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  1. #1
    Share the road. bugly64's Avatar
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    which rear derailleur?

    I have already purchased 10spd 105 cranks, cassette, and Ultegra brake/shifters, but I don't know which derailleur to buy. The long or the short one. What's the difference? What's preferable for racing and part time commuting?

  2. #2
    blithering idiot jhota's Avatar
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    depends on your cassette.

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    it more depends on if you have a double or triple. For most applications, you use a short cage for a double

  4. #4
    jack of one or two trades Aeroplane's Avatar
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    If you are going single ring you can probably go with a short-cage. The only way to know for sure is to look up how many teeth of difference your derailleur can take up, and figure out what the maximum difference your setup has. For instance, if you are running a 36-44 and a 12-28 cassette (random numbers), your minimum is 48 (36 + 12) and your max is 72 (44+28), so you will need a derailleur that can take up 24t (72-48) of slack.
    Quote Originally Posted by Dr Irwin Goldstein
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  5. #5
    Team Water Andy_K's Avatar
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    Just to make explicit what I don't see explicitly above, you want the shortest cage derailleur you can get away with. If I'm not mistaken this is because a longer cage derailleur gives you a bit more chain slack in many gear combinations and the shorter cage derailleur deliver crisper shifting.

    To give an example, I've got a 50-39-30 front crank (triple, because like you I also use it for commuting). I'm looking to upgrade to an Ultegra rear derailleur. I've got a 12-27 cassette. Going to the Shimano web site, I get the following data:

    Ultegra GS
    Maximum Sprocket = 27T
    Minimum Sprocket = 11T
    Maximum Front Difference = 22T
    Total Capacity = 37T

    Ultegra SS
    Maximum Sprocket = 27T
    Minimum Sprocket = 11T
    Maximum Front Difference = 16T
    Total Capacity = 29T

    Looking at this, I can't get the SS (short cage) derailleur because I have too much front difference (20) and require too much total capacity (35). So I'd have to get the GS (medium cage). But when I use this bike for cross racing, I never get onto the big chainring, so if I wanted to, I could take it off and set up the front derailleur to keep the chain on my 30-39 rings. Then I'd have a front difference of 9 and a total capacity of 34, and I could happily use the short cage derailleur.

    Notice also, that if I had a nine speed setup and wanted to run an 11-32 cassette, I'd need a MTB derailleur regardless of capacity because I'd be exceeding the max sprocket size for road derailleurs.

    Also, you can also calculate the total capacity as the front difference (50-30) + the rear difference (27-12). It's mathematically the same, but when you're thinking about things like reconfiguring cranks and cassettes, it's sometimes easier to juggle.

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    Quote Originally Posted by Andy_K View Post
    Also, you can also calculate the total capacity as the front difference (50-30) + the rear difference (27-12). It's mathematically the same, but when you're thinking about things like reconfiguring cranks and cassettes, it's sometimes easier to juggle.
    It's not just about chain slack, though, but also whether there's interference between the jockey pulley and cassette.
    http://www.sheldonbrown.com/gloss_ca-g.html#capacity

    Since the mfgrs publish max cog and max tooth difference, might as well just follow their numbers.

    Sheldon says there's no real reason to use a short-cage: "SS cage derailers don't work any better than GS models, just a couple of grams lighter."

    Short answer: Medium cage up to 28t, large cage ("mountain" or "touring") for anything bigger.

  7. #7
    jack of one or two trades Aeroplane's Avatar
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    Quote Originally Posted by flargle View Post
    Sheldon says there's no real reason to use a short-cage: "SS cage derailers don't work any better than GS models, just a couple of grams lighter."
    I hate to disagree with Sheldon, but in my experience short cages are a LOT less likely to pick up a stick, and we all know how a stick in the derailleur can ruin a ride.
    Quote Originally Posted by Dr Irwin Goldstein
    Men should never ride bicycles. Riding should be banned and outlawed. It is
    the most irrational form of exercise I could ever bring to discussion.

  8. #8
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    Quote Originally Posted by Aeroplane View Post
    I hate to disagree with Sheldon, but in my experience short cages are a LOT less likely to pick up a stick, and we all know how a stick in the derailleur can ruin a ride.
    Fair enough.

    I can't say it's been a problem for me. Also, I think the amount of chain slack would play into the equation, too. Shortening the chain as much as possible is a good idea anyway, for cross especially.

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