Condensed Answer:
A 9-speed SRAM or Shimano cassette can be installed on an 11-speed MTB SRAM or Shimano hub right away.
A 9-speed SRAM or Shimano cassette will require a 1.85mm spacer to be mounted securely on an 11-speed Shimano or SRAM road hub.
A 9-speed Campagnolo cassette will fit on an 11-speed Campagnolo road hub without the need for a spacer.
Freehub Body Width
The freehub body is the part of the rear hub on which the cassette slides. It’s also the element containing the pawls that brush against an engagement ring to create the buzz sound that freehubs make.
11-speed MTB freehub bodies are approximately 1.85mm narrower than 11-speed road freehub bodies.
11-speed hubs are wider so that the last sprocket can be further away from the spokes.
Road bikes have a higher low gear than MTBs (e.g., 28t big cog). Subsequently, the edges of the largest cog sit low and dangerously close to the spokes. When you add the chain, you have a recipe for chain rubbing that could quickly result in an “eaten wheel”.
To prevent this outcome, 11-speed cassettes are made wider. The hubs have no choice but to reflect this.
Conversely, 11-speed MTB cassettes and hubs can remain narrower because the low gear is larger. (It’s not uncommon to find an MTB cassette with a 42t large cog. The larger the cog, the higher its periphery in relation to the spokes’ base. Thus, there’s no chance of chain rub.
The MTB segment takes advantage of this property and successfully keeps 8/9/10 and 11-speed cassettes and hubs cross-compatible with close to non-modifications. This is very beneficial when making upgrades or simply mixing different bike parts.
That said, the extra width of 11-speed road hubs is not a big problem to overcome when installing a 9-speed cassette on it.
All you have to do is install a 1.85m spacer before sliding the cassette on the hub. Many 11-speed road hubs come with such a spacer to make the unit compatible with more cassettes.
Conclusion 1: A 9-speed MTB cassette can be put on an 11-speed MTB hub right away.
Conclusion 2: A 9-speed MTB or road cassette can be installed on an 11-speed road hub, but only if the user mounts a 1.85mm spacer before sliding the cassette on.
FAQ: What would happen if I don’t use a spacer?
If you don’t use a spacer when installing a 9-speed cassette on an 11-speed road hub, one part of the cassette will be separated from the smallest cog.
Also, the smallest cog will end up inline or below the freehub body. Consequently, the lock ring connecting the cassette to the freehub body won’t have enough surface area to push onto the smallest cog and tighten the entire cassette.
In order for the lock ring to secure the cassette, the smallest cog has to be ever so slightly taller than the freehub body.
Therefore, if you don’t use a spacer, the cassette will be wobbling, and the entire drivetrain will be unstable.
FAQ: Why don’t Campagnolo 9-speed cassettes need a spacer?
Campagnolo 9-speed cassettes do not need a spacer when installed on an 11-speed Campagnolo hub because the two cassettes are of very similar widths. The 11-speed model is about 37.5mm whereas the 9-speed one is 36.4mm.
This is achieved via the following methods:
- The cogs on the 11-speed cassette are closer together (smaller cog pitch)
- The big sprocket on the 11-speed cassette is dished inward to clear the spokes. Аs a result there’s no need to add extra width to the cassette.
The Benefits of Running a 9-speed Cassette on an 11-speed Hub
Parts Recycling
If you already have an 11-speed hub but want to run it on a 9-speed drivetrain, the combination will save you money and eliminate the need to buy a new hub.
Future-proofing
A 9-speed MTB hub can accommodate an 11-speed MTB cassette but isn’t compatible with an 11-speed road cassette.
If you have a road bike and plan to upgrade to 11 speeds eventually, and you can source an 11-speed hub for cheap, use it with your current 9-speed drivetrain and eventually build an 11-speed transmission.
Cheaper Components
The parts used by 9-speed drivetrains (derailleurs, cassettes, chain…etc.) are cheaper than the 11-speed equivalents.
The Downsides of Running a 9-speed Cassette on an 11-speed Hub
Additional Adjustments
As far as the installation is concerned, the only downside is the need to run a 1.85mm spacer if you have a road hub.
Bigger Gear Jumps
9-speed cassettes come with larger jumps between the gears. As a result, the rider has a harder time maintaining high cadence at all times.
Cadence is the number of crank rotations per minute. It’s accepted that a high cadence (e.g., 90RPM) increases one’s average speed while minimizing fatigue and lowering joint stress.
Having said that, 9 gears are still plenty smooth, especially for recreational riders.
Summary: What You Need To Know
- Shimano and SRAM 9-speed cassettes can be installed on Shimano and SRAM 11-speed MTB hubs.
- Shimano and SRAM 9-speed cassettes need a 1.85mm spacer when installed on Shimano and SRAM 11-speed Road Hubs
- Campagnolo 9-speed cassettes can be installed on 11-speed Campagnolo hubs right away.
