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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 subtracting 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 the manufacturer.

A great source for all of this is Sutherland's Handbook for Bicycle Mechanics (Repair Manual) 7th Edition.

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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.
    – Darth Egregious
    Dec 9, 2011 at 18:50
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    A lot of MTB cassettes now have a max cog of 36 teeth. Care to update your answer?
    – cherouvim
    Sep 5, 2013 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, 2015 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, 2015 at 15:11
  • The manufacturer specs are much more conservative than what you can actually achieve and keep as usable: sheldonbrown.com/deakins/lowgears.html
    – gps
    Jul 24, 2016 at 19:06
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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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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, 2015 at 15:10
  • Gidday and welcome to SE Bicycles. Good answer, I look forward to your future site contributions.
    – Criggie ♦
    Nov 21, 2015 at 21:51
  • I ran a 12t-30t with my 105 SS derailleur for a year. Worked fine. But it was obvious that it was not working well if cross chained. So long as you/rider are aware of that and avoids cross chaining it's fine. With my previous 12-28t it was no problem. But derailleurs are cheap. Just get a GS.
    – gps
    Jul 24, 2016 at 18:57
  • Getting the chain too short can cause serious damage. THat's what the MFG is trying to avoid. If you think about it... once that chain is grabbed by the cog, your weight will make sure it revolves. The results? Hopefully the R.D. sacrificing itself is enough to keep from dog-earing the crank and dropping your face into the headset... or worse... destroying the rear triangle or other drivetrain woes or hidden problems. Don't downplay the risks. Sure they are avoidable (e.g. set your chain length correctly so it gets a little slack on little/little) But new players don't know these tricks!
    – david1024
    Oct 26, 2016 at 19:06
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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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I have found that the length of the derailleur from the bolt centre (rear axle) to the first jockey-pulley's outer edge (when the derailleur is in its lowest gear), must be greater than the radius of the largest rear cog (with the chain on it), i.e. an RD M592 9-speed will work on a 40 tooth maximum cog but not on a 46 tooth maximum cog. Thank you

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  • How does this take into account front shifting?
    – ojs
    May 6, 2019 at 11:01
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The manufacturer's specs are total BS. The max cog sizes and capacties can he extended ALOT! Flipping the B tension screw over can get a Shimano Short Cage RD up to 36t max cog size (vs 27t spec) and 39t is the real capacity (vs. 29t spec). The Specs are leaning WAY TOO PARANOID on the safe side. The manufacturers are TERRIFIED of lawsuits. So in essence you are lost and at the mercy of blogs and youtube videos to find THE REAL SPECS. Lacing up a chain so the chain doesn't drag or drags just a tiny bit in small/small combo, then flipping the B tension screw so the head of the screw leans against the derailer hanger stop and max extending it, and then experimenting with the max tooth count cassette cog that'll fit w/o the upper jockey pulley grinding the largest cassette in small/big and the chain not jamming in big/big shift will determine your true RD max cog size and total tooth capacity. It's that simple.

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  • I agree with Michael Greene but there is an easy way to do it. Buying a derailleur hanger entension for $5 and you will be suprised of how big you can go on your cassette. Just watch out for your small rear small front combination that will be your limitation if the chain starts to sag.
    – Mon
    Nov 6, 2018 at 15:45

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