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  1. #1
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    Broken Front Derailluer

    What causes this type of failure?



    1st time back in October, I bought another derailleur off of Ebay and installed it. While I was waiting for the new one to arrive I welded the broken one back together to use on a ride the next morning. The new one (both Tiagra) is having a similar issue. They break when shifting from big to little and then it is noticed the next time a shift is made from little to big. This is what happens, I don't know why.

    1. Riding in the big ring.
    2. Shift is needed to the little ring.
    3. Shift is actuated and derailleur moves.
    4. Chain hesitates in the big ring.
    a. this would appear to just be a noisy shift.
    5. Front derailleur is distorted sometimes breaking, sometimes just bending,

    No idea what causes this. I chalked up the first one to inexperience or damaged by the previous owner. I had just came around a right hand corner and attempted to shift when it failed.

    2nd time. Climbing and tried to shift under a load. (just bent, did not break). I bent the mechanism back into place and it continued to work. I didn't trust that it would continue to work and replaced it with my crappy looking welded spare.

    3rd time. My welds don't give. Shifting at a low cadence at the bottom of a descent just before starting the next climb.

    For the sake of argument letís just assume that if I have the skill set required to weld one back together, that it is likely that I can install it properly also. I followed Shimanos tech docs to the T.
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  2. #2
    wim
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    The most common cause for this type of failure is shifting while a large amount of force is applied to the pedal. This tensions the top run of the chain to such a degree that the derailleur cage bends a bit before it finally manages to derail the chain off the chain wheel. After the cage has been bent for a certain number of times, it snaps.

    There's no way I can tell from the picture or your description if this applies to you. But the fact remains that people who always reduce pedal force when shifting never break front derailleur cages.

  3. #3
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    Re: Broken Front Derailluer

    Is there s compatable derailluer that can handle the abuse?
    People sleep peaceably at night because rough men and woman stand ready to do violence on their behalf. George Orwell

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  4. #4
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    Quote Originally Posted by matfam View Post
    Is there s compatable derailluer that can handle the abuse?
    I do not know if any derailleur's will take too much shifting under weight. My guess is Tiagra is one of the better ones at that because of its weight. You would probably run into more problems with a light weight one like Sram Red.

  5. #5
    tlg
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    Quote Originally Posted by matfam View Post
    Is there s compatable derailluer that can handle the abuse?
    No since they're not really designed for that type of abuse. If you shift properly it isn't an issue.

    Also make sure your derailleur height is adjusted properly. If too low, it could be causing excess force or catching on the teeth of the large ring. You should be able to fit a penny in there.


  6. #6
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    Quote Originally Posted by tlg View Post
    No since they're not really designed for that type of abuse. If you shift properly it isn't an issue.

    Also make sure your derailleur height is adjusted properly. If too low, it could be causing excess force or catching on the teeth of the large ring. You should be able to fit a penny in there.

    I liked that trick. I always just measured 2 mm (IIRC)
    It's a fire road.............
    I'm on a road bike..........

    They have enough in common to blast down it.

  7. #7
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    Even if you have a more robust front derailleur something is still going to have to give when excess forces take place, ie: chainring, chain

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