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I recently bought a beat-up old mountain bike for commuting. I've tried to clean and adjust the derailleurs and everything seems to work well enough in my garage. But when I put heavy weight on the pedals (say, starting across an intersection after a stop light) the rear gear momentarily derails with a crash (or do you say the chain derails?), I flounder for a second, there's another ugly clashing sound, and then things go back to normal.

Is this something that I can adjust my way out of or more likely an indication of a worn-out part?

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It may be that the derailer simply needs adjustment, or it may be that the chain or rear cluster is worn out. On an old bike this can also be due to simple lack of lubrication of the chain or derailer. –  Daniel R Hicks Jan 26 '12 at 23:09
    
Could you tell us if the chain "jumps" to another gear, or stay in the same gear? –  heltonbiker Jan 27 '12 at 1:56
    
Another thing to do is to lube the shift cables. Sticky cables can cause the derailer to jump. –  Daniel R Hicks Jan 27 '12 at 13:18
    
I agree that your drivetrain sounds worn and you need to replace some parts, but applying some gentler shifting technique also can't hurt. –  Neil Fein Jan 27 '12 at 15:19
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3 Answers

up vote 4 down vote accepted

It sounds like the combination of your chain and cassette / sprocket are at the end of their lives. Examine them for signs of wear.

To check a cassette for wear, look for pointy teeth. A non-worn cassette will have teeth with a fairly flat ends.

A worn cassette will have teeth that have become pointier.

To check your chain for wear, ideally you'd use a chain wear tool to check how much your bushings have been worn away. If you don't have one of these tools, you can kindof tell when a chain is worn out because it will feel like the bushings are a bit loose.

Alternately, a drivetrain generally wears out at around 10000 kms (~6000 miles). You can rack this amount up pretty quickly as a regular commuter.

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Bike shops have (or should have) a wear gauge for sprockets. Everyone who does their own maintenance should have an inexpensive chain wear gauge. –  Daniel R Hicks Jan 27 '12 at 22:50
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Yet another possible cause is frame flex. In some frames the shifter cable is routed under the bottom bracket with part of the shifting cable exposed. In this kind of kike, the cable has not full length casing, instead the casing goes from the shifter to some cable stop in the frame, then the inner cable goes to another cable stop and enters another casing length finally going to the derailleur.

Some frames can flex under stress and pedaling hard from a full stop can be enough to momentarily deform the frame so the distance between certain cable stops changes, thus changing the cable position inside the casing which in turn changes derailleur position by just millimeters.

This position change is too small to complete a full gear shift, but is enough to make the chain "want to shift", hence the rider can feel skipping or "ghost shifting".

The solution for this is either create or use a different cable routing (Some frames may have alternate cable routing) or simply use a single casing from the shifter to the derailleur.

References:

http://sheldonbrown.com/gloss_g.html#ghostshifting

http://sheldonbrown.com/autoshift.html

http://sheldonbrown.com/frame-materials.html

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Also, I've seen bikes where there was a problem of a similar nature and it turned out that, on close inspection, the frame was cracked. –  Daniel R Hicks Jan 30 '12 at 21:40
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another possible reason and something to check:

From my experience - Stiff chain. Often an old chains become stiff, in that the links are not very free to rotate around their pins. This means that if you bend the chain around something (jockey wheels on the derailleur for example), it will want to stay in that shape until straightened out. When you accelerate you are effectively feeding a lot of chain to the bottom of the derailleur and cassette / rear cluster quickly, and if the chain is stiff, it'll miss several of the teeth and jump about.

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But note that generally you only get a stiff chain if the bike is exposed to the weather for a length of time (long enough to wash off existing chain lube) with inadequate maintenance and little or no use. And once the chain is properly lubed and all the stiff links are worked loose, they won't re-stiffen if the bike is ridden regularly. –  Daniel R Hicks Jan 27 '12 at 12:28
    
@Daniel R Hicks - Yes agreed. I wondered if it was in bad shape because Larry calls it a "beat-up old mountain bike" and I have visions of terrible neglect by pervious owner :) –  Mere Development Jan 27 '12 at 12:32
    
Yeah, hard to say -- not clear what he cleaned, and whether he lubed anything. –  Daniel R Hicks Jan 27 '12 at 13:17
    
As far as lubrication goes, I've essentially wiped off old grease and sprayed everything with WD-40. I don't think the bike was "terribly" neglected but it lives oceanfront in Hawaii, so it's a pretty corrosive environment. The chain isn't actually rusty though. –  Larry OBrien Jan 27 '12 at 19:20
    
@LarryOBrien WD-40 isn't considered a lubricant for bicycles. It helps cleanup the chain, but you should follow up with chain lube. –  BPugh Sep 9 '13 at 19:36
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