One More Annoying Bike Problem Demystified

from Rookie’s keyboard

Hello, friends

Today, I will demystify one more bike issue that rookies face from time to time – a cassette/freewheel that wobbles

The Causes

In most cases, that culprit is among the following:

The Cassette/Freewheel Isn’t Secured

This is the first technical issue that has to be ruled out as it carries the greatest risk. If the cassette or freewheel isn’t adequately connected to the rear hub, it can unscrew itself and begin to wobble. In the worst case, the cassette and the chain may jam the wheel…and the rest I’ll leave to your imagination, fellas.

Remove the rear wheel and check if the cassette/freewheel is moving back and forth. Even if there’s slight movement, the unit isn’t secured.

  • Slide a cassette/freewheel tool into the cutout of the cassette
  • Grab the tool with a wrench or a socket depending on the model
  • Rotate the tool clockwise
  • Check for movement again

Inaccurate Hub Machining 

In a perfect world, the body of the hub and the freehub body (the section on which the cassette slides) will be straight. And the flanges (sides) will be parallel to one another.

In practice, the hub body is often off on one side by less than 1mm. Consequently, the cassette goes up and down by the same amount when the wheel is spinning. 

If that’s your situation, there’s nothing you can do other than looking some ultra-high-end hub machined by a magician. 

Personally, I wouldn’t bother and would just ride the imperfect one I already have. 

A Damaged Rear Axle

The hubs rotate around an axle that remains immobile. The most common axle types are – solid and hollow. The hollow axle is needed for quick-release skewers whereas the solid one operates with locknuts.

Axles are tough and more often than not remain strong and intact. But if an axle is bent, the wheel and the hub will rotate around a crooked axis. The cassette/freewheel will follow.

If that’s the case, you can replace the axle (if you can find one and the hub is serviceable) or get a new hub (a costly procedure that will also require re-lacing of the wheel).

Absence of Spacers

If the bike isn’t new and has been maintained by too many rookies (no offense, brothers), chances are that the cassette “assembly” might be incorrect.

A lockring keeps the cassette on the hub. And for the lockring to do its job, the final cog on the cassette has to stick out ever so slightly above the freehub body. Otherwise, the lockring won’t be able to sufficiently tighten the cassette.

If someone has installed a cassette that’s too narrow for the hub, the lockring will fail to do its job unless a spacer is added to push out the cassette.

For instance, you can’t combine a 7-speed cassette with an 8-speed hub without a spacer due to the aforementioned issue. The 7-speed cassette is narrower and leaves a small gap when installed on an 8-speed hub unless there’s also a spacer.

Note: There’s a possibility that the last cog of the cassette is not seated correctly as it’s separate from the rest of the body. Check its alignment and then tighten the lockring again.

Hub Bearings

If you have a cup and cone rear hub bearing, it’s possible that it’s assembled too loose after service and has play in it.

To reverse the issue, you don’t have to fully disassemble the hub. But you will have to tighten the “pre-load” nut and then the locknut. (The process is a bit too long to explain in a non-dedicated article).

A Myth

There’s a myth according to which the wavering of a cassette or freewheel facilitates shifting. Some pushy salespeople even use this as a pitch when hyping a cheap bike.

The truth is that wobbling is a bug rather than a feature and doesn’t help shifting. 

Washing Your Bike

Very often the deviation of the cassette/freewheel becomes more apparent after washing your bike. Some “professors” might conclude that this happens due to water getting inside the cassette/freewheel, but that’s incorrect.

The truth is that a clean cassette makes it easier to see the underlying problem whereas a contaminated one camouflages the issue.

Freewheels Are Worse

The oscillation of cassettes is smaller than the one of freewheels. The first reason for that is that cassettes slide onto the freehub body whereas freewheels are threaded on. 

The thread that freewheels use introduces another variable that may be inaccurate. A crooked thread won’t position the freewheel perpendicularly to the hub and axle. And you will see the oscillation.

Also, freewheels (apart from some old-school models) are reserved for low-end bikes and manufacturers have no incentive to invest the money needed for super-precise machining.

A Practical Conclusion

If you’re having this issue, your highest priority should be to determine whether the cassette/freewheel is properly installed. Inspect the axle too. If it’s bent or broken, it will be obvious. 

After ruling out those two issues, the rest is trivial and you might just as well ignore the issue. Truth be told, you will be hard-pressed to find a cassette that doesn’t bounce up and down ever so slightly. If the shifting is good, the cassette is secured and the hub is not broken, it’s all good. 

Until next time

– Rookie






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