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Condensed Answer:

A 9-speed chain can technically operate with a 7-speed cassette, but the combination is not recommended as it results in “weird” shifting. The incompatibility is due to the internal and external narrowness of 9-speed chains.

For best performance, it’s recommended to use a dedicated 7/8 speed chain.

The Connection Between Chain Width and Bike Speeds

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 external plates (image below).

• The inner width of 7 and 8-speed chains is larger than that of 9/10/11/12 models.

• 9/10/11/12-speed chains have that same inner width (2.18mm)

• The inner chain width is kept the same because the thickness of the cogs remains identical or close to it even when the number of gears increases.

• However, the outer width of multi-speed chains decreases with each gear increase. This happens because the overall width of bike cassettes is kept the same even when the number of gears and respectively cassette cogs goes up.

• Since the thickness of cassette cogs doesn’t change, the only way to fit more of them within the same space is to decrease the spacing between each cog. The decreased spacing requires a narrower chain or else the chain’s outer plates will rub against the cogs.

• Since the inner width of the chain doesn’t change much or at all, the only way to achieve a narrower chain is to thin out the external plates. For that reason, when all parameters are equal, chains designed for fewer speeds are stronger.

The table below contains the data for 7/8/9/10/11/12-speed chains.

The next table contains the sprocket thicknesses and cog pitches of 7/8/9/10/11/12 cassettes. (The cog pitch is the center-to-center distance between two cogs on a cassette.)

Conclusions:

The inner width of 9-speed chains is 2.18mm. Consequently, they aren’t compatible with Shimano’s IG 7-speed freewheels.

IG stands for Interactive Glide – an older Shimano technology. If you have a 7-speed IG freewheel or cassette, you won’t be able to use a 9-speed chain because its inner width is only 2.18mm and thus insufficient for the thickness (2.35mm) of 7-speed IG sprockets. In that case, you will need an IG chain.

7-speed IG freewheels aren’t super common, however. Chances are that you have a 7-speed HG model. In that case, the sprockets are narrow enough (1.85mm) to fit within the inner plates of a 9-speed chain.

The external width of 9-speed chains is up to 0.6mm less than that of 7-speed chains. The chain will not rub against the cogs, but its narrowness results in latency because the derailleur has to travel more to reach the chain and move it onto the next cog.

If you want optimal shifting, it’s recommended to use a dedicated 7 or 8-speed chain.

Potential Problems At The Front

If the chainrings are also designed for a 7-speed drivetrain, the spacing between them will be ever so slightly larger too (to make space for the wider chain). Consequently, a 9-speed chain might get stuck between the chainrings when shifting at the front.

This outcome isn’t guaranteed, but it’s technically a possibility and deserves to be mentioned.

Summary: What You Need To Know

• 9-speed chains are too narrow for Shimano’s 7-speed IG freewheels and cassettes. In that case, the only option is an IG chain.

• 9-speed chains are compatible with Shimano’s 7-speed HG freewheels and cassettes, but the user could experience latency due to the extra distance than the derailleur has to travel to initiate a shift. Another issue could be a stuck chain between the chainrings.

• For best performance, it’s wiser to use a dedicated 7/8-speed chain.