Condensed Answer: A 12-speed chain can operate with a 10-speed cassette. The only exception are Shimano’s new 12-speed chains. Those don’t mix well with 10-speed chainrings.
Understanding Chain Width
A bicycle chain has an outer and an inner width. The outer width is the distance between the external sides of the plates. Or in other words, the outer width is the total width of the chain.
The inner width is the distance between the inner plates surrounding the chainring and the cogs.
The inner width of a chain changes very little with each gear increase or decrease.
However, the outer width decreases by a notable amount with each gear jump. The purpose of this engineering is to avoid rubbing of the chain against the cassette cogs.
As the number of gears increases, so does the density of the cassette. To ensure that a cassette is compatible with a maximum number of hubs, the overall width of the cassette doesn’t change much, however.
Consequently, the only way to fit cassettes with more gears within the same dimensions is to decrease the space between the cogs.
For that reason, chains designed for more gears have thinner outer plates and are inherently weaker than models designed for fewer speeds. (Ultimately, single-speed chains are the thickest and strongest.)
The outer width of a 12-speed chain is roughly 5.25mm whereas that of a 10-speed chain is 5.9mm. That’s a 0.65mm difference.
Meanwhile, the inner width of both 12 and 10-speed chains is about 2.18 mm. Thus, the only dimensional differences between the two types of chains are the total widths.
You can go up, but you can’t go down.
It’s possible to use a chain designed for more speeds on a cassette with fewer gears than what the chain is originally intended for (e.g., a 12-speed chain on a 10-speed cassette.) But the opposite cannot be done, because chains made for fewer speeds are thicker and will cause gear jumping and rubbing.
The Downsides of Using a 12-speed Chain on 10-speed Cassettes
12-speed chains are thinner and consequently need slightly more time to reach the desired cog when used on a 10-speed cassette. The result could be reduced responsiveness in comparison to a 10-speed chain.
That said, it’s hard to say whether the rider will notice a difference. It depends on the entire setup.
- Shimano’s 12-speed Chain Incompatibility
Shimano’s new 12-speed chains have been redesigned to improve downshifting and chain retention. This goal is accomplished via narrower/extended inner plates and ramps on the chain.
The extended inner plates of 12-speed chains make the chain too narrow for a regular 10-speed chainring.
Consequently, a 12-speed Shimano chain cannot properly “grab” a 10-speed chainring.
- Weaker Chain
A quality 12-speed chain may be stronger than a cheap 10-speed one. However, when all parameters are equal, a 10-speed will be thicker and consequently stronger.
12-speed components tend to do more expensive. For that reason, it makes little sense to purchase a 12-speed chain for a 10-speed drivetrain.
The Advantages of Using a 12-speed Chain On 10-speed Cassettes
The narrow profile of 12-speed chains completely eliminates chain rubbing and thus results in silent operation. Hence why some people purposefully use chains designed for more speeds.
- Potential Money-savings
If you already have a 12-speed chain from another build, using it on a 10-speed system will save you some money that could be invested elsewhere.
- One more chain option
If you have a 10-speed drivetrain and for some reason, you can’t find a 10-speed chain in-store, a 12-speed chain will save the day as long as it’s compatible with the chainrings you have.
FAQ: What about using a 10-speed chain on a 12-speed cassette?
A 10-speed chain is 0.65mm thicker than a 12-speed one and will therefore rub against the cogs of the cassette. This results in poor shifting and premature component wear. Given the price of 12-speed components, it makes no sense to perform similar experiments. If you have a 12-speed system, the best choice is to get a 12-speed chain compatible with it.
Summary: What You Need To Know
- 12-speed chains, apart from Shimano’s newest models, can operate with 10-speed cassettes.
- It’s possible to combine a 12-speed chain with a 10-speed cassette because 12 and 10-speed chains have the same inner width (the distance between the inner plates of the chain).
- The total (outer) width of 10-speed chains is greater than that of 12-speed chains. Consequently, a 10-speed chain creates rubbing and component wear when integrated into a 12-speed drivetrain.
- The main downsides of using a 12-speed chain with a 10-speed cassette are the extra cost and potential delay/slow responsiveness.