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I bought a brand new bike last May (2015 Brodie Elan) and proceeded to put 11,000km on it by November (cross Canada ride plus some commuting).

Around the 11,000km mark I brought it back to the shop and they checked the chain wear, using some sort of spring-loaded Park Tool, which indicated that the chain was totally dead as well as the cassette.

I'm not clear if the chain wear gauge directly indicated dead casette, or if it was more "if the chain is this worn the casette is definitely dead too". Either way this did not surprise me as I had been experiencing some gear/ tooth hopping that I could not correct with cable tension adjustments. I asked them to replace both casette and chain.

After getting the chain and casette off they found the bottom bracket was also dead. This also was not surprising as I had been experiencing some "gravely" feel while pedalling.

So they replaced chain, BB, and casette. However, they said the derailleurs and chainrings still had life in them and did not need replacement.

I'm wondering if that makes sense, i.e. would derailleurs and chainrings be expected to long outlast chains and casettes with which they were purchased? Or should I be worried that those could be in a heavily-worn-but-not-quite-dead state that in turn would be wearing out my new chain and casette faster? How would I check?

For reference, here is what was originally on the bike when new:

CRANKS: Shimano Deore 26/36/48T
CASSETTE: Shimano Deore 11-30t
FRONT DERAILLEUR: Shimano Deore
REAR DERAILLEUR: Shimano Tiagra
BOTTOM BRACKET: Not published but I'm guessing low end Shimano

Thanks.

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    11,000 km quite likely means that the chain and casette have been in service for about twice the expected distance. Derailleurs and chain rings can survive a number of cassette and chain changes.
    – Kaz
    Commented Apr 20, 2016 at 21:09
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    The chain has gone about twice it's expected life at 11,000 km (6800 miles), and the rear sprockets are likely dead at that distance too, if not because of simple wear then because of being run with a worn-out chain. This is to be expected. Normally a set of front rings is good for maybe twice the 11,000 km, though being run with a worn-out chain will have increased the wear rate. The only parts that really wear on the derailers are the jockey wheels on the rear, but those are generally good for a long time. A better bike shop will have a gauge for checking sprocket wear.
    – Daniel R Hicks
    Commented Apr 21, 2016 at 11:57

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The load is spread across more teeth on the chainrings than on the cassette, so they tend to last quite a bit longer. If they said your chainrings are fine, then they are probably no problem. derailleurs don't really have many parts that can wear out. There isn't much load being placed on the jockey wheels, so they can last quite a long time. The spring which returns the derailleur when shifting should also last very long. There's no reason to assume that the wear on the cassette, chain, and bottom bracket, would mean that the chainrings and derailleurs are about to die.

That being said, if you are planning on doing another long cross country tour, you might want to replace them ahead of time. I wouldn't want to be stuck in the middle of Saskatchewan trying to get replacement parts. If you're just going to be doing short trips there's no reason to replace these parts prematurely.

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