Manufacturers Defect or Symptom of Progressive damage?
 

So the cog stack on my fiance's bike does something really wierd... it wobbles. Most noticeably when the bike is coasting. We're talkin' 2-3 mm of wobble.

What I've been told by some LBSs is that this is an artifact of poor manufacture on the part of the manufacturer.

Is this true?

Or is this something that I can fix? And how?

now this is not just a minor thing. when she leaves an intersection, for example, and REALLY needs to pick up the pedals and hoof it (and therefore putting ALOT of force onto her pedals) her chain will rythmically skip. now... my theory is that this is caused by the cog-stack wobble.

Thoughts?

Thanks

Is the cog set on a freewheel or a Cassette? I've found that cassettes are pretty decent, but the freewheel units do tend to have some 'wobble' in them, especially the cheaper ones.

it's a cassette

This is normal even for a cassette. I have yet to seen one not wobble when coasting.

Mine don't and I've never had one that does. I've worked on dozens of bikes and never seen enough wobble to cause chain skip unless something was seriously wrong.

Either the cassette's lock ring isn't tight, there is a spacer missing, the freehub body is damaged or the freehub body isn't tight on the hub shell.

What make and model is the hub?

I stand corrected. I guess I was thinking of a freewheel. I apologize for my mis-information.

it's a generic shimano IIRC. the lockring is damn tight, all the spacers are accounted for. How do I diagnose and/or fix the freehub body?

The freehub body is fastened to the hub shell with a hollow internal bolt. You have to remove the axle and bearings first, then insert a 10 mm hex wrench into the center of the freehub to see if the retaining bolt is tight.

Also check to see if there is the correct amount of ball bearings in the hub.

Wobble is caused by the gap in the bearings; a little is normal but too much might mean someone left a ball out when building or servicing the hub.

rmfnla: checked the axel as one of my diagnostic measures (worried that it was bent... it wasn't) and all the bearings were present and accounted for.

HillRider: thanks for the info.

This question comes up so frequently, it should be made a "sticky". All freewheels wobble to some extent when coasting.There's nothing wrong and it doesn't hurt anything. Here is an explanation:

"Freewheels and hubs are made as concentric as
machining reasonably permits. However, threads do not constitute a
good centering mechanism and since freewheels are made of concentric
components that are mounted on concentric threads, they can easily
acquire some wobble, that as you must have noticed, does not affect
performance at the slow rotational speeds of a bicycle. That is why
the bicycle industry can use these designs that high speed and
automotive assemblies cannot.

Jobst Brandt"

Sorry, but Jobst is just plain wrong. The wobble in a freewheel is from the aforementioned gap in the ball bearings. Unfortunately, this does not help the OP because we have established that his fiance's bike has a cassette.

Severian, I think the chain skip has nothing to do with the wobble, especially since the intersection scenario would involve lots of pedaling pressure but not high speed. I suggest you check for cog/chain wear.

I've heard that also. Whatever the cause, it's harmless. I noticed it years ago, but concluded it's irrelevant because it only happens when coasting. It's nothing to do with the chain skipping, which is more likely to do with worn teeth.

followup: yes, this only occurs while coasting, I observed her while she road and this had no effect on her pedaling.

coming clean: she does have a screwed up derailleur from two prior accidents. The jockey-cage is bent as are the jockey wheels.

so much for wishful thinking eh?

The cogset is a year old, so no dice on cog wear, the chain is completely new. this has happened before and after the chain was replaced.

who knows then.

I think you nailed it with the messed-up derailleur.

Try swapping that out; I'd be really surprised if that doesn't fix the problem.

Chances are that it is cog wear after all, per rmfnla's suggestion. Many casual cyclists spend most of their time riding around in the middle front ring/small rear cog or small front ring/small rear cog combination. Since the rate of cog wear is inversely proportional to the number of cog teeth, the chain can start skipping on the smallest cog after comparatively few riding miles. The cheapest fix (assuming this is the problem) is for her to stay away from the smallest cog unless she's in the big ring in front.

If she's had a couple of crashes and the rear derailleur is visibly bent, the derailleur hanger may have been knocked out of alignment. Having an experienced bike mechanic check and correct the dropout alignment is cheap insurance against the possibility of running the derailleur into the spokes.

Trakhak: dropout's fine.. it's been re-aligned at least two times in my presence. The Jockey cage itself is visibly bent. As for cog wear... well... I'll check that too I guess.

But the chain skips from the middle of the stack and alwas goes to a cog with more teeth (inboard to the wheel). not skipping on the cog itself.

If it's skipping up to a lighter (more teeth) gear, there's either too much tension on the der. cable (unless it's a rapidrise derailleur, in which case there would be too little tension) or the derailleur's damaged. I also doubt that the cassette's wobble is causing the skipping. Try an undamaged derailleur off of another bike if possible in order to avoid having to buy one just to find out whether that's the issue.

Bent RD cage is probably the culprit.

The alignment of the cage determines the feed of the chain to the cog.

Bad alignment = bad angle, hence chain mis-feed.

New RD is an easy fix and everybody's favorite upgrade.

Why do I detect sarcasm in the comment about RD cages being everyone's favorite upgrade?

Or perhaps my sarcasmometer is broken...

ahh well.. back to the drawing board.