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How do you tell when it's time to replace a cassette?
Are there any specific wear patterns on the teeth to watch for or symptoms specific to a worn-out cassette?

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There is a tool that bike shops have for checking the cogs. Ask to have yours checked if you're having skipping, missed shifts, etc. But if you're symptom-free there's no need to replace the cassette, even if it's technically worn out. It may slightly increase the rate of chain wear, but chains are a "consumable" item anyway. –  Daniel R Hicks Apr 24 '12 at 0:19
@DanielRHicks this comment would make the perfect answer to the question –  heltonbiker Apr 24 '12 at 15:47

4 Answers 4

up vote 7 down vote accepted

Without experience, or obvious damage, a cassette gauge like the one from Rohloff, or mileage are your best options for deciding when to replace a cassette. For me, a good rule of thumb has been:

  • 10 chains = 2 cassettes = 1 set chain rings

That is, I change my chain every 1200-1500km. I change the cassette on the 5th time I change the chain. And I change the chain ring set every other time I change the cassette.

This is an example of an obviously worn chainring.

Worn Campy Chain ring.

This is an example of a worn cassette.

Worn cassette

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Yeah, 10-2-1 is about right, though I probably come closer to 10-3-1. –  Daniel R Hicks Apr 24 '12 at 10:53
My cassette was starting to look like the second picture - probably a good thing I'm getting it replaced :) –  Nate Koppenhaver Apr 24 '12 at 16:48
+1 for the pics. FWIW, one will notice a degradation in shifting performance before sprockets and chainrings get that noticeably bad! –  Angelo Apr 24 '12 at 16:54
@Angelo: Absolutely. But I wanted pics that would make it obvious what we were looking at. –  zenbike Apr 24 '12 at 18:25

The wear pattern I have usually noticed is that the teeth become more "pointy" or "hook" shaped. Also, Rohloff makes a tool called the HG-Check tool for Shimano cogs. Not sure if it works for other brands.

The main symptoms I've noticed are rear chain skipping and poor rear shifting. However, these symptoms have multiple causes, so you want to rule those out before spending the money on a new cassette. (For example those symptoms can indicate chain, cable, and derailleur issues.) And at times, one may only need to replace a cog or two, rather than the whole cassette.

A major key to cassette lifespan is keeping the chain clean and lubed.

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EDIT: this answer uses an extreme example, perhaps irresponsible, I admit, but which tried to state some things which I think are valid considerations regarding the question asked:

  • Sprockets can be used for very long provided you change the chain frequently; That COULD mean there are a lot of usable cassettes being thrown away that could still give more kilometers per buck if properly managed;
  • Replacing a sprocket is necessary more because of safety than because it doesn't work anymore, or because it could damage other components (the example shown crossed the safety issue while still working fine);
  • There is a limit to how worn a sprocket can be even if it still works. The photo shown is an extreme example (caricatural, in a way). Thanks to the fruitful discussion (see comments), I could not feel like advising anyone to use any sprocket to the point the one I show in this photo. Anyone doing this would be at his own risk, and that is Bad Thing. It shouldn't be done.

I hadn't, and would not recommend, to measure the wear on the cassette, but on the chain.

If you put a new chain with a cassette, and it doesn't skip, the cassette is ok, even if the teeth are a bit thinner than new. That is possible if you change the chain often, always for a new one: then the cassette wears out slowly, but the teeth are never deformed like a ramp (shark teeth). I elaborated a lot on this in another answer: http://bicycles.stackexchange.com/a/7880/2355

Of course, when the teeth are SO THIN that they are pointy, or its width is significantly reduced, perhaps it's time to convert the cassette into a set of ninja shuriken (just kidding).

There could be problems with the indexing of gears if you are a fan of ultra-precise shifting, but that usually is not a great problem, and as far as I know don't cause damage to other parts of the shifting system, at least if it is not necessary to force the lever to shift up (could stress the pulleys - usually this is not the case).

At last, the proper way to measure for the CHAIN wear is the distance between pins, that should be exactely half an inch for each consecutive pair of pins. This can be measured in two ways:

  • With a chain wear measuring gauge/rule. This is very easy and precise.
  • With a regular ruler. You must measure one foot of chain (12 pairs of links) while it is slightly under tension. If the distance increased by more than the thickness of a pin, then it's time to replace it.

Just for the record, this is a current picture of my internal-gear-hub bike with surely more than 25 thousand kilometers with the original cog. I must have used some eight chains on it, always replacing via measuring, either with a gauge or with a ruler. It works without issues (no skipping, no falling off, no grinding sensation). This is too much, I recommend not going so far. I has paid itself a long long time ago.

enter image description here

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Measuring the chain and replacing it at proper intervals will allow your cassette to last far longer, but the cog pictured above is an accident waiting to happen. You need to examine your cogs and chain rings each time you change the chain. The photo above is the most extreme example of worn cog teeth I've ever seen. It should have been replaced at least 15000 km ago. –  zenbike Apr 24 '12 at 3:45
And last, why would anyone want to ride something that is so far damaged? You said, " I was considering it just a matter of proper working...". Isn't that a good enough reason by itself? Riding is supposed be a release from all the nonsense of the day. My bike is not great, but it's perfectly tuned and maintained, precisely because I want the bike to disappear and the ride to be what I notice. I don't want to feel like I might not get to ride home at all. –  zenbike Apr 24 '12 at 14:51
Thank you. I'll remove the downvote when you remove references that make it appear that the cog is usable and safe. –  zenbike Apr 24 '12 at 18:29
This is a perfect example of letting the thing break before you get around to pretty much normal maintenance. –  user313 Apr 25 '12 at 1:03
I think an important point is being missed here. The picture is of a single fixie cog, not a regular multi-cog cassette. With the standard cassette you'll experience shifting problems when the cassette is worn, and the teeth will tend to wear away to a point, not develop the long, narrow shape pictured. –  Daniel R Hicks Apr 25 '12 at 15:36

I agree with the comments that say that chain wear governs. But be sure to test after changing the chain for skipping. Put the chain on the cogs you use the most and pedal vigorously. It will be obvious if the casette needs replacing.

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