Shimano: what's the difference between medium cage and long cage?
A long cage is used for drivetrains with a wide range. A wide range would be a typical triple front crank or a wide ratio cassette (11-46 tooth) with a double crank. To determine what you need you need to determine the tooth capacity of the derailleur (check Shimano web site for tech information). Add the difference of the cassette tooth count (42-11=31) crankset(36-26=10) for a total tooth count of 41. You may be thinking that a long cage would always be a better choice for the flexibility of range. However the long cage can be an issue for a mountain bike where logs, rocks and such love the big target of the long cage. This is especially important with 26" wheels where everything is lower to the ground.