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My chain looks as though it's 'stretched' - by the time it's fed around the large front chainring it appears to be 'longer' than when on the other chainrings.

Therefore I'm trying to work out whether the chain/links have stretched, or whether the teeth on the chainrings have worn.

How could I tell which I need to address?

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Get a chain length gauge and measure. Every parts shop should have a cheap variant for next to nothing. –  arne Oct 22 '13 at 14:41
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Worn chains flex like a rope, while new chains are stiff side-ways. Worn cogs are detected by visual inspection: the profile on the two sides of each tooth is different for a worn cog. –  Vorac Oct 22 '13 at 14:59
    
I have only ever seen el-cheapo chain rings wear seriously. On a decent quality bike, the chain will wear lo-o-ong before the chain ring. That said, the way to wear the front ring or back sprockets is to ride with a worn chain. Replace it first. –  andy256 Oct 23 '13 at 6:34

1 Answer 1

up vote 4 down vote accepted

Measure the chain. You can use a chain gauge, or just a metal ruler.

Sheldon describes it in detail, but assuming you have a ruler with inches (and fractions of an inch), you can just measure 12 pins' worth of the chain (under tension). If it it's 1/16th past the inch mark, he recommends changing it.

That linked page also has some information on checking for a worn sprocket: if the chain is new (or you measured and it hasn't stretched), and the sprocket is worn, the chain rollers won't sit evenly between the teeth when under tension.

If you can see light between the sprocket and chain, it's probably time to replace.

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I replace the sprocket if, after replacing the chain, it starts skipping badly. –  Romulo Oct 22 '13 at 17:28
    
@Romulo That's the latest point at which you need to replace the sprockets, but you will probably already ruin one or two chains by accelerated wear before. –  arne Oct 23 '13 at 4:34

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