Know Your Derailleur Limits

Brad Quartuccio

Rear derailleurs have limits on gear range and movement, each important when choosing or setting up a drivetrain. Derailleur capacity is a manufacturer’s specification that determines the gear range that the cage can accommodate, derailleur limit screws adjust the physical limits of the derailleur’s motion to fine tune indexing and prevent over
shifting.

 

Derailleur Limit Screws

Derailleurs have a pair of limit screws. The L for “low gear” screw limits the motions of the derailleur past the largest cog, the H for “high gear” screw limits the motion past the smallest cog. The high limit serves as a starting point for your indexing, the low limit prevents over shifting the chain into the spokes.

Rear Derailleur Capacity

On a spec sheet capacity is a maximum number, representing the amount of difference between the big/big and small/small gear combinations that the derailleur cage can accommodate, with longer cage rear derailleurs having a larger capacity.

Capacity = (largest chainring - smallest chainring) + (largest cog - smallest cog)

For example, consider a bike with a 50/34 crankset and an 11-32 cassette.

(50-34) + (32-11) = 37

In this example a long cage derailleur with a listed capacity of 37 or greater is required to work properly. For a single ring setup, simply subtract the smallest cog from the largest cog to determine required capacity spec.