Rear Derailleur Capacity

Rear DerailleurI am not a small person and don’t resemble a Tour de France competitor in any shape, form, or fashion. As a result, I have had to change the gearing on most of my bicycles in order to get a reasonably high spin (cadence) going up hill in order to protect my knees.

One concern when changing the gear ratios on a bicycle is whether or not the rear derailleur will work with the new gears. Rear derailleurs come in 3 sizes: small, medium, and long (some manufactures may have a slightly different name). How do you determine what size fits a particular application, you ask? Good question, I answer!

Rear Derailleur Requirement

  1. Chainring Gears: Subtract the smaller gear from the larger gear. i.e. 52 – 30 = 22
  2. Cassette Cogs: Subtract the smaller cog from the larger cog. i.e. 34 – 11 = 23
  3. Rear Derailleur Requirement: Add the two number together. i.e. 22 + 23 = 45

Rear Derailleur Capacity

  1. For SRAM derailleurs
    1. Short Cage
      1. largest cassette cog: 34 tooth for mountain bikes, 28 tooth for road bikes
      2. maximum capacity: 32 for mountain bikes and 31 for road bikes
    2. Medium Cage
      1. largest cassette cog:  34
      2. maximum capacity: 37
    3. Long Cage
      1. largest cassette cog: 34
      2. maximum capacity: 45
  2. For Shimano derailleurs
    1. Short Cage (SS designation)
      1. largest cassette cog: 27
      2. maximum capacity: 29
    2. Medium Cage (GS designation)
      1. largest cassette cog: 34 tooth for mountain bikes, 27 tooth for road bikes
      2. maximum capacity: 33 for mountain bikes and 37 for road bikes
    3. Large Cage (SCS designation)
      1. largest cassette cog: 34
      2. maximum capacity:  45
  3. Check the manufacturers information for other rear derailleurs brands.

In the above example, the largest cassette cog is 34 tooth, and the derailleur requirement is 45. In checking the guidelines, for both SRAM and Shimano, the example gear setup would need a Long Cage rear derailleur.

In practical application, it is never recommended that the largest chainring and largest cassette cog(s) or smallest chainring and smallest cassette cog(s)  be used at the same time. Running a gear combination like this is sometimes referred to as cross chaining (see the diagram). Cross Chaining If you follow this recommendation and don’t cross chain then you can exceed the manufacturer’s capacity guidelines by several numbers, if necessary. When exceeding the guidelines make sure the derailleur is adjusted properly and test all gear combinations under little or no load.

Good luck and safe riding!

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