As referred to in the last post about derailleur cage lengths, rear derailleurs have a number listed as capacity to determine what gear range is acceptable for a given model. Or more accurately, what cage length is required for a given gear range.

It’s simple math…

Capacity = (biggest chainring - smallest chainring) + (biggest sprocket - smallest sprocket)

On a spec sheet, capacity is a maximum number. For an example, consider a typical 9-speed mountain bike with a 22/32/42 chainring setup and a 12-34 cassette.

(42-22) + (34-12) = 42

This means that a long cage LX rear derailleur with a listed capacity of 45 would work fine, yet the mid length cage version with a capacity of 33 would not. A 2×9 setup would likely be able to use the mid length cage, whereas a 1×9 setup could get away with a short cage road deraileur depending on that max cog size.