Condensed Answer: Most 10-speed rear derailleurs aren’t compatible with a 7-speed indexed shifter and cassette. There is one exception, however.
Shimano’s 10-speed Road derailleurs can operate with a 7-speed Shimano shifter and cassette. In all other cases, you will need a derailleur of fewer speeds for crisp index shifting.
If you use friction shifters, all 10-speed derailleurs will work with a 7-speed cassette.
Rear Shift Ratio
The rear shift ratio of a derailleur indicates how much the unit moves per 1mm of gear cable pulled or released by the shifter.
Indexed shifters move a predetermined, unchangeable amount of cable per click/shift. This is the so-called cable pull.
The amount of distance traveled by the derailleur during each indexed shift can be calculated via the following formula:
Traveled Distance = Cable Pull x Rear Shift Ratio
In order for a 10-speed derailleur to perform well when coupled with a 7-speed cassette and shifter, it has to have the same travel distance as a 7-speed derailleur.
To meet this conditions, the 10-speed derailleur must have the same rear shift ratio as the 7-speed models.
If the rear shift ratios are different, the movement of the derailleur across the cassette will be inaccurate and will cause irregular shifting and chain rubbing against the cogs.
The rear shift ratio of 7-speed Shimano derailleurs is 1.7. This means that the derailleur moves 1.7mm per 1mm of cable pull or release initiated by the shifter.
The cable pull of 7-speed Shimano shifters is 2.9mm.
Thus, the 7-speed derailleur moves – 1.7 x 2.9mm = 4.93mm per shift.
Meanwhile, the rear shift ratio of Shimano 10-speed MTB derailleurs is 1.2. Thus, a 10-speed MTB derailleur would move only 3.48mm (1.2 x 2.9mm) when combined with a 7-speed shifter, and the shift won’t be complete.
Only 10-speed Shimano Road derailleurs can be effectively combined with a 7-speed Shimano shifter and cassette because they have a 1.7 rear shift ratio that matches that of 7-speed Shimano derailleurs.
Another important property for derailleur and cassette compatibility is range. If the cassette is too big for the derailleur, the mech will fail to reach the largest cog.
In the case of 7-speed cassettes, however, this is rarely an issue because the largest 7-speed cassette usually goes up to cogs with 34 teeth – not a problem for 10-speed derailleurs.
Therefore, the only requirement in this scenario remains the rear shift ratio.
Rear Shift Ratio Table
The table below contains the rear shift ratios of popular 10 and 7-speed derailleurs. It shows clearly that only Shimano’s 10-speed road models are compatible with an indexed 7-speed derailleur.
|Brand||Number of Speeds||Type||Rear Shift Ratio|
|Shimano||7||MTB & Road||1.7|
|SRAM||10||MTB & Road||1.3|
Friction Shifters Do Not Care About Rear Shift Ratio
Friction shifters are old-school shifters normally found on retro road bikes and touring models.
Unlike indexed shifters, friction shifters are unrestricted. They move smoothly and do not click. It’s up to the rider to determine when to stop shifting and choose a position for the shifter.
This property of friction shifters provides a lot of flexibility by making rear shift ratios irrelevant because the shifter can compensate for all the differences.
If you plan to build a bike of mixed MTB and road parts, friction shifters will greatly facilitate this task.
If you have a 10-speed derailleur of any kind, it will work with a 7-speed cassette as long as you use friction shifters.
That said, friction shifters have their shortcomings, namely:
- Non-user friendly
Friction shifters are a bit like a manual car. They provide flexibility, but it comes at a price – more input is needed for the proper operation of the machine. Many cyclists find it annoying to trim the derailleur after every shift.
Ultimately, friction shifters aren’t loved by beginners and are absent from most commercial bikes.
- Slow and unstable
Friction shifters are installed on the downtube or stem (retro road bikes), the end of the handlebars (many touring bikes), or on the handlebars (modern models).
Downtube, stem, and bar-end shifters come with inherent instability whether they’re indexed or not.
However, even when the friction shifters are placed on the handlebars they still aren’t as quick as common indexed shifters.