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  1. #1
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    What is the difference between front derailleurs?

    Well I'm guessing top pull means cable comes from the top tube down the seat tube instead of the down tube. I've been reading some derailleurs say that they can cater to both. I am looking at the Shimano XT front derailleur I was wondering are they all top pulls? I see a M770 and a M771 they are both XT derailleurs is there any difference? Also how do I determine the size clamp I need. I have a Trek Wahoo is there a standard size?

  2. #2
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    FD-M770 is top swing type (cage above the clamp).
    FD-M771 is down-swing (bottom swing) type (cage bellow the clamp).

    Both are dual-pull (cable can came from above or from below).

    Clamp size is for 34.9 mm seat tube on both. This is the standard size on current MTB aluminium frames.

    If you have a 28.6 mm or a 31.8 mm seat tube, you need the small size, respectively medium size adapter for the clamp.

  3. #3
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    Both derailleurs that you mention are dual-pull: they can function as either top-pull or bottom-pull. I believe Shimano now designs all their front derailleurs that way.

    The difference is that the M770 has the clamp lower than the cage, whereas the M771 has the clamp higher than the cage. On some frames, you can go either way. On other frames, you will want one or the other due to the location of other items such as bottle-cage bolts or suspension pivots.

    The photos on Shimano's website should make the difference clear: M770, M771

    Offhand, I'm not sure about clamp diameter on your bike. I believe it might be 34.9mm, but don't hold me to that. The common clamp sizes are: 28.6/31.8/34.9mm. Just measure the diameter of your seat tube where the derailleur clamps on. That's how you can tell which size you need. Aftermarket deraillers often come with shims to let you use them with whatever clamp size you happen to have.

  4. #4
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    So if my seat post is 27.2 I ned a 28.6 clamp?

  5. #5
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    Quote Originally Posted by nyczbubba
    So if my seat post is 27.2 I ned a 28.6 clamp?
    You really need to measure the seat tube just above where it hits the bottom bracket. Or look at the inside of your current derailleur clamp. Sometimes the diameter is stamped on the inside.

  6. #6
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    Its the frame material as much as anything that influences clamp size. Steel only would be 28.6 as the material is stronger so can have a thinner wall (they often run smaller post sizes too), alu needs to be a bit thicker and has 31.8 , and carbon fibre often has gigantic tubes and a massive bb area so runs a 34.9
    Rimmer - "There's an old human saying - if you talk garbage, expect pain"

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