Bicycle Mechanics - Chain Length Determination

According to Sheldon Brown, the best way to determine the necessary chain length for a drive train is to wrap the chain around the large sprockets front and rear (as I recall), without running the chain through the RD. My question is: Does this mean the chain can be the same length for a bike with a long cage RD as well as a medium or short-cage mech?

My question is: Does this mean the chain can be the same length for a bike with a long cage RD as well as a medium or short-cage mech?

Yes. If you're sizing the chain for a specific drivetrain, as is the case with the big/big (plus one inch) sizing method you describe, then the cage length isn't a factor in determining the length of the chain-

the alternative is to run it small x small, through the derailer and cut the chain to a length where it won't droop.

advantage to this method, is if you have a medium or long cage RD, you can swap out the cassette and not have to get a new chain.

the alternative is to run it small x small, through the derailer and cut the chain to a length where it won't droop.


As long as your drivetrain is within the derailleur's chain wrap capacity, that's fine. But if not, and you do it this way, the chain won't be long enough to cover the big/big combo. The results can be disastrous, much more so than if the chain is indeed long enough but the derailleur doesn't quite have enough chain wrap capacity-

yep, but usually not a problem if you use a road medium (37T capacity) or mtb long cage (45T capacity).

either way, the wrong RD length is being used if the chainrings + cassette teeth difference is exceeding the RD capacity.

My method is to thread chain thru RD and around small cog and small ring.
Take enough off so upper jockey pulley doesn't rub.
Been doing this for many years and works for me YMMV.

Does this mean the chain can be the same length for a bike with a long cage RD as well as a medium or short-cage mech?

Yup. If you use the big/big method when you're in the big/big the derailleur arm, regardless of length, will be pulled nearly horizontal virtually parallel with the bottom chain run. Consequently, arm length doesn't matter.

The big/big sizing method (which is what I happen to use) yields the shortest chain that's safe to run with a particular chainring, cassette combination. The small/small sizing method gives the longest chain that will work with your derailleur.