RT

I'll be moving from a 12-34 SRAM 9-speed cassette to an SRAM 11-26. Which chain would you recommend, and will I have to shorten it to accommodate the smaller cassette?

SweetLou

Yes, you will need a shorter chain. I have never tested this, but I have heard that you should replace the chain when you get a new cassette, unless the chain has just a small amount of distance on it. I'm not sure if this is true, just something I have heard.

JiveTurkey

Actually, if you keep the same derailleur on, you don't have to shorten the chain. Your drivetrain won't know the difference, it'll all be the same except it will just think you're not using the biggest few cogs. That being said, I'd shorten it anyway because there's no need for the extra length. Edit: Like SweetLou said, you should get a new chain if the cassette has a decent number of miles on it, otherwise the new cassette will wear faster to match the stretched pitch of the old chain. You can keep the old cassette and chain as a backup or sell them together for a few bucks. Oh yeah, and most any 9-speed chain will work, but SRAM will say their cassettes and chains work best together.

RT

Thanks guys. Is there a way to calculate based on my max/min number of total teeth how long my chain should be?

JiveTurkey

I use Sheldon's method: https://sheldonbrown.com/derailer-adjustment.html#chain

Wrap the chain around the largest chainring and largest cog (bypassing the rear derailleur). Find a length that is just long enough to connect (inner plate to outer plate), then add one full link (inner plus outer).

Zouf

The Sheldon Method is basically the SRAM Method, also known as "Big + Big + 2", i.e. wrap the chain (without dérailleur) on the big chainring + big cog, and add 2 links (what you call "one full link (inner + outer)". Interestingly, the Shimano Method is different, and insensible to largest cog dimension: big chainring + smallest cog, ensure the 2 RD pulleys are in a vertical line. Also interesting: for "smallest cog of 15 or 16T"(!), they revert to the SRAM Method (big + big + 2).