Ask a Mechanic | Understanding Rear Derailleur Cage Length

Jenny wants to know, “What is the difference between short, medium, and long cage rear derailleurs? How do I know which one I need?”

Thanks for the question Jenny. Derailleur cages are there to take up excess slack in the chain when shifting into different gears. Most manufacturers offer rear derailleurs in three different cage lengths: short, medium, and long. The most simple and straightforward way to think about cage length is as follows: the larger the range of gears you have, the larger the cage you’ll need.

For those unsure about what “range” is, range is the difference between your biggest and smallest cog. A wide range is typically defined as any cassette with more than 32 teeth, or any road or mountain triple crankset.

To use an example, if you have a triple crankset with teeth ranging from 22 to 44, or a cassette with a large cog bigger than 32, you’ll need a medium or long cage rear derailleur to take up the extra slack between your biggest and smallest gears. Conversely, any road bike using a double crankset or a cassette with 28 teeth or less will utilize a short cage rear derailleur.

Here are some general rules to follow: if you’re riding a mountain bike or run a 3x road crank, you’ll need a medium or long cage derailleur, which Shimano calls “SGS” and SRAM simply denotes in the product title. Road bikes with 2x cranksets use short cage derailleurs, which Shimano calls “SS” models and SRAM denotes in the title.

Finally, many brands have Medium cage options on their rear derailleurs that is typically used for “in-between” road drivetrains and mountain bikes that are in the 28-36 tooth range. Cyclocross bikes and many gravel grinder builds are prime examples of when to use a Medium cage rear derailleur.

If you are in-between the capacity ranges of the two, which is often the case with mountain bikes, then note the following: long cage derailleurs will accommodate a wider gear range and deliver a better chain line, while medium cage derailleurs are less likely to hit rocks on the trail. Though wide range cassettes are becoming the new standard, if you can use a medium cage derailleur without exceeding its tooth capacity, then we’d recommend going that route.

If you’re truly unsure about which rear derailleur you might need to purchase, you’ll need to first find out how many teeth are on your rear derailleur’s biggest cog. If you can’t find any markings on the cassette, there’s nothing wrong with simply counting the teeth. From there, simply cruise onto Art’ and check our product description to make sure that the derailleur you want to buy will fit the cassette that you already have.

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