Condensed answer: An MTB shifter can technically be used on a road bike. Ideally, the derailleur and the cassette will be designed for an MTB too. Otherwise, the shifter will not move the derailleur accurately and will create a poor shifting experience. There are a few exceptions.
Friction and Index Shifting
There are two types of shifting mechanisms – friction and index shifting.
Friction shifting is the old school system found on bikes before the late 80s and early 90s. Friction shifter move as much as the rider wants them to. In other words, they are “free” and therefore it’s up to the rider to find the correct position of the shifter for a shift to occur.
Friction shifting is slower and fairly difficult for a beginner to learn. However, it has one major benefit – a friction shifter is compatible with both road and MTB derailleurs precisely because it’s not locked in a pattern.
In the late 80s, friction shifting was replaced by index shifting to make the process faster and simpler.
Indexed shifters have segregated clicks. Each click moves a pre-determined amount of cable (cable pull) which in return moves the rear derailleur.
The rider does not guess anything, and if the derailleur and the cable tension are set adequately, a shift occurs immediately.
The ease and precision are based on three variables:
- Cable pull
- Rear derailleur ratio
- Cog pitch
The rear derailleur ratio describes how much the derailleur moves per 1mm of cable pulled or released by the shifter. For example, if the rear derailleur ratio is 1:1.7, then the derailleur moves 1.7mm per 1mm of cable pulled or released by the shifter.
The cog pitch is the center-to-center distance between two cassette cogs.
The cable pull, the rear derailleur ratio, and the cog pitch have to be “made for each other” for index shifting to work.
If one of the elements has a non-matching value, then index shifting is either not ideal or a complete failure.
The cog pitch is somewhat consistent among MTB and road systems. However, the cable pull and the rear derailleur ratios differ significantly by type (MTB or road), brand, and number of gears.
Road and MTB Rear Derailleur Ratios
In order for an MTB shifter to fit in an indexed road transmission, the following requirements have to be met:
- Shifter’s speed = number of cogs on the cassette
- The rear shift ratio of the current derailleur has to match that of the MTB rear derailleur that the shifter is designed for.
The table below contains the rear shift ratios of road and MTB derailleurs:
| Brand | Number of Speeds | Rear Shift Ratio (MTB) | Rear Shift Ratio (Road) |
| Shimano | 8 | 1.7 | 1.7 |
| Shimano | 9 | 1.7 | 1.7 |
| Shimano | 10 | 1.2 | 1.7 |
| Shimano | 11 | 1.2 | 1.4 |
| Shimano | 12 | 1.2 | 1.4 |
| Campagnolo | 8 | – | 1.4 |
| Campagnolo | 9 | – | 1.5 |
| Campagnolo | 10 | – | 1.5 |
| Campagnolo | 11 | – | 1.5 |
| SRAM | 8 | 1.1 | 1.1 |
| SRAM | 9 | 1.1 | 1.1 |
| SRAM Exact Actuation | 10 | 1.3 | 1.3 |
| SRAM Exact Actuation | 11 | – | 1.3 |
| SRAM X-Actuation | 11 | 1.12 | 1.12 |
| SRAM X-Actuation | 12 | 1.01 | 1.01 |
The next table contains the cable pull of many MTB and road shifters:
| Brand | Speeds | Cable Pull (road) | Cable Pull (MTB) |
| Shimano | 8 | 2.8 | 2.8 |
| Shimano | 9 | 2.5 | 2.5 |
| Shimano | 10 | 2.3 | 3.4 |
| Campagnolo | 8 | 3.5 | – |
| Campagnolo | 9 | 3.0 | – |
| Campagnolo | 10 | 2.8 | – |
| Campagnolo | 11 | 2.6 | – |
| SRAM | 8 | 4.3 | 4.3 |
| SRAM | 9 | 4 | 4 |
| SRAM Exact Actuation | 10 | 3.1 | 3.1 |
| SRAM Exact Actuation | 11 | 3.1 | – |
| SRAM (X-Actuation) | 11 | – | 3.48 |
Conclusions:
- Shimano
Shimano’s 8 and 9-speed MTB/road rear derailleurs have a 1.7 rear shift ratio. Shimano’s 10-speed road derailleurs have a 1.7 rear shift ratio too.
Consequently, it’s possible to combine an 8/9 MTB Shimano shifter with an 8/9 road Shimano derailleur.
However, the fact that the rear shift ratios of Shimano’s 10-speed road derailleurs and Shimano’s 8/9 MTB derailleurs match, does not mean that one can use a 10-speed MTB shifter with a 10-speed road derailleur right away.
Why? Because 10-speed MTB shifters have a different cable pull. Consequently, when you combine a 10-speed MTB shifter and a 10-speed road derailleur, it’s also necessary to get a 10-speed MTB cassette. Otherwise, the derailleur will not move to the right place due to the dissimilar cable pull.
- SRAM
SRAM’s Exact Actuation 10 and 11-speed derailleurs have the same rear shift ratio (1.3). The 10 and 11-speed Exact Actuation shifters have a matching cable pull too. Consequently, it’s possible to use a 10/11 speed MTB shifter with a 10/11 speed derailleur from that group.
- Campagnolo
Campagnolo derailleurs have a different rear shift ratio than Shimano and SRAM. Consequently, they cannot be combined with shifters outside of the Campagnolo family. (Campagnolo only produces road parts.)
(Of course, even Campagnolo derailleurs will work with a friction MTB shifter.)
Cassettes
For best performance, it is recommended to get an MTB cassette too. In some cases, the difference in cog pitch will cause poor shifting.
The Advantages of Installing an MTB Shifter On a Road Bike
There are two main advantages to installing an MTB shifter on a road bike, namely:
- Non-drop bar compatibility
The brake-shifters found on road bikes are specifically designed for drop bars. In certain situations, they can be integrated into some alternative bar systems (e.g., bullhorn handlebars), but if you intend to install flat bars, for example, a standard MTB shifter will be much better.
- Integration of MTB cassettes
MTB cassettes offer much lower gearing making climbing easier. For example, a standard road cassette may have a large rear cog with 25 teeth whereas that of an MTB cassette is 34+ teeth.
The largest MTB cassettes (42 or more teeth) cannot operate with most road derailleurs. The cages of road derailleurs are simply too short. As a result, the derailleur cannot climb the cassette. There are some exceptions, of course. And it’s also possible to use a third-party derailleur-hanger extender. That said, it’s more efficient and aesthetically pleasing to simply use the correct derailleur for the cassette.
The Disadvantages of Installing an MTB Shifter On a Road Bike
The conversion has the following downsides:
- Limitations
As already explained, the indexed system comes with many limitations when trying to perform this conversion without replacing the entire drivetrain (cassette, chain, derailleurs…etc.)
- No longer a road bike
Conversion to an MTB shifting system will turn a road bike into a hybrid machine. This isn’t bad if the new style suits the rider’s goals. However, one will no longer enjoy the aerodynamics and aggressiveness that a road bike has to offer.
