Why is the rear mech jumping irregularly?

[swee'pea99]

Went out with my youngest and noticed that her chain was occasionally jumping...couldn't see exactly what was happening, but the bottom line was, it wasn't running smoothly. Every couple of seconds or so it seemed to have a little spasm. Didn't jump sprockets or anything, just carried on, but then...there it goes again.

I took off the chain and found a couple of stiff links...and it was a pretty old chain generally, and not much cared for, so I decided that was it, and got a new chain. Fitted the new chain and...same old same old.

The cassette looks pretty worn too, so my plan is to replace that (I probably should anyway, what with the new chain) but I just wondered if anyone might have any other ideas or suggestions for what might be causing it.

 

[avsd]

Cable tension on the shifter/ watch this video/

View: http://www.youtube.com/watch?v=SkzvfCaIbyQ

 

[swee'pea99]

Thanks for that, and it's a good vid, but doesn't apply to me. I should've said, this is an old, no-indexed mech, so I know the jumping isn't caused by it being not quite in gear (because without indexing, you can fine-tune that infinitely with the shifter). It is exactly in gear...but still doing this hoppity-skippety thing. Good vid tho' - that could've saved me many hours of 'fun' over the years.

 

[ufkacbln]

Worn chains match the worn cogs as the two wear together, with new chains, the cogs wear will mean that the chain does not fit and will skip

 

[swee'pea99]

Thanks. That tends to confirm my suspicions...I'll get a new cassette and see how it goes.

 

[RecordAceFromNew]

Given it happened with the old chain and old cassette, twisted hanger or mech could also be the reason, where the sprocket and top jockey wheel have different ideas of what the correct plane (either looked from above or from the back) for the chain is.

 

[swee'pea99]

Thanks. I'll look out for that if the new cassette doesn't sort it.

 

[Moodyman]

I've gone through this whole rigmarole recently (new chain,cassette and chainset) and discovered that it was the freehub. Took it apart, cleaned and oiled and no improvement. So replaced it and the skipping stopped. The skipping Only happened when moving from stationary or giving it some uphill. The pawls had worn out, which meant cassette wobbled which caused chain to skip gears

 

[swee'pea99]

Ah...interesting. Because the cassette does wobble slightly. I suspect I could do with some kind of a shim. (I'm using a 7-speed cassette, and I think it may be a bit small for the freehub....I don't think the hub is excessively worn.) I shall look into that if I still have wobbling with the new one. Many thanks.