Why does the rear hub (cassette) moves on the axle when I pedal?

Question

I am new to this website. I am also new to the terms, and names related to bicycle components.

I have a bicycle called a Burner SuperCycle.

I went to the repair shop twice in the last year because things started breaking down. I am not using my bike in an extreme environment. Just in the street of my city. What I have noticed is, after a while, I start hearing a strange noise when I pedal. And with time it gets worse and worse and now it's just too much.

A couple days ago I decided to investigate by putting my bicycle upside-down and pedaling manually. There was no strange noise but the rear hub/cassette was moving up and down in a circular manner on the little bar that goes trough it (I don't know the name, sorry).

So I have a strong feeling this is where the sound is coming from.

What should I do next? Take off the whole wheel? Tighten up something?

Answer 1

It's almost certainly a bent rear axle. I'm willing to guess that you have a freewheel design, where your rear chainrings have the freewheel mechanism integrated. The assembly then screws onto the rear wheel. This design was universally used from the mid-80s back, and is still used on low-end bikes. The design works ok for single-speed bikes, but as you add more gear sprockets on the back, the drive-side wheel bearings (at the edge of the wheel hub) must be further and further away from the frame. The resulting forces are notorious for bending axles, even with good quality axles. I've broken one axle cleanly through because of this issue. On a low-end department store bike like yours, the axle is probably even weaker, so it's not surprising that it would bend, even if you haven't been abusing the bike badly.

You should be able to replace the rear axle, assuming that there is no additional damage. It's probably best to take the bike to a bike shop, since the work is a bit messy and is a little finicky with reassembling and adjusting the hub.

If you wanted to do it yourself, you should do a bit of reading on how cup and cone rear hubs work (Sheldon Brown's site has some nice diagrams and advice: http://sheldonbrown.com/cone-adjustment.html and http://sheldonbrown.com/tooltips/hubs.html). Then you could disassemble the wheel bearings, pull out the axle, find a matching replacement, then reassemble. You would need wrenches to remove the rear wheel nuts, cone wrenches and grease to reassemble and readjust correctly.

Answer 2

Depending on how much chainrings are on the cassette the problem may has different sources.

If ≤ 6 — you have a freewheel, and maybe the axle us curved (cheap wheels can obtain such problem in one year)

If 7 — may be freewheel (see upper), but may be freehub (see lower)

If ≥ 8 — it's freehub, maybe all can be fixed by tighten hub together, but maybe hub is ruined.