I’m planning to use two different cassettes: a 12-27 and a 11-21 depending on the rides. 75% of my rides will be done with the 11-21 and I’ll switch to the 12-27 the rest of the time.
As you will see in the formula below, the Length of the chain varies directly proportional to the largest rear cog. If I setup for R = 21 then the chain may be too short for the R = 27.
Will the difference be relevant enough to setup up two different chains or can I get away with one chain? And if I can get away with one chain should I setup to the R= 27 or R = 21?
Formula:
L = 2 (C) + F/4 + R/4 + 1
L = Chain length in inches. Round the final result to closest whole inch figure.
C = Chain stay length in inches, measure to closest 1/8”
F= Number of teeth on largest front chainring.
R= Number of teeth on largest rear cog.
For simplicity the two scenarios are:
L=51.5 rounded to 52 when R=21
L=53 when R=27
Is a 1 inch difference too significant?
Set it up to work with the 27. The chain shouldn’t be ‘too long’ for the 21, as the comparision for ‘too long’ will be made using the smallest cog - not the largest. Ex - you’re comparing the 11 cog on the 11-21 to the 12 on the 12-27 - that’s the slack the derailler has to take up.
You’ll have a few extra links in your chain over what you’d have if you were just running an 11-21, and possibly slightly more chain slap due to the extra length - but nothing more than you’d experience when you were in the 39/21 combo on the 12-27 cassette. And it’s a whole lot worse to have a chain that’s too short than one that’s too long. One forgetful shift with a short chain could mean the end if your frame, derailler and rear wheel.
It makes sense. Thanks a lot!