Shifter and mech must be designed together
Each shifter pulls a set amount of cable per click, and the mech is built to move the right distance for that pull. If they don't match, the gears won't line up.
- Same brand, same number of speeds is the safe rule.
- Road and MTB usually don't mix. Shimano road and MTB shifters often use different cable pull, even at the same speed. Check the maker's compatibility chart before mixing.
- Electronic groupsets (Di2, AXS) need a mech from the same electronic system.
Cage length and capacity
The cage is the arm that holds the two small pulley wheels. A longer cage can take up more chain, so it handles a wider spread of gears.
| Cage | Shimano name | Typical use |
|---|---|---|
| Short | SS | Road bikes with close-ratio cassettes |
| Medium | GS | Wider road cassettes, compact chainsets, gravel |
| Long | SGS | MTB and touring with wide ranges |
Capacity check: (biggest chainring − smallest chainring) + (biggest sprocket − smallest sprocket). The total must be no more than the mech's listed capacity.
Example: 50/34 chainset with an 11-32 cassette = (50 − 34) + (32 − 11) = 37 teeth.
Biggest sprocket
Every mech also has a maximum sprocket size. For example, a Shimano 105 R7000 SS mech takes up to 30T, while the GS version takes up to 34T. Fit a bigger cassette and the top pulley can hit the sprockets.
How it fits to your frame
| Mount | What it is |
|---|---|
| Standard hanger | A replaceable hanger bolted to the frame. Most bikes use this |
| Direct mount | A Shimano MTB/gravel design that moves the mech slightly back. Some hangers support it |
| UDH | SRAM's Universal Derailleur Hanger. Needed for SRAM Transmission mechs |
If you bend a hanger, replace it with one made for your frame. A bent hanger is one of the most common causes of poor shifting.
FAQs
Can I fit a 12-speed mech to my 11-speed bike?
Only if you change the shifter, cassette and chain too.
Do I need a new cable?
It's worth fitting a fresh inner cable with a new mech so the shifting is crisp.
Why won't my new mech reach the biggest sprocket?
Check the limit screws and cable tension first, then that the cage and max sprocket suit your cassette.