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Condensed Answer: In most cases, a 10-speed chain can be successfully used with a 9-speed crankset. The dimensional differences between 10-speed and 9-speed chains are not substantial enough to create complete incompatibility.

Understanding Chain Width

A chain has 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 and outer width of a chain depend on the number of speeds that the unit is designed for. The table below contains the inner and outer width of 7-12 speed chains:

As the data shows, 10-speed chains have the same inner width as 9-speed chains but are narrower overall by about 1mm.

Why?

The overall width of a cassette changes very little with each gear increase. The purpose of this engineering is to maximize the number of cassettes that a hub is compatible with. Additionally, the thickness of cassette sprockets remains similar across all gears too.

Consequently, the only option left to fit all sprockets within roughly the same dimensions is to decrease the so-called cog pitch. The cog pitch is the center-to-center distance between two cogs.

The decreased cog pitch requires a narrower chain or else the chain’s outer plates will rub against the cogs.

For the reasons above, the outer width of bike chains decreases when the number of speeds increases. This is achieved by thinning out the outer plates of the chain.

The varying outer width of chains requires minor changes to the front derailleur and the crankset.

For example, a front derailleur designed for 10-speed chains will have a narrower cage than a derailleur made for 9-speed. Also, the distance between the chainrings will be smaller too.

If a 10-speed chain is combined with a 9-speed front derailleur, there will be lower chances of rubbing between the cage and chain thanks to the extra space that a 9-speed FD cage comes with. However, that extra space could theoretically make shifting slower/less snappy than what’s necessary for optimal performance.

Furthermore, 9-speed chainrings have slightly more space between them because they’re made for a wider chain. The extra space necessitates a front derailleur with a different pull ratio. The pull ratio shows how much the derailleur moves per 1mm of cable pulled or released by the shifter. Since in this case, we have more space between the chainrings, the derailleur needs a longer travel.

For the reasons above, combining a 10-speed chain with 9-speed cranks works best in the following scenario:

• The drivetrain has a single chainring or the front derailleur is made for 9-speeds. If a 10-speed front derailleur is used, its pull ratio could cause shifting issues because the derailleur will not be moving to the appropriate location.

• The bike uses a friction shifter for the chainrings. A friction shifter is not segmented into clicks and moves as much as the rider wants it to. Consequently, it’s possible to combine all kinds of bike parts because friction shifting makes shift ratios irrelevant. The rider quickly learns how much the derailleur has to move for a shift to occur.

That being said, the differencе between 9 and 10-speed chains is still small. And since chainrings and front derailleurs have lower compatibility requirements than cassettes and rear derailleurs, combining a 10-speed chain and 9-speed cranks is expected to work in the vast majority of cases.

FAQ: Can a 10-speed chain be combined with a 9-speed cassette?

Yes. A 10-speed chain can be combined with a 9-speed cassette because it’s narrower and won’t rub against the cogs of the cassette. That said, the opposite (a 9-speed chain on a 10-speed cassette) is not a good combination because the 9-speed chain is wider than the 10-speed unit and will rub against the cogs.

The downside of using a 10-speed chain on a 9-speed cassette is that it will potentially shift slightly slower. Whether this effect will be perceivable depends on the entire drivetrain.