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Over the past year, my road bike's crankset has gotten pretty bent up and out of wack. Not due to abuse, just wear and tear I guess. It's pretty bent and has a nice wobble that I can see as I look down at it while pedaling.

So I'm interested in replacing the crankset. But I've read there are various rules of thumb to follow when replacing cranksets, chains and cassettes. When you replace one you have to replace the other or something along those lines. Something about teeth wearing evenly.

Do I need to replace anything else if I replace my crankset?

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Stupid question, are you sure that your whole crankset is bent, not just bottom bracket worn out? –  Davorin Ruševljan Feb 12 '13 at 7:49
    
Well I put in a new BB like a year ago. Unless it's bent already or something... –  Jakobud Feb 12 '13 at 18:09
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4 Answers 4

If the bike is a good bike, a good fit and you like riding it I would replace the chain, cassette and chain set with a higher quality upgrade. If you do that you can ensure that the gearing will not jump on a worn cassette and to guarantee my gear changes I would change my derialleur jockey wheels.

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Your new crankset will have new chainrings so you'll be set on that front. It's also possible that you need to change your chain, cassette, or both.

Generally speaking, as a chain stretches the cassette stretches with it. The chain rings on the crankset can stretch too but they take longer due to the stress being distributed across more teeth, thus less strain on any one tooth.

You can get an estimate of how much your chain has stretched by measuring a full link (which consists of two half links, an inner and an outer). They should measure exactly one inch. A tenth of an inch longer means it's worn and should be replaced.

There isn't any really foolproof method that I know of for checking the wear on the rear cassette other than try it and see if it works. You can try to eyeball it, but that's not entirely accurate even if you know what you're looking for.

For all you could ever want to know, check out the always helpful Sheldon Brown. http://sheldonbrown.com/chains.html

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There is a tool for checking cassette wear. A reasonably-equipped bike shop should have one. –  Daniel R Hicks Feb 20 '13 at 16:10
    
When measuring the length of chainlinks it's much easier (and more accurate) to measure 10 or 12 and average the length. –  Trengot Mar 24 at 8:49
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There is a similar thread that addresses the chain\cassette question.

Bottom line is it depends.

Here are the steps I would follow in determining what to do if you were to bring your bike to me:

  1. Make sure the crank is not loose (this can cause crank wobble).
  2. Make sure there are no loose bolts that connect the crank to the chain rings (this can also cause wobble).
  3. If you need a new crank get one. If you buy a crank that does not come with new chain rings (higher end cranks can be purchased without the chain rings) you'll probably want to purchase new ones (unless your current chain rings are not heavily worn\damaged).
  4. Check your chain for wear. A chain wear indicator tool is a great investment as it will save you from using a worn out chain (and possibly ruining your cassette and chain rings).
  5. If your chain needs to be replaced do so.
  6. Check your cassette for wear. There are tools for this. Your LBS can also check this. If you have been using a worn out chain (step 5) for a while then there is a good possibility that it damaged your cassette.
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If you replace the crankset you almost certainly get new front sprockets. Compared to the cost of the crankset and sprockets a new chain is chicken feed, so get a new chain.

Whether you need to replace the rear cassette is a whole 'nother question, pretty much independent from the front. If the cassette is badly worn it may cause your chain to wear slightly faster, but not enough to worry about. And you may find that the rear shifts poorly with the new chain, or suffers some "chain suck", but probably it will be OK, if it is now.

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The cassette generally wears out faster than the chain rings. If you're replacing the crankset and chain, it makes sense to replace the cassette at the same time. –  Trengot Mar 24 at 8:48
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