If the hub is 7-speed specific, installing a 9-speed cassette on it is impossible because the freehub body is too short.
If the hub isn’t 7-speed specific and uses a spacer along with the cassette, a 9-speed cassette can be installed on it after removing the said spacer.
What Is a 7-speed “Specific” Hub?
7-speed “specific” hubs are designed solely for 7-speed cassettes and are shorter than 8/10/11/12-speed hubs.
7-speed cassettes are approximately 31.9mm wide whereas the width of 8-speed models is about 35.4mm. This is a 3.5mm or 11.86% difference.
Consequently, if the hub is designed specifically for 7-speeds (a rarity these days), it will be approximately 3.5mm shorter than needed for an 8-speed (or a larger) cassette.
In that case, the only way to install an 8/9/10/11/12-speed cassette on your bike would be to get a new hub.
Hub replacement is not the cheapest procedure. The new hub will have flanges of a different width and will therefore require new spokes and an entire wheel rebuild. An advanced home mechanic can perform the procedure with the help of online material, but it will take a fair amount of time.
That said, the developed skills will be helpful in the future.
In the past, I rebuilt the rear wheel of my retro road bike two times without a truing stand. I used the frame as a stand and zip ties as calipers to true the wheel. It worked out fine, but the procedure is much easier if you have a dedicated stand of decent quality.
7-speed Cassettes On Non-7-speed Hubs
If your 7-speed cassette is installed on a hub that doesn’t have a spacer before the cassette, then the hub is 7-speed specific.
If your 7-speed cassette is on a hub that has a small spacer of approximately 3.5mm before the cassette, then the hub will become compatible with 8/9/10-speed cassettes when you remove the spacer.
The newer your bike is, the more likely the second outcome is. 7-speed specific hubs are quite rare and aren’t mass-produced anymore. Thus, manufacturers rely on 8/9-speed hubs with spacers to sell 7-speed bikes.
Do I Have a Freewheel Or a Cassette?
If you have a 7-speed freewheel, your hub is not compatible with any cassette.
Freewheels come with a ratcheting mechanism and screw onto the hub. Meanwhile, cassettes slide onto the splines of the freehub body and have no ratcheting mechanism in them.
You can determine whether you have a freewheel or a cassette by examining the lock ring (the front of the cassette or freewheel).
A freewheel has two parallel dots on the front whereas a cassette does not.
The images below show both a 7-speed cassette and a 7-speed freewheel.
Parts Needed For a 7 to 9-speed Conversion
If you want to convert from 7 to 9-speeds, the following parts will be needed:
A. No Hub Replacement
If the hub is 9-speed compatible, you will need:
- a 9-speed cassette
- a 9-speed chain (You can’t use a 7-speed chain because it’s too wide and will rub against the surrounding cogs.)
- a 9-speed shifter
- a new gear cable + housing
B. Hub Replacement
If you have to replace the hub, the conversion requires more “sacrifices”, namely:
- а 8/9/10-speed hub
- а 9-speed chain
- а 9-speed shifter
- a new gear cable + housing
The Benefits of a 7 to 9-speed Conversion
A conversion from 7 to 9 speeds comes with the following benefits:
- Smaller gear jumps
More gears on a cassette equal smaller gear jumps. As a result, the rider has an easier time maintaining a high cadence.
Cadence is a term referring to the rotations of the cranks per minute. High cadence (e.g., 90RPM) is associated with greater average speed and minimal energy expenditure.
- Larger range
A 9-speed cassette can offer substantially lower gearing than 7-speed models. Some 9-speed MTB cassettes have a large rear cog with over 40 teeth.
- 1x Drivetrain
Large cassettes make 1x drivetrains a more viable option because the large cog can compensate for the lack of a smaller chainring.
The Downsides of a 7 to 9-speed Conversion
Conversion from 7 to 9 speeds has the following downsides:
- Higher Price
You will have to buy many parts to make the conversion possible.
You will have to spend 1-2 days removing the old components and installing the new ones.
Summary: What You Need To Know
- If you have a hub designed specifically for 7 speeds, it will be too short to accept a cassette with more gears.
- If your 7-speed cassette is installed on a hub with a spacer, then the cassette can accept a cassette with more cogs. In that case, you will be able to install a 9-speed cassette.
- If you have a 7-speed freewheel, you won’t be able to install any cassette on that particular rear hub