Originally Posted by
Cougrrcj
In the past, I've always subscribed to the 'arc' method of chain wear. When the chain, when off the bike when held sideways, exceeds a 8-10" arc over it's length, it is time for replacement. The chain is held somewhere near the middle and the 'droop' is measured... I have well over 40k miles on my vintage Fuji...
That said, I've worn cheap chains out in a few thousand miles of riding in nasty wet weather, and yet others have lasted 5k miles or more. FWIW, my current chain has over 8k miles on it, and is probably due for replacement, but I'm not looking forward to replacing the 'dinosaur' (20+-yr-old) 6-spd Shimano Ultraglide chain... because I'm an old fossil myself!
If using the arc method, remember that derailleur chain is designed to "arc." You WANT a laterally-flexible chain in order to get easier shifts.
1/8" track chain, on the other hand, is designed to NOT arc. With one chainring and one cog, you want that chain running as straight as possible with no lateral movement that could unship the chain.
This is the main reason for using 1/8" chain on fixies and IGH-equipped bikes, or why it would be silly to use a 3/32" chain on your fixie.
Modern 3/32" derailleur chains would also bear some watching once you put more that 5,000 km (3,000 miles) on them. Because the pins are flush with the side plates, you don't have a "margin of error" if the pin starts to drift. I used to replace the timing chain on the tandem once every three drive chain changes, until I had the timing chain break on a ride. This leaves the stoker to get us both home under her sole power, and she's not capable of that! This is also why you never reconnect a modern derailleur chain (9 spd or higher, but a good idea for 8 spd as well) with anything but a master link or Shimano pin. The timing chain on my tandem is made from 6- or 7-spd chain. Cheaper, and with longer pins, so some margin of safety.
I guess there are two strategies to chain & cassette replacement:
1) replace the chain before the cassette cogs wear out because chains are cheaper than cassettes.
2) wear them both out together, and replace both, but after thousands of miles.
I think (1) works better if you are concerned with performance. Also, the cogs on a cassette will wear at different rates depending on your gearing preferences. So at some point you may get one cog skipping.
I try to use a combination of the two strategies. I will replace derailleur chains when they get worn, but I try to only replace the worn cogs on the cassette. If you use Shimano cassettes, you may need to drill out the pins that hold the cog segments together. And then good luck finding replacement individual cogs!
Luis