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I have an old bicycle that I would like to repair. Basically the chain is always "clicking/jumping" when I pedal. I know this is either a worn chain or cog. My question is what worns first?

If it's the chain I might actually give it a try and switch it. If it's the cogs I don't think it's worth the money and I doubt I can switch it myself and I don't want to spend too much money with this.

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  • While the chain will wear quicker, if you have an old bike it is probably best to replace both. Start with a clean slate, so to speak.
    – PeteH
    Commented Feb 10, 2015 at 11:42
  • My general rule is that a chain lasts about 2000 miles, a rear cluster about 5000, and your larger front chainrings 10-15k. But of course a lot depends on the quality of the components and your riding style. And if you've let the chain go too far it wears the rear cluster faster, so you probably need to replace both.
    – Daniel R Hicks
    Commented Feb 10, 2015 at 12:36

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Tricky one.

You will go through several chains before going through a cassette.

A worn chain can be measured quite easily with a gauge.

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Measuring wear on a cassette is more difficult. It is usually the middle set of sprockets which wear first - due to their more frequent use. If examined carefully - you may notice the teeth on them thinning.

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  • You will go throguh several chains before going through a casette if you change your chains regularly.
    – Batman
    Commented Feb 10, 2015 at 13:53
  • Definitely. And you will go through a significantly fewer number of chains if you clean your chain and sprockets regularly. And keep it lubed. My tip for chain / sprocket cleaning is a degreaser - but using the degreaser sparingly on the cassette as you don't want it running into the hub.
    – OraNob
    Commented Feb 10, 2015 at 14:11
  • Another sign of wear on a sprocket is that the teeth no longer have square corners (or whatever their original shape was).
    – PJTraill
    Commented Jan 7, 2019 at 22:08
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Given that it is an old bicycle, it is likely that the chain or the cassette (or even chain ring) are worn down to the point that it's causing your symptoms. However, it's also possible that the derailleurs are mal-configured and your gears need to be indexed correctly. It may be worth checking this out too as it's a relatively quick fix.

As pointed out by OraNob, chains generally wear much quicker than cassettes and as a rule of thumb would be replaced more frequently. Chains stretch with use and if left too long can begin causing issues with your cassettes and front chain ring(s). As the teeth don't mesh with the chain properly, the result is a rapid wear of the teeth and will ultimately end up in both needing replacement.

Switching cassettes is not really much harder than changing your chain, you just need the right tools for the job. You can pick up relatively cheap cassettes from your local bike shop or online, so cost shouldn't really be an issue here. You need a chain whip to keep the cassette / cogs still, and another tool for unscrewing the cassette. These will depend on your make/model of cassette as there are differing standards.

Indexing bicycle gears

Changing a bicycle cassettte

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