Condensed Answer: In most cases, 9-speed chains can be used on 8-speed cassettes.
9-speed chains are slightly narrower, but their dimensions do not make them completely inadequate when integrated into an 8-speed drivetrain.
That said, it’s recommended to get a dedicated 8-speed chain for the best performance.
Inner and Outer Chain Width
Bicycle chains have an inner and outer width. The inner width is the distance between the inner plates whereas the outer width is the distance between the external plates.
The inner width of a chain does not change much or at all with each gear increase or decrease.
However, the outer width diminishes as the number of gears climbs higher.
This engineering is necessary because:
- The thickness of cassette sprockets changes little with each gear increase. For that reason, the inner width of the chain cannot change much either.
- The overall width of a cassette doesn’t change dramatically with each gear increase. This is done to increase the number of cassettes that a rear hub is compatible with. Consequently, the space between sprockets gets smaller. Or in other words, the cassette gets denser.
The increased density requires a narrower chain to avoid contact between the outer plates and the cogs adjacent to the gear currently in use.
The table below contains the outer width of 7-12 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) |
| 9 | 2.18mm | 6.5-7mm |
| 10 | 2.18mm | 5.88-6mm |
| 11 | 2.18mm | 5.5-5.6mm |
| 12 | 2.18mm | 5.3mm |
Conclusion:
a. The inner width of 9-speed chains is 0.2mm narrower than that of 8-speed chains.
b. The outer width of 9-speed chains is about 0.8mm narrower.
The outer width difference is not expected to cause issues. The inner width discrepancy, however, could make the chain bind ever so slightly to the cassette.
Whether this will be noticeable depends on the entire drivetrain and especially the thickness of the sprockets. Some 8-speed cassettes have thinner sprockets. In that case, the problem will not manifest.
The table below contains the sprocket thickness of various cassettes:
| Number of Speeds | Sprocket Thickness |
| 7 | 1.85mm |
| 8 | 1.85mm |
| 9 | 1.78mm |
| 10 | 1.6mm |
| 11 | 1.6mm |
Conclusion: The sprockets on 9-speed cassettes are 0.7mm narrower. The chains have no choice but to reflect that property and therefore have a narrower inner width.
The thickness of 8-speed sprockets is 1.85mm whereas the inner width of 9-speed chains is 2.18mm. Consequently, there’s enough space for the sprocket. The downside, however, is that the chain isn’t as free to move. This could result in slightly accelerated drivetrain wear. It’s also possible for the shifting to be less snappy than expected.
That said, the difference isn’t dramatic enough to make the combination (9-speed chain + 8-speed cassette), inadequate.
FAQ: What about using an 8-speed chain on a 9-speed cassette?
An 8-speed chain is too thick for a 9-speed cassette. The outer plates of the chain will rub against the cassette cogs. The result will be poor shifting, noise, and premature drivetrain wear.
