Condensed Answer:
A 12-speed cassette can be used on an 8-speed hub when:
- The hub is made by Shimano or SRAM (HyperGlide or PowerGlide series) and is combined with 12-speed cassettes made for HyperGlide hubs (by any brand) such as SRAM NX and SX 12-speed cassettes.
The Requirements For Compatibility Between a Cassette and a Hub
There are two requirements for a cassette to be compatible with a particular hub
- The splines of the hub should match the cutout on the cassette. If that condition is not met, there is no way to slide the cassette onto the hub.
- The cassette should be narrow enough to fit on the freehub body.
The splines on a hub are the grooves on which the cassette slides. If the cutout of the cassette is not made for the splines of the hub, the two units are incompatible.
Shimano’s hubs from 8 to 11-speed fall into the HyperGlide category (image above). (SRAM’s equivalent is called PowerGlide).
Shimano introduced a new hub with different splines for the 12-speed series. The hubs are part of the so-called Microspline family and are not compatible with non-12-speed cassettes.
Or in other words, a 12-speed Microspline cassette cannot be installed on an 8-speed hub.
That said, there are third-party companies such as SunRace that produce 12-speed HyperGlide cassettes.
Those models can fit on a standard HyperGlide/PowerGlide hub made for at least 8-speeds.
The second compatibility requirement is width.
The table below contains the dimensions of 8 to 12-speed cassettes:
| NUMBER OF SPEEDS | TOTAL WIDTH |
|---|---|
| 8 | 35.4mm |
| 9 | 36.5mm |
| 10 | 37.2mm |
| 11 | 40.6mm |
| 12 | 41.2mm |
As you can see, 12-speed cassettes are about 5.8mm wider than 8-speed models. The difference between the freehub bodies is about as much.
Thus, if we go simply by the numbers, 12-speed cassettes will be too wide for an 8-speed hub.
In reality, however, the hub can accept a 12-speed cassette because the largest (last) sprocket of 12-speed cassettes is not in contact with the freehub. Instead, it’s riveted to the cassette itself.
Consequently, the hub itself does not support the last cog. As a result, the hub does not have to be wider to accept an 11 or a 12-speed cassette.
Above you see a SRAM NX Eagle cassette. Note that the last two sprockets do not reach the freehub body.
What are the advantages of installing a 12-speed cassette on an 8-speed hub?
- Money savings
If you have an 8-speed hub that can accept a 12-speed cassette, a transition from 8 to 12 speeds will be cheaper since you won’t have to buy a new hub and re-lace the wheel.
- Smoother Transitions
More gears on a cassette equal smaller gaps between the cogs and smoother transitions. As a result, the rider can maintain a higher cadence with greater ease.
The term cadence indicates the number of crank rotations per minute. High cadence (e.g., 90RPM) is associated with efficient and consistent output.
What are the downsides of installing a 12-speed cassette on an 8-speed hub?
- Limited choice
The main downside of keeping an 8-speed hub when switching to 12 speeds is the limited number of cassettes that you can use.
