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I bought a new cassette last week and they said that I also need to replace my chain. I only used that chain for 4 months and only rode my bike every sunday for 50 kilometers at max. Do I really need to replace my chain? I used the ruler method on testing chain wear and it's still okay.

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    That's about 50×4×4 = 800 km of total mileage for that chain then. If used in bad conditions (wet winter, much mud etc.) I'd say it could be in well-used state. If it was used together with a very worn cassette, that could have contributed as additional wear. Apr 29 at 9:13
  • A worn cassette does not cause accelerated chain wear. The converse if true: a worn chain causes accelerated cassette wear.
    – juhist
    Apr 29 at 16:20
  • Replace the chain but keep the old one so you can use it up later. Chains are pretty cheap usually compared to the cassette
    – JoeK
    Apr 30 at 12:58
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You need a chain wear gauge or other accurate chain wear/length measurement to be sure. Chains shouldn’t be used past 0.75% elongation. For modern 11 or 12 speed chains I’ve even heard 0.5%.

A worn chain will accelerate cassette wear, especially if the cassette is new.

On new chainrings a worn cassette can cause chain suck.

50km per week over 4 months is 800km. That’s usually not enough to reach 0.5% elongation unless you were riding in adverse conditions (e.g. mud, sand, dust, rain) with high power and/or didn’t clean and lube the chain.

If your current chain is only somewhat worn you could get a new one for the new cassette but keep the used chain for later. After the cassette has been through one or two chains you can use the used chain.

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    Chain wear gauges are a couple of dollars, every rider doing more than a couple of km a week should have one (and use it).
    – mattnz
    Apr 29 at 8:14
  • I use 0.5% as the limit for even 8-speed chains.
    – juhist
    Apr 29 at 16:20
  • @juhist: At that point, how much cassette lifespan do you really gain? In my experience around 0.75% is also the time when the chain starts to get noisy and shifting performance drops, so I tend to replace at 0.75%. A cassette easily lasts me 4 or 5 chains. But I mostly use the larger sprockets.
    – Michael
    Apr 29 at 18:13
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    @juhist The standard recommendation is 0.5% for 11s and above, 0.75 otherwise. Naturally, nothing is stopping you from replacing earlier.
    – Weiwen Ng
    May 3 at 22:13
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The rules of replacing drivetrain components are:

  • If you replace a chainring, you may need to replace the chain
  • If you replace the chain, you may need to replace the cassette

From these it will follow that if you replace a chainring and find you need to replace the chain, which may further necessitate you to replace the cassette.

Whether the replacement is necessary depends on the wear state of various components. For example, a new chainring will probably work with a little used chain but not a chain near its end of lifetime.

So, as summary: when changing a drivetrain component, everything in rear of that component may need changing too.

In your case, you replaced the cassette. The chain is in front of the cassette, not in rear of the cassette. Thus, the old used chain will work just fine. No problems there.

However: if the chain is very near the end of its lifetime, it will cause accelerated sprocket wear on the cassette. So even though it will work for a long time, it may cause so much sprocket wear that the cassette after little use cannot be used with a new chain anymore. 800 km is hardly on the end of the lifetime of a reasonable chain used for road riding in dry conditions.

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  • The "if you replace ... you may need to replace ..." statements also go the other way round: Cassette replacement may force chain replacement, which may force chainring replacement. It's basically: If you replace one drivetrain item, you may need to replace the one(s) in contact with it. Apr 30 at 14:47
  • @cmaster-reinstatemonica Disagree. I have never seen a chain so worn that it won't work on a new cassette but works on a worn cassette.
    – juhist
    Apr 30 at 16:16
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    Well, I did. I even destroyed that new chain within weeks by not replacing the rear cog. True, that case was rather extreme, but the point is not whether the setup works, but rather whether the new part takes fast damage from the worn old part. If you put a new chain on an old cassette, it may work alright, but the chain will lengthen quite a bit faster than if it had been used on a new cassette. Apr 30 at 17:42
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    One of the mechanisms that damage the parts is, that a mismatch between tooth distance and chain link length inevitably concentrates the entire force on a single tooth/link all the time. Apr 30 at 18:01

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