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?

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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:

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

2. 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

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

4. 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

NOTE - THIS INFORMATION IS SUBJECT TO CHANGE BY MANUFACTURER

And a great source for all of this is Sutherland's 7th edition

http://www.sutherlandsbicycle.com/7th_Edition.html

Hope that helps

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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. – user973810 Dec 9 '11 at 18:50
A lot of MTB cassettes now have a max cog of 36 teeth. Care to update your answer? – cherouvim Sep 5 '13 at 12:35
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. – Craig Hicks Jun 10 '15 at 23:08
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). – Craig Hicks Aug 7 '15 at 15:11

You should be able to estimate the tooth capacity of a rear derailer by installing it on a bike, threading a chain through (and over a cluster), anchoring one end of the chain so it can't move, and pulling on the other end. You'd first pull just enough to achieve "minimum" tension, then pull until the derailer was stretched close to tight. Count the chain links that are pulled past a fixed spot (ie, don't just count how many links roll off the jockey wheel) as you pull between these two positions.

You'd probably want to try this 2-3 times on different cogs of the cluster.

Definitely takes a bit of judgment, but should come within 1-2 teeth of the "right" answer.

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I'm building a new touring bike and finding the capacity specs far below what I've used on the old one I put together. The crank is 44-20 and the cassette is 36-11, which is a 49 capacity. It works perfectly with old friction shifters, and an extra long derailleur screw. and yes, this low gearing really has been useful/necessary with a loaded bike in the mountains.

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Some nice detail in the big answer there. However I should add that on my fancy road bike, I'm running a Shimano Ultegra 6700 SS (i.e. short cage) Derailleur on a 10 speed cassette that is 12-30. (I have a compact 50-34 at the front.)

Not that I do, but crossing the chain isn't a problem either, except for rubbing on the front derailleur.

So I don't understand the limits that Shimano give, except for - either covering their asses if somethign does go wrong - or more likely, wanting to sell more kit on the back of incompatibility.

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It's basic engineering (safety margins). You can exceed the capacity by a bit, but you're on your own for that. – Batman Nov 21 '15 at 15:10
Gidday and welcome to SE Bicycles. Good answer, I look forward to your future site contributions. – Criggie Nov 21 '15 at 21:51