Condensed answer: An 8-speed cassette can be installed on an 11-speed hub. If the hub is designed for an MTB, the cassette should fit right away. If the hub is made for a road bike, a 1.85mm spacer will be needed for a secure fit.
Cassette and Freehub Body Width
It’s logical to assume that 11-speed cassettes are much wider than 8-speed models and thus require a hub of greater width.
However, this isn’t really the case. The widths of 8/9/10 and 11-speed cassettes are very close. The main difference is the cog pitch (the center to center distance from one cog to the other).
Cassettes with more speeds have a shorter cog pitch in order to fit more cogs within the same space. (For that reason, chains designed for cassettes with a greater number of speeds have narrower outer plates.)
This property makes MTB 8/9/10 and 11 cassettes compatible with 8/9/10 and 11-speed freehub bodies.
The freehub body is the part of the hub on which the cassette slides. It also contains the engagement pawls which brush against a ratcheting ring inside the hub and create the popular buzz sound that rear hubs make.
Road cassettes are a bit different. The 8,9 and 10-speed cassettes and hubs are similar, but the 11-speed hubs are wider by about 1.85mm.
This is done intentionally to push away the largest sprocket of the cassette away from the spokes and prevent rubbing.
Road cassettes have a smaller large sprocket (e.g., 28t) than MTB models. The periphery of the large sprocket is very close to the base of the spokes inserting into the hub. As a result, the chain could rub against the spokes and slowly “eat the wheel”.
MTBs do not face this problem because they have a much larger low gear (e.g., 42t) which easily clears the spokes.
Therefore, if you want to install an 8-speed cassette on an 11-speed road hub, you will need a 1.85mm spacer.
Why a 1.85mm Spacer?
To find out the width of the spacer, we need the following data:
- The width of a 10-speed freehub body
- The width of an 11-speed freehub body
The difference between the two widths determines the size of the spacer.
11-speed freehub bodies are about 36.75mm whereas 10-speed freehub bodies are 34.95mm.
The difference is 1.8mm.
FAQ: What will happen if I don’t use a spacer?
Without the spacer, you won’t be able to fully tighten the cassette. Also, the cassette will move across the freehub body and cause poor shifting experience and overall drivetrain instability.
FAQ: What are the advantages and disadvantages of running an 8-speed cassette on an 11-speed hub?
The main pros of running an 8-speed cassette on an 11-speed hub are part recycling and money savings. The combination allows you to forgo the purchase of a new hub while also giving you the ability to run an 8-speed chain and derailleur which are a lot cheaper than the 11-speed versions.
The downsides are the required rearrangements as well as the larger gear jumps that come with 8-speed cassettes. The bigger the jumps between the sprockets, the more difficult it becomes to maintain consistent cadence (rotations of the cranks per minute). Thus, cyclists obsessed with optimal cadence might consider this combination a massive downgrade.
FAQ: Can I install an 8-speed Campagnolo cassette on an 11-speed Hub?
No, because 8-speed Campagnolo cassettes were designed for different freehub bodies than the 11-speed models. Therefore, an 8-speed Campagnolo cassette won’t fit on an 11-speed hub.
Having said that, it’s still possible to put 8 speeds on an 11-speed hub even when using Campagnolo components. The catch is that you will have to use a hack.
You can run an 8-speed system on a Campagnolo wheel if you remove the largest sprocket from a 9-speed cassette (Veloce/Mirage/Miche) and then add a spacer to make up for the gap.
Related Question: Can you fit an 11-speed cassette on an 8-speed hub?
Summary: What You Need to Know
- An 11-speed MTB hub can accept an 8-speed cassette right away.
- 11-speed road hubs are made wider to push the largest sprocket away from the spokes.
- An 11-speed road hub needs a 1.85mm spacer when combined with an 8-speed cassette.
- By default, 8-speed Campagnolo cassettes are not compatible with 11-speed hubs.
- 8-speed cassettes are more cost-efficient but come with larger gear jumps making it harder to maintain a consistent cadence.