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a geek question?

i'm looking to buy a new shimano rear derailer, how do i know if ineed a short cage or a long cage?


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    TesseractTesseract Posts: 280
    I think it's generally short cage for road/ race bikes, long cage is for mountain?
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    Jack HughesJack Hughes Posts: 1,262
    The longer your arm the further you can reach.

    The rear derailleur is an "arm". A large diameter sprocket requires a long reach to get round it. E.g. if you want one with more than 30 teeth, you need a longer arm.

    You tend to get big sprockets on mountain bikes where you need nice low gears. A road or TT bike will have much smaller ones - and quite a small range between the smallest and largest, so a shorter arm is needed.

    So, you probably need a shorter one, unless you have realllllllyyyyyyy looooooowww gears
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    BopomofoBopomofo Posts: 980
    You need a long arm if your chain has to cope with wildly different cog sizes, like Jack says. But he was only talking about the back... you'll probably need a long arm if you're running a triple at the front, too.

    The arm has to take up all the slack when you wimp out and go for the granny cog.
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    that seems simple enough, thanks for your help, just started to build a bikeso be ready for more siily questions.
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