Short answer:
- A 9-speed Shimano or SRAM cassette can be installed on a 9-speed Shimano or SRAM hub right away.
- A 9-speed Campagnolo cassette cannot be installed on an 8-speed hub.
MTB Cassettes
8/9/10 and 11-speed MTB cassettes are extremely close in overall width and are therefore cross-compatible when made for the same type of hub.
This is achieved via the following modifications:
- Shorter distances between cassette sprockets
With each gear increase, the sprockets get closer to one another. Or in other words, the cassette becomes denser. The center-to-center distance between two sprockets is known as cog pitch.
The table below contains the cog pitches of 8,9,10 and 11-speed MTB cassettes.
| Number of Speeds | Cog Pitch | Total Width |
| 8 | 4.8 mm | 35.4mm |
| 9 | 4.35mm | 36.5mm |
| 10 | 3.95mm | 37.2mm |
| 11 | 3.9mm | 40.6mm |
The total width difference between 9 and 8-speed cassettes is only 0.9mm. This value isn’t substantial enough to make 9-speed cassettes too wide for 8-speed hubs.
The 9-speed cassette is narrower, but its smallest cog is still at the sweet spot – ever so slightly taller than the freehub body. As a result, the cassette lock ring can successfully tighten the sprockets to the freehub body.
FAQ: What is a freehub body?
The freehub body is the part of the hub on which the cassettes slide.
The freehub body has splines guiding the cassette and acting as anchor points for the cogs.
Also, the freehub body contains spring-loaded pawls which brush against a ratchet ring to create the popular buzz sound that rear hubs make.
- Thinner Sprockets
Another factor that influences cassette width is sprocket thickness. With each gear increase, the sprockets of a cassette get thinner.
The table below contains the data for 8,9,10 and 11-speed MTB cassettes:
| Number of Speeds | Sprocket Thickness |
| 8 | 1.85mm |
| 9 | 1.78mm |
| 10 | 1.6mm |
| 11 | 1.6mm |
Conclusion: The sprockets of 9-speed cassettes are only 0.7mm narrower than those of 8-speed models.
FAQ: What about road cassettes?
The above applies to 8 and 9-speed road SRAM and Shimano cassettes. However, the case of Campagnolo is different.
8-speed Campagnolo cassettes have a different spline pattern (cutout) than the one we see on 9/10/11 cassettes.
The 8-speed hubs are made for that pattern and thus cannot accept a 9-speed cassette from Campagnolo or any other brand.
What Are The Advantages of Installing a 9-speed Cassette on an 8-speed Hub?
- Money-savings
If you already have a compatible 8-speed hub and want to upgrade to 9-speeds, using your existing hub will make the transition cheaper and easier.
- Smoother cadence
Cadence = the number of crank rotations per minute. A high cadence such as 90RPM is associated with more efficiency. A 9-speed cassette has smaller jumps between the gears than an 8-speed model and thus makes it easier to transition smoothly between the cogs while pedaling at a high cadence.
- Low gearing
Some 9-speed cassettes have an incredibly low first gear making it easier to climb hills.
What Are The Cons of Installing a 9-speed Cassette on an 8-speed Hub?
- Weaker chain
9-speed cassettes require a 9-speed chain. 9-speed chains are thinner than 8-speed models and are therefore weaker when all parameters (quality, materials…etc.) are equal. If an 8-speed chain is used on a 9-speed cassette, the outer plates of the chain will rub against the adjacent cogs.
Summary: What You Need to Know
- A 9-speed Shimano or SRAM MTB or road cassette can be installed on an 8-speed hub made for the same brands.
- A 9-speed cassette cannot be installed on an 8-speed Campagnolo hub.
- 9-speed cassettes have slightly smaller jumps between the gears and thus provide smoother transitions making it easier to maintain high pedaling cadence.
