Condensed answer: Chainrings are technically speed specific because the outer width of the chain changes with each gear increase or decrease.
Those changes directly influence the width of the front derailleur’s cage and the distance between the chainrings. As a result, the distance between the chainrings varies too.
There are also custom chainrings made for very specific chains.
The Connection Between Chain Width and The Number of Gears
Chains have an inner and an outer width. The inner width is the distance between the inner plates; the outer width is the distance between the outer plates.
The inner width of a chain either doesn’t change or does so very slightly with each gear increase or decrease.
The outer width, however, decreases with each gear increase. Since the inner width remains the same, this is achieved by thinning out the outer plates.
This engineering is necessary because the overall width of a cassette changes very little with each gear increase. (This is done to ensure that one rear hub can accept a great number of cassettes.)
Consequently, to fit an extra cog, the space between individual sprockets has to decrease. The decreased distance necessitates a narrower chain or else the chain’s outer plates will rub against the cogs.
The varying width of the chain influences the front mech of the bike too. The dependencies are listed below:
- A narrow chain requires a narrow front derailleur cage for optimal shifting.
- The wider the chain, the more space there is between the chainrings.
- The greater the inner width of a chain, the thicker the chainrings can be.
Consequently, it’s technically correct to consider chainrings speed (or chain) specific.
Chain Width Table
The table below presents the inner and outer width of 7 to 12-speed chains:
|Number of Speeds||Inner/Roller Width||Outer width|
|7||2.38mm||7.3mm (Shimano), 7.1mm (SRAM)|
|8||2.38mm||7.3mm (Shimano), 7.1mm (SRAM)|
- 7 and 8-speed chains have a notably greater inner width than 9/10/11/12-speed chains. For that reason, chainrings made specifically for those chains can be slightly thicker.
- The inner width of 9/10/11/12 chains is identical. Consequently, the chainrings made for them can have matching thicknesses. That said, the outer chain width gets progressively narrower. This change necessitates narrower derailleur cages and influences chainring proximity.
The distance between the chainrings matters too when the gearing is indexed. Indexed shifters move the derailleur a pre-determined number of millimeters with each click. As a result, shifting is fast and easy.
To achieve precision, derailleurs have a specific shift ratio. The shift ratio indicates how much the derailleur moves per 1mm of cable pulled or released by the shifter. That shift ratio is specific to the number of gears that the bike has.
If the derailleur does not have the correct shift ratio for the particular drivetrain, the shifting process won’t be as smooth, and the chain could rub against the cage.
Due to this dependence, one can see cranks (rather than chainrings) as speed specific. If a rider uses a set of cranks that positions the chainrings at locations that aren’t correct for the indexing of the particular derailleur, it’s possible to experience less-than-ideal shifting.
Note: Friction shifters do not create this issue and allow the user to mix all sorts of bike parts because it’s up to the rider to determine how much the shifter moves. This freedom makes it possible to eliminate all discrepancies.
There are also chains made for specific chainrings. For example, Shimano’s 12-speed chains are optimized for downshifting and have a smaller inner width than normal. Subsequently, the chain has a hard time grabbing chainrings that aren’t compatible with Shimano’s HG+ system. In some cases, the chain gets stuck and fails to move along the chainring.