cassette/chain relationship

[keram]

does a new casette require new chain or does new chain require new cassette or both and if so why? I could never get this one.

[C.P.]

Depends on how elongated the worn chain is (your removing).
Some cases you can install a new chain, but keep an old cassette(if the removed chain has chain stretch between 1/16th inch and under 1/8 inch).

It's best practice when replacing a cassette to install a new chain.

Do you know how to check if your chain is worn out?

An easy answer is if your getting ready for a new season of cycling, and you want things hassle free - just replace both...

[bdee]

Don't forget the chainrings, typically we mtbers spend most of our time in the middle ring, unless you're PK and all your time is spent in the big ring. If the chain is really stretched and the cassette is real worn out cahnces are your favorite ring is gone too. You can get away with a new chain and cassette but you'll wear out the chain faster, also you could develop a nasty case of chainsuck. And according to Murphy's Law, section 7, this will ALWAYS occur during a race, typically when you are about to take the lead and ride to glory ;)