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.
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.
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.