Originally Posted by
techsensei
I prefer to size to the small-small combination, so that the chain doesn't drag on the top pulley/cage plates. I know sometimes that ends up longer than the more accepted method of running the chain on the large-large without weaving through the rear derailleur plus two links, but my rational for having a slightly longer chain is if I ended up getting a bent or broken link, I can shorten the chain without issue. I've never broken a chain out on a ride, but I do have a portable chain tool in the bag just in case.
The other advantage of small-small is that because you are getting the longest-possible chain size, you can change to larger cassettes up to the rated capacity of your derailleur without needing a new chain.
Originally Posted by
Metaluna
Also don't forget to check the chain in big-big to make sure it's long enough. If it's still too short after using the small-small sizing method, then your gearing choice exceeds the chain wrap capacity of the rear derailer.
For such a scenario to occur...I can only imagine it if you are not properly doing small-small (allowing FAR too much space between the chain and the RD arm in small-small)...or if you are using a cassette beyond the rated chain wrap of your RD.