Condensed answer: Standard BMX chains are 1/8″ (3.2mm) wide and cannot be used on a geared mountain bike because the outer plates would rub against the adjacent cogs of the cassette.
A BMX chain can be installed on a mountain bike only when the bicycle has a single-speed drivetrain including a cog and a chainring of a size compatible with the chain.
Bicycle chains have two widths: inner and outer. The inner width is the distance between the inner plates; the outer width is the distance between the outer plates. (image below)
Usually, the chain size on the label indicates the inner width.
The default size of single-speed chains is 1/8″ (3.2mm). There are also narrower 3/32″ (2.4mm) chains designed to save weight. That said, narrower chains aren’t always lighter because the final weight depends on the model.
It’s also possible to find extra-wide 3/16” (4.8mm) BMX chains. The additional thickness provides strength and reduces the chances of chain failure when performing various stunts.
Single-speed chains are significantly thicker than geared chains for the following reasons:
- The rider cannot shift to a lower gear. Consequently, the chain is under great stress, especially when climbing. To reduce the chances of failure, the chain has to be stronger.
- The chain’s width isn’t restricted by the rear cog.
Chains designed for multiple gears controlled by a derailleur system decrease in width with each gear increase.
Or in other words, an 11-speed chain is narrower than a 9-speed chain.
This happens because the overall width of a cassette doesn’t increase dramatically with each extra gear. The goal is to keep rear hubs compatible with a greater number of cassettes so that the user can change/upgrade their drivetrain without having to replace the hub.
To preserve the overall width of a cassette the same, the rear cogs are positioned closer to each other and made ever so slightly thinner.
The table below contains the width of various chains designed for geared bicycles:
|Number of Speeds||Internal Width|
|1 (single-speed)||1/8″ (3.2mm), 3/32″ (2.38mm)|
The data reveals that 6,7 and 8-speed chains have the same internal width as 3/32″ single-speed chains.
The external width of 3/32″ single-speed chains is also close to that of 6,7 and 8-speed chains, and in some cases, the chain can operate without rubbing against the cogs of the cassette.
However, the width of a chain is not the only source of incompatibility. The shape of the chain’s plates matters too.
Chains designed for shifting have inner plates with a shape facilitating the movement of the chain from one cog/chainring to the next. Single-speed chains, on the other hand, have no such architecture because the chain isn’t meant to move sideways.
The center-to-center distance between two cogs is known as cog pitch. The cog pitch of a cassette decreases as the number of speeds increases.
For that reason, it’s not recommended to rely on a chain designed for fewer speeds (the extra width causes rubbing).
However, it’s possible to use a chain designed for more gears than the bike has. (e.g., using an 11-speed chain on a 10-speed drivetrain).
The table below contains the cog pitches of various cassettes. The data is for mountain bikes. (Road cassettes often have a different pitch.)
|Number of Speeds||Shimano||SRAM|
Single-speed Mountain Bikes
Single-speed mountain bikes are fairly rare because they don’t have the ability to cover diverse terrain. The lack of low gearing makes climbing difficult and in some cases impossible.
Most of the time, single-speed MTBs are used for stunts or recreational purposes.
A single-speed MTB can operate just fine with a BMX chain as long as the chain is compatible with the cogs and the chainring.
A standard 1/8″ (3.2mm) chain is wide enough for pretty much every cog and chainring. However, if the cog and the chainring are designed for a thinner chain, the chain will not be as secure as possible and there may be some side-to-side movement.
If the chain is 3/32″ (2.4mm), a 1/8″ cog and chainring won’t fit through it.
The Benefits Of Combining a BMX Chain With an MTB
There are two main incentives to install a BMX chain on an MTB:
Strength. BMX bicycles are designed for aggressive riding and come with tough components. A BMX chain offers a lot of strength and is less likely to fail than a thinner geared chain.
Looks. Some people like the looks of BMX components and install them on MTB bikes. Of course, this practice is more common when the MTB in question is used for street riding.