1. ## Calculating capacity?

When deciding if my short cage derailleur will take a 12-30 in the rear and a 36-46 up front, I calculate this by capacity of 34T for a 5701 105 short cage rear derailleur by getting the differences of the cassette (in this case 18t) and the chain set up front (in this case 10t) and adding them together to get 28t difference which is acceptable to that rear derailleur, right?

2. According to United Bicycle Institute:

Determine Maximum Chainring Difference by subracting the number of teeth in the smallest chainring from the number of teeth in the largest chainring

Determine Maximum Cassette Cog Difference by subtracting the number of teeth on the smallest cassette cog from the number of teeth on the largest cassette cog

Determine Total Drivetrain Capacity by adding Maximum Chainring Difference to the Maximum Cassette Cog Difference

Record the Maximum Cassette Cog (the number of teeth on the largest Cassette Cog)

For Shimano:

SS - Short Cage Road Double - Maximum Cassette Cog is 27 and Total Capacity is 29

GS - Medium Cage MTB/Road Triple - Maximum Cassette Cog is 34(MTB)/27(Road) and Total Capacity is 33(MTB)/37(Road)

SGS - Long MTB - Maximum Cassette Cog is 34 and Total Capacity is 45

For SRAM:

Short - Maximum Cassette Cog is 34(MTB)/28(Road) and Total Capacity is 32(MTB)/31(Road)

Medium - Maximum Cassette Cog is 34 and Total Capacity is 37

Long - Maximum Cassette Cog is 34 and Total Capacity is 45

For Campagnolo:

Short - Maximum Cassette Cog is 26 and Total Capacity is 27

Medium - Maximum Cassette Cog is 29 and Total Capacity is 36

Long - Maximum Cassette Cog is 29 and Total Capacity is 39

3. Seems the short cage is good for 30T cog and 34T capacitiy. Your 28 teeth is correct and should work fine.
When to use a Long Cage vs Short Cage Derailleur?

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