shifters – in case you like mix-matching. 🙂

– T.O.C. –

0. Basic terms and what to pay attention to

Most modern bicycles have index shifters, that work with a certain number of clicks. One click per gear change. That is moving the chain from one sprocket/chainring to another with each click.

In order for this to work, these things need to be in tune:

• Length of cable that shifter pulls/releases with each click. This is called shifter cable pull.
• Distance that rear derailleur (RD in the remainder of this text) moves laterally per 1 mm of cable movement. This is called rear shift ratio (or “derailleur movement ratio“). E.g. if RD moves for 2 mm for 1 mm cable movement (pull or release), then the rear shift ratio is 2.
• Rear sprocket spacing.

Not directly related to shifter-derailleur compatibility, but it is also crucial to use proper cables and housing. I wrote a post explaining shifter (and brake) cable and housing standards.

When RD compatibility is mentioned, it is all about it’s tune with the shifter cable pull and rear sprocket spacing. When it is all set up correctly, one click of the indexed shifter shifts exactly and correctly one gear (i.e. one sprocket) at the rear. Compatibility will be explained per number of gears and per manufacturer. If it is not noted otherwise, it is understood that shifter make and number of gears match the RD make and the actual number of rear sprockets. There are three major RD and shifter manufacturer standards (and at least one exotic – by Rotor):

1. Shimano
2. SRAM
3. Campagnolo
4. Rotor’s hydraulic derailleurs

– T.O.C. –

1. Shimano

6, 7, 8 and 9 speeds

Rear shift ratio is 1.7, that is for 1 mm of cable pull/release, RD is moved left/right by 1.7 mm. Shimano calls this 2:1 ratio for marketing reasons.

All these RDs are compatible and any 6 to 9 speed RD will work perfectly with either 6, 7, 8 or 9 speed shifter. Regardless whether it’s a MTB, or road shifter, or RD. They are also compatible with Shimano 10 speed road shifters, except the Tiagra 4700 series.

Shimano Dura Ace from 1984 to 1996 period (6 to 8 speeds)

Rear shift ratio is 1.9. They are compatible only with Shimano Dura Ace shifters from the same period (that is for 6, 7 and 8 speeds).

10 speed ROAD RD

Rear shift ratio is 1.7, so it’s compatible with all the 6 to 9 speed shifters, as well as road 10 speed shifters. Same rear shift ratio. MTB 10 (and 11 speed) shifters won’t work well.

Exception is Shimano Tiagra 4700 10 speed road RD – it has the same shift ratio as Shimano road 11 speed RDs, so it will work only with Tiagra 4700 shifters and Shimano 11 speed road shifters.

Another exception is the new “gravel” group (with hydraulic brakes): Shimano GRX. Caple pull wise it’s the same as Tiagra 4700.

10 speed MTB RD

Rear shift ratio of this RD is about 1.2. It is only compatible with Shimano 10 speed MTB shifters and nothing else. Shimano calls this system Dyna-Sys.

There are a few caveats here. See below, my comment on 10/11 Shimano MTB RDs.

11 speed ROAD

Rear shift ratio is around 1.4.  Only compatible with Shimano 11 speed road shifters.

The same shift ratio is used for Tiagra 4700 10 speed groupset, so it’s compatible with it as well, but not with other 10 speed road groupsets.

Rear shift ratio of 1.4 matches old Campagnolo ratio, but I haven’t tested this in practice.
Update: According to Peter’s feedback, it won’t work with Campagnolo.

11 speed MTB

Rear shift ratio is around 1.1. Compatible with Shimano MTB 11 speed shifters. Shimano calls this system Dyna-Sys, same as 10 speed MTB, but they’re not compatible! most current models are compatible and will work with Shimano MTB 10-speed RDs in practice.

It has similar rear shift ratio to SRAM 1:1 standard (explained later in this post), so it should match it as well, but I haven’t tried it yet!

11 and 12 speed MTB – Hyperglide+ (XTR M9100)

From summer of 2018 Shimano introduces a new Hyperglide+ 11 and 12 speed MTB standard. It comes with a new XTR M9100 rear derailleur, that only works with a matching shifter (which has an integrated 11, or 12 speed operation switch!) and Hyperglide+ 11 and 12 speed cassettes.

– T.O.C. –

2. SRAM

SRAM has four RD families, with 4 different amounts of RD movement per cable movement (actuation ratios). So the only thing to pay attention to is actuation ratio. As long as the RD matches shifter’s actuation ratio (and vice versa), it will work fine.

First, less popular family of SRAM RDs are the ones compatible with Shimano 2:1 standard. Rear shift ratio of 1.7 means they will work with Shimano (and SRAM 2:1) shifters for 6 to 9 speeds (both road and MTB) and road 10 speed ones, except Tiagra 4700 10 speed road shifters.
RDs made in this standard were marketed as 6, 7, 8 and 9 speed models, though this is determined by the shifter – RD doesn’t care how many rear sprockets there are.

Second, more popular SRAM standard is 1:1, with rear shifter ratio of 1.1. This shift ratio is the same as Shimano MTB 11 speed RD (and shifter), so they should be compatible, but I haven’t tested this yet.
RDs made in this standard were marketed as 7, 8 and 9 speed models.

Third, newer standard is Exact Actuation. Rear shifter ratio is 1.3.
Marketed as 10 speed road and MTB , as well as 11 speed road.

Fourth family is X-Actuation, with 1.12 ratio for 11 speeds, and 1.01 for 12 speeds.
Available as 11 and 12 speed MTB RDs 

EDIT: Based on this comment (on this very article) and provided links to SRAM’s website, Exact Actuation and 1:1 are the same thing. Will have to measure, check and confirm this.

– T.O.C. –

3. Campagnolo

Campagnolo has two standards of rear shift ratio. Old 1.4 and new 1.5. Also, not every shifter pulls the same amount of cable with each click. With 10 speed for example, there is more cable pull for “slower” speeds, than there is for the others.  2.5 mm pull five times (from smallest 1st to the 6th sprocket), 3 mm for 7th and 8th, then 3.5 mm for 9th and 10th. 2.83 mm average cable pull per click.  🙂

Apart from that, as of 2014 (don’t take my word for the exact year), campagnolo introduces another 11 speed standard. Previous Revolution 11 and the newest Revolution 11+. They are not compatible.

Since Campagnolo cassettes sprocket spacing differs from other manufacturers’, it is hard to combine Campagnolo RDs with anything but the appropriate Campagnolo shifters and in most cases cassettes too.

In 2018, Campagnolo introduced their own 12 speed standard – not compatible with any other.

– T.O.C. –

4. Rotor

Rotor’s hydraulic rear derailleurs are only compatible with their hydraulic shifters.

– T.O.C. –

5. Table of RD movement ratios for various standards

The table shows how many mm derailleurs move laterally (left-right – on average across their movement range) per one mm of caple pull or release by the shifter. The smaller the movement ratio – the more cable you need to pull/release to make a gear shift (all else being equal).

– T.O.C. –

6. Problems preventing you from EVER tuning your derailleurs properly – video

– T.O.C. –

7. BikeGremlin’s comment – explanation

This was originally posted as a reply to a comment, but there are almost 200 comments to this article now, so I’m moving it here:

Due explanation:

I had managed to get some things working OK, even though they don’t match looking by the manufacturer’s specs.

When it comes to specs, this is what Shimano says:
First they say (said) 9, 10 and 11 are all non-compatible with each other.
Now, on the very same page, they claim otherwise, under 10 speed rear drivetrain compatibility section.

Their Zee (RD-M640-SS), and Saint (RD-M820-SS) RDs are noted as 10 speed only.
While the Deore you noted does say 10/11 speeds.
To make things more interesting (and confusing), Deore 11 speed series has:
RD-M5100-SGS that says 1×11, and RD-M5120-SGS that says 1×10, 2×10, and 2×11.

My goal when writing this series of articles was to help myself, by having all the combos that work (in spite of the manufacturers’ specs. stating otherwise) in one place.
The no-go stuff was written down to help when mix-matching, to know what to rule out from the start.

A problem I face is, even when I make a combo from this no-go zone to work, I can’t say with enough certainty that it works on most bicycles.
Like here, when pairing an 11 speed MTB cassette with an 11 speed road groupset.
For me, it would take at least 5 more bikes, tried and (field) tested to be able to say: 11 speed MTB cassettes work with road 11 speed shifters & RDs, in spite of the obvious and measurable differences in pitch.

The above noted Shimano tech. and product specs. certainly don’t make things less confusing (at least for me).

Also, with so many what/ifs and gotchas (by Shimano), it seems the only thing to do is make a database driven app where you enter your shifter, and cassette models, to get a list of RDs that work with those.
Same goes for other combos (cassette, and RD to get a matching shifter model).

Because stating all the possible combos on one page would make it very, very long and almost unreadable (shifter models x cassettes x RD models… it grows exponentially for each new combo, even if only listing the combos that work).

Bottom line:
Combos that are noted here to work – do work, i.e. I’m yet to find a case when they don’t (poor tuning by the user/mechanic aside, of course).
Combos that are not noted to work – sometimes they work, more or less perfectly. If the differences are slight enough, it’s usually worth giving it a try (as we did with the MTB cassette and 11 speed road shifters, noted above).

Having said all this, I do try to keep this as correct and up-to-date as possible. So will add notes regarding 10/11 MTB RDs (and shifters) in the matching sections/articles. Even if not providing a definite answer.

Related post – Bicycle rear hub compatibility:

Compatibility posts are also available in eBook (printable and Kindle) and paperback editions on Amazon:

Author

Relja Novović

Relja is an avid cyclist, a pretty good bike mechanic, and a professional computer systems administrator – with decades of experience in both those fields (computers and bikes 🙂 ). Relja Novović’s credentials and a short biography

The existing comments (questions and answers) posted to this article over the years have been moved to this BikeGremlin forum thread:
https://www.bikegremlin.net/threads/compatibility-04-rear-derailleurs-article-comments.100/

– T.O.C. –