Super long cage and long cage are terms applied to Shimano rear derailleurs for bicycles. Super long cage and long cage derailleurs are common on mountain, commuter and touring bikes where the rider desires a wide range of gears to deal with varied terrain.
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Rear derailleurs typically come in three cage lengths: short, medium and long. As of 2010, Shimano typically sells rear derailleurs designated as short, long or super long sizes, or SS, GS and SGS, respectively. Shimano sometimes refers to the GS size as medium and the SGS size as long. On some models, Shimano uses S or L for short or long. The mega long designation was used on the discontinued Nexave series.
All derailleurs include several basic parts. The cage holds two pulley wheels, around which the chain is threaded when the derailleur is installed on the bike. The cage is attached to the body of the derailleur, which is spring-loaded and moves the cage in response to shifter cable tension changes, controlled by your shifter lever. The derailleur attaches to your bike via a screw in the upper part of the derailleur, which screws directly into the frame or into a derailleur hanger in your axle dropout.
The body of a modern rear derailleur is slanted to keep the cage's upper pulley wheel at the same distance from each sprocket as you shift gears. Shimano generally designs long or super long derailleurs to handle 7-speed or greater rear cassettes and large gear ranges, such as 11-tooth small sprockets and 36-tooth large sprockets. Road or racing bikes typically have smaller ranges requiring smaller derailleurs, such as 5- or 6-speeds with 13- to 26-tooth sprockets. Long cages move more easily and control the pulley wheel distance from the sprockets better over a wide range of gear sizes.
A common measurement for derailleur size is to subtract the number of teeth on the smallest front chain ring from that of the largest chain ring, and add that to the difference between the largest and smallest rear sprockets. This is called the capacity of the derailleur. All Shimano derailleurs have a set capacity, but it is possible to exceed this capacity if you are knowledgeable about gears and gear shifting. A long or super long cage size has a capacity of around 36 teeth.
You don't necessarily need a super long cage or even a long cage rear derailleur if you start using a cassette or freewheel with a large number or a large range of gear sizes. Sheldon Brown, the late bicycle mechanic and writer, recommended screwing in the B-screw on the back of the derailleur as far as it will go to increase the angle of the derailleur body to mimic a long or super long derailleur. However, the best shifting occurs when the pulley wheels are close to but not touching the cogs. If you can't get the pulley wheels away from the cogs, you need a longer derailleur.