JRA, things do break...- Mtbr.com
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  1. #1
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    JRA, things do break...

    I thought I'd take a nice loop on the way home, but as I was cranking up a short rise, "snap!". Broken axle.





    With the chain off, I could coast slowly (~running pace), any faster an the rear wheel would wobble out of control. It was a short ride, and a long walk.


    It's an aftermarket 10mm chromoly solid rear axle for Shimano hubs. The threads are cut (not rolled) and it looks like the initial crack occurred in the tread where the cone race ended on the drive side. Then progressive fracture propagation until there was ~1/4 of the axle left before it let loose.

    JRA. Honest!

  2. #2
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    axle too long for a front hub fixed gear conversion?
    everything sucks but my vacuum cleaner.

  3. #3
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    It was not the JRA that cause the axle to break.

  4. #4
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    Good thing you were just "JRA" and not flying down some hill in the woods full of roots, trees, rocks...when it happened. I suspect that it was degenerative if you will in that it happened initially and simply got worse before giving way.

    Sorry, I just had to.

    <iframe width="420" height="315" src="http://www.youtube.com/embed/jJvTDmvrDQI" frameborder="0" allowfullscreen></iframe>
    Last edited by 1SPD; 09-28-2011 at 07:52 AM.

  5. #5
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    Dude, first off that wheel is filthy. Get your self some shop rags.
    Secondly that is long term fatigue on a piece of metal that was doing a hell of a job on a non designed purpose. To second Fishcrick's observation.

  6. #6
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    Quote Originally Posted by S:Drive
    It was not the JRA that cause the axle to break.
    I think it was - about 1.5 years of JRA. That's the nature of fatigue failures.


    Quote Originally Posted by can't get right
    Dude, first off that wheel is filthy. Get your self some shop rags.
    Secondly that is long term fatigue on a piece of metal that was doing a hell of a job on a non designed purpose. To second Fishcrick's observation.
    Quote Originally Posted by fishcreek
    axle too long for a front hub fixed gear conversion?
    Dirt: more time riding, less time cleaning. Besides, dirt on the outside does not negatively affect performance or longevity.

    Axle: this one is a "purposely built" axle to convert a Shimano front M756 to rear fixie hub. I have done this conversion with low end Shimano solid axles, and none of those have broken in the last 7 years.

    Older "freewheel" MTB hubs (OK, now I'm dating myself) had the cone & bearing inboard of the 7-speed freewheel, the distance is about the length you see on the non-drive side of the above hub. Besides, the axle broke on the shorter side (drive side).

    My guess is: fatigue failure initiated by a notch stress riser and brittle material. The part is ductile chro-moly (EN 19), silver passivated for corrosion resistance. I don't know if it was heat treated, but the alloy may have been more brittle (hardness) than spec'ed. The machined threads have a sharp groove, and this along with a sharp load (ie: riding up on a curb, root, or off a ledge sometime) may have initiated it.

    I'm getting another one, if you don't hear from me (about this) in the next 2 year, then this one was probably a fluke.
    Last edited by itsdoable; 09-29-2011 at 06:17 PM.

  7. #7
    CB2
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    Quote Originally Posted by itsdoable View Post

    Axle: this one is a "purposely built" axle to convert a Shimano front M756 to rear fixie hub. I have done this conversion with low end Shimano solid axles, and none of those have broken in the last 7 years.



    .
    I thought older XT front hubs had 9mm axles?

  8. #8
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    Quote Originally Posted by CB2 View Post
    I thought older XT front hubs had 9mm axles?
    Yes, the first XT front hub with a 10mm axle was the HB-M755, which came out in 2000 (available in late 1999).

  9. #9
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    The low end Shimano axle would be better as you said the cromo is more brittle where the low end one will bend or stretch as forces are put on it. Do you really need to use a front axle on the rear?
    2011 Kona unit with some carbon.

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