The Dangers of a Cracked Fork....

Discussion in 'Mountain Bikes' started by Westie, Jul 26, 2003.

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  1. Westie

    Westie Guest

    Because I'm well out of town I sent my bike in to the LBS a few weeks ago to have the Skareb forks
    fixed. There had been a big crack around the casting where the arch meets the lower leg - right next
    to the oil seals. Today I visited the LBS and was talking to the techie about the fork. He was
    telling me that there had been seven other cases of it and that the agents thought that it may have
    been caused by the arch pressing against the side of the carton during shipping. "Wait a minute", he
    says, "I've still got the legs out the back. I'll show you." He hunts for a minute and comes back
    with one lower leg with arch attached. The other leg is snapped off, of course. We look at it and I
    point out what looks to be a small crack forming in the same place on this remaining leg. "Crikey!"
    he exclaims (in much the same was as Steve the Crocodile Hunter might say it) and promptly grabs leg
    in one hand and arch in the other and starts twisting and bending as hard as he can. He certainly
    looked like he was wrestling an alligator. Suddenly the crack cracks. Hands, arms and alloy go in
    all directions. Moments later the blood starts flowing from a startled and shocked technician. He's
    sliced his hand and punched a small hole into the outside corner of his left eye! A quarter inch
    inwards and it would have been a different matter all together. Luckily, no stitches are needed but
    an icepack on the eye is. Fifteen minutes later he's sporting a fine black eye, but is happy as
    Larry and already planning a story for the g/f when he gets home tonight.

    If broken alloy gives way so suddenly and completely I'm glad I wasn't riding those forks.

  2. Andy Chequer

    Andy Chequer Guest

    Fatigue failures progress in two distinct stages - initiation and propagation. The initiation stage
    is the part of the service life before the crack nucleates and accounts (in most cases) for the
    vast majority of the service life. Once the crack nucleates, the applied load required to grow the
    crack is much lower as stress is concentrated at the crack tip; as a result therefore crack can
    propagate through the material relatively easily, hence your wrenchman being able to break it with
    his bare hands. The resulting fracture surfaces can be razor sharp but I suppose it's a bit late
    for some, eh......

    So yeah, don't ride a cracked fork.

    Andy "fatigue is the primary failure mechanism in 90% of service failures, y'know" Chequer
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