The Reasons Behind a Wobbling Bike Cassette (ELI5 explanation)

Condensed Answer:

The common sources of cassette wobble are:

  • Hub imperfections during manufacturing
  • A narrow cassette combined with a wide hub without the use of spacers
  • The cassette hasn’t been tightened properly
  • A bent rear axle
  • Hub bearing play
  • The last cog isn’t seated properly
  • A bent Lockring

Hub Imperfections

In the ideal scenario, the body of the hub and the freehub body (the part on which the cassette slides) will be perfectly straight and its flanges (sides) will be parallel to one another.

Very often, however, the hub body is off on one side by a small amount (less than 1mm). As a result, the cassette ends up going up and down by the same amount when the wheel is rotating.

The graph above shows a rear cassette hub and the hub body. If the hub is ever so slightly crooked, the cassette has no choice but to reflect the imperfection.

Obviously, this issue is more common among low-end hubs where the main focus is quantity rather than quality.

No Spacers

The cassette is secured to the freehub body via a lockring. If the final cog on the cassette doesn’t stick out ever so slightly above the freehub body, the lockring won’t be able to sufficiently tighten the cassette.

This can happen when the cassette is too narrow for the hub in question. One example would be using a 7-speed cassette on an 8-speed hub. The 7-speed cassette is slimmer and thus leaves a small gap when installed on an 8-speed hub. As a result, it cannot be secured on that hub unless the user installs a spacer.

The Cassette Isn’t Fully Tightened

If the cassette hasn’t been fully tightened, it may unscrew and start wobbling.

To diagnose a loose cassette, remove the rear wheel and try to slide the cassette back and forth. If it moves even a little bit, it’s not tight enough.

To tighten the cassette, do the following:

  • Remove the rear wheel and quick-release
  • Slide a cassette tool into the cutout of the cassette
  • Use a wrench or a socket to grab the tool depending on the model
  • Rotate the cassette tool clockwise

A Bent Rear Axle

The hub and consequently the wheel and the cassette rotate around an axle.

There are two main types of axles – solid and hollow. The hollow axle is used on quick-release bikes whereas the solid one requires nuts to secure the wheel.

If the axle is bent, the wheel and the hub will be rotating around a crooked axis. As a result, the entire wheel will be wobbling.

If the axle is bent, the only solution is to replace it. If you can’t find one that matches your hub, you will have to buy a new hub.

Hub Bearing Play

The rear hub relies on ball bearings or sealed bearings to rotate smoothly around the axle. In order for the bearings to do their job, they have to be “free”.

If the bearings are too tight, the hub won’t rotate smoothly. If they’re too loose, the hub won’t be as stable as possible. It’s often quite difficult to find a setting where the bearings are tight enough to eliminate all play while also being capable of rotating freely.

The Final Cog Isn’t Seated Properly

The last cog of the cassette is separated from it. As a result, people often do not align it properly before tightening the lockring. This makes the cassette unstable.

A Bent Lockring

If the lockring is bent, it will fail to secure the cassette and may cause wobbling.

Note: It’s also possible that the thread of the lockring is damaged.

Is Cassette Wobbling a Problem?

If the wobbling is the result of manufacturing imperfections and doesn’t cause shifting problems (e.g., chain slipping even though the derailleur hanger is straight and the gears are set properly), the wobble isn’t much of an issue.

In fact, almost all cassettes and freewheels wobble to an extent regardless of what one does.

Is Wobbling Better for Shifting?

There’s a rumor that the wobbling of a cassette or freewheel is actually a built-in function that facilitates shifting. Some sellers even use this as a sales pitch when promoting a cheaper bike.

The truth is that wobbling is a bug rather than a feature and doesn’t facilitate shifting. If the cassette was perfectly straight, the derailleur would shift just as well.

FAQ: I washed my bike and my cassette began wobbling. It was fine before. What happened?

Very often people notice that their cassette wobbles after cleaning their bike and conclude that both processes are related.

This is rarely the case, however. More often than not, it’s simply easier to notice the wobbling when the cassette is free of contaminations “hiding the view”.

Also, people become a lot more vigilant of a component’s performance upon cleaning or servicing it.

FAQ: Do cassettes wobble less than freewheels?

Yes. The oscillation showcased by cassettes is much smaller than the one seen on freewheels.

The reasons for this are:

  • Cassettes don’t thread onto the hub. Instead, they slide on the freehub body. Conversely, freewheels thread onto the hub. If the thread isn’t straight, the freewheel won’t be perfectly perpendicular to the hub and axle.
  • Freewheels are found on cheaper bikes. As a result, the quality control isn’t on the highest possible level.

Summary: What You Need To Know

  • Some cassette wobbling is expected due to hub imperfections during manufacturing. If there are no structural problems with the bike, the wobbling isn’t a concern.
  • However, before concluding that the bike is safe to ride it has to be examined because the wobbling cassette may be a symptom of a deeper issue such as a bent axle, an improperly fitted cassette, a broken hub, or lockring.

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