from Rookie’s keyboard
Hello, friends
I decided to start a post during my lunch break. I don’t think I will have time to finish it, but let’s see.
Today, I will talk about “transplanting” an MTB cassette on a road machine as some of you may have more than one bike and wonder whether you can mix parts.
Before continuing with specific information, a generic explanation of the index-shifting process is in order.
Here’s how index shifting happens:
1. The rider presses or pulls the shifter.
2. The shifter pulls or releases a pre-determined amount of cable.
3. The rear derailleur jumps either to the left or right and brings the chain onto a new cog.
The process appears incredibly simple, but many parameters have to be respected to ensure smoothness.
Essential Terms(bike theory)
- Cable pull
As I’ve said before, index shifters pull or release a pre-calculated cable length called cable pull.
The value of the cable pull cannot be changed.
The rest of the transmission (cassette+rear derailleur) is engineered to work with a specific cable pull and will fail miserably if another shifter is used.
- Rear shift ratio
Derailleurs have a rear shift ratio showing how much the rear derailleur moves per 1mm of cable pull.
For instance, a Shimano 9-speed derailleur has a 1.7 rear shift ratio. So, the derailleur moves 1.7mm per 1mm of cable pull.
- Cog Pitch
That is the center-to-center distance between two neighboring cogs on a cassette.
This value of the cog pitch is critical.
If the cog pitch differs from the one that the derailleur and the shifter are designed for, the shifting will suck.
As an example, the cog pitch of a Shimano 10-speed MTB cassette is 4mm. If you combine that cassette with an 8-speed shifter, the cogs wouldn’t be where the derailleur “expects” them to be.
Now that the theory lesson is over, let’s return to the topic at hand – the compatibility of MTB cassettes with road (and gravel) bikes.
To make a definitive“diagnosis”, we need the following data:
1. The rear shift ratios (RSRs)
2. The cable pull
3. The cog pitch of the cassettes
Table A1: Shimano Derailleurs
| Number ofSpeeds | Rear Shift Ratio(MTB) | Rear Shift Ratio(Road) |
| 6 | 1.7 | 1.7 |
| 7 | 1.7 | 1.7 |
| 8 | 1.7 | 1.7 |
| 9 | 1.7 | 1.7 |
| 10 | 1.2 | 1.7 |
| 11 | 1.1 | 1.4 |
Important: The RSR of Shimano road and MTB derailleurs is the same (1.7) up to 10 speeds.
Then, the RSR of road derailleurs remains 1.7, but that of MTB derailleurs changes to 1.2.
So, I can make the following conclusion (take a pen and a piece of paper to write it down).
6,7,8, 9-speed MTB/Road Derailleurs + 10-speed Road derailleurs travel the same amount per 1 millimeter of pulled or released cable.
Table A2: Shimano Shifters Cable Pull
| Number of Speeds | Cable Pull(MTB) | Cable Pull(Road) |
| 6 | 3.2 | 3.2 |
| 7 | 2.9 | 2.9 |
| 8 | 2.8 | 2.8 |
| 9 | 2.5 | 2.5 |
| 10 | 3.4 | 2.3 |
| 11 | 3.6 | 2.7 |
Conclusion: The cable pull of Shimano shifters is the same up to 9 speeds.
Final Statement Regarding Shimano Cassettes
An MTB cassette can work on a road machine only if it has the same cog pitch as its road equivalent.
If the cog pitches don’t match (or are extremely close), the shifting would be bad.
The cog pitch formula is:
Cog Pitch = Cable Pull x Rear Shift Ratio
In other words, two cassettes can have an identical cog pitch only when they’re built for derailleurs with the same RSR and shifters with the same cable pull.
In Shimano’s case, this condition is present only on 6,7,8, and 9-speed drivetrains.
As mentioned earlier, Shimano’s 10-speed road rear derailleurs have the same RSR as 6,7,8 and 9-speed MTB derailleurs. Thus, you can combine an 8 or 9-speed MTB derailleur with a 10-speed road cassette.
But, you can’t use a 10-speed MTB cassette on a road machine because the cog pitches of 10-speed MTB and Road Cassettes are different.
Table C1: SRAM Derailleurs
| Number ofSpeeds | Rear Shift Ratio(MTB) | Rear Shift Ratio(Road) |
| 7 | 1.1 | – |
| 8 | 1.1 | – |
| 9 | 1.1 | – |
| 10 | 1.3(Exact Actuation) | 1.3(Exact actuation) |
| 11 | 1.12 | 1.3 |
Takeaways
- 10-speed road and MTB Exact Actuation SRAM derailleurs have matching RSRs.
- 11-speed road SRAM derailleurs have the same RSR as 10-speed road and MTB Exact Actuation derailleurs
Table C2: SRAM Shifters Cable Pull
| Number ofSpeeds | Rear Shift Ratio(MTB) | Rear Shift Ratio(Road) |
| 7 | 4.5 | – |
| 8 | 4.3 | – |
| 9 | 4.0 | – |
| 10 | 3.1(Exact Actuation) | 3.1(Exact Actuation) |
| 11 | 3.5 | 3.1 |
Final Statement Regarding SRAM’s Cassettes
SRAM cassettes built for 10-speed exact actuation MTB and Road derailleurs have a matching cog pitch (3.1×1.3=4.03).
Consequently, SRAM’s 10-speed MTB and Road cassettes are interchangeable when used with SRAM components.
SRAM’s 11-speed road cassettes have the same cog pitch as SRAM’s 10-speed MTB and Road cassettes
But SRAM’s 11-speed MTB cassettes have a different cog pitch than 11-speed Road Models, and the two aren’t interchangeable.
A Word On Maximum Cog Capacity
MTB cassettes have a much lower gearing than road models. In some cases, the largest cog has 52 teeth. A similar cassette requires a dedicated MTB derailleur with a high maximum cog capacity.
If your current road derailleur cannot operate with the MTB cassette that you want to install, you have the following choices:
1. A derailleur hanger extender
This is a little adapter that attaches to the original derailleur hanger to lower the derailleur and increase its range.
2. Get an MTB derailleur
As you already know (if you were paying attention), 6-9-speed MTB derailleurs are compatible with up to 10-speed road shifters except for the Tiagra 4700 series (those series are downgraded 11-speed models and differ from the rest of the 10-speed components).
Friction Shifting
Friction shifters are “free”. They move as much as the user wants them to. A friction shifter doesn’t care what’s the RSR of a derailleur and allows you to mix all sorts bike parts.
As a result, the rear shift ratio of the derailleur becomes inconsequential and so does the cog pitch of the cassette. If the derailleur has the capacity to work with the cassette, and the cassette fits on the hub, you can use it.
Well, there you have it. Another rookie tutorial that will make your cycling life a little easier. I hope.
Until next time,
– Rookie
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