Rear derailleurs are available in different cage lengths - short, mid and long. This refers to the distance between the pulleys, and influences a few aspects of your drivetrain. Most mountain bike derailleurs are available in mid and long cage varieties, with short cage derailleurs few and far between. Road bikes have short or mid cage lengths.

Short cage derailleurs take up less chain slack, yielding a slightly more taut chain and crisper shifting but limiting the overall gear range to a degree. Longer cage derailleurs have the ability to take up more chain slack, allowing one to use a wider gear range with a sacrifice in shift speed and ground clearance.

Mid cage

Long cage

Spec sheets for rear derailleurs list a max cog size and a capacity, a measurement of the gear range that will give acceptable performance. The max cog size is pretty self explanatory - don’t exceed its listed number of teeth and your derailleur won’t run into the top of your largest cog. Most mountain bikes have wide ranging drivetrains (12-34 or so out back, 22/32/42 or so in front) that need the capacity of a long cage derailleur. Road bike drivetrains with a double chainring setup use a short cage, with road triples using a mid.

But what about capacity? Next post…