How to calculate the capacity of a rear derailleur
It's easy to calculate what capacity of a rear derailleur will be needed based on the size of the big and small chainring and the big and small cog. What about the converse question, how to calculate the capacity of a derailleur?
I was comparing some short-cage rear derailleurs I have lying around, and I noticed that the cage length and pulley sizes varied enough that they probably had different capacities.
Related question: is there a fairly comprehensive place to look these up?
If you are asking how to calculate the maximum capacity of the chainrings and cogs, based on looking at the derailleur, then it's not going to be as easy as just looking at them.
Finding a derailleur to fit your chainrings/cogs based on just the chainrings/cogs is going to be a lot easier than finding chainrings based on looking at your derailleur. HOWEVER, there are some standards for manufacturer, I have listed them below. I have also included the formula to find out your capacities, since I misread your question initially and decided to answer the part you weren't concerned with. I am not deleting it because, well, it took a while to type.
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)
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
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
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
NOTE - THIS INFORMATION IS SUBJECT TO CHANGE BY MANUFACTURER
And a great source for all of this is Sutherland's 7th edition
Hope that helps
Excellent answer. Exactly what I was looking for. I suppose it would be wishful thinking to find similar information for defunct manufacturers (e.g. exage, ofmega) anywhere. It should be within the same ballpark though. Thank you.
A lot of MTB cassettes now have a max cog of 36 teeth. Care to update your answer?
It's 2015 and Shimano short cage derailleurs are seemingly intended to work with 11-28 cassettes (at least for 10 speed). The original question asked for references, and that would be useful to see the updated information.
I am running (and apparently it is common to do so) a road triple chainring (50/39/30) with an MTB derailleur (SGS=long) and sprocket (11-34).
The manufacturer specs are much more conservative than what you can actually achieve and keep as usable: http://sheldonbrown.com/deakins/lowgears.html
I don't think that updating this answer is reasonable--It changes every year. With the recent rise of the compact double for road (50/30 and 50/34's...) coupled with 10/11 speed cassettes... (Campy has an 11spd 11-32 ROAD cassette in 2016) medium-load touring is possible without needing a triple!