Left crank arm problems

bornsinner
bornsinner Posts: 3
edited September 2014 in MTB workshop & tech
Hello everyone, just signed up to the forum. :oops:

Having some problems with my recently purchased Falcon bicycle, although it may not be specific to mountain bikes I've posted here because it's a mountain bike. Maybe a mod could move this if need be.

I've done less than 100 miles on this bike and the left crank arm has become lose and fallen off. After putting it back on and tightening the nut I've been out and done another 4 miles or so and it's come off again. Today I took a trip to Halfords to buy a new left arm and fitted it, but after another few miles today it's come off again. I don't know much about bikes and I'm now stuck as to where to go next.
I've uploaded some pictures of the nut, crank arm and left side of the bottom bracket but not sure if I'm allowed to post links or not?

Any help would be greatly appreciated,
Thanks.

Comments

  • The Rookie
    The Rookie Posts: 27,812
    You need a new crank arm, the alloy arm is much softer than the steel axle and will have been damaged when ridden with it loose.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • cooldad
    cooldad Posts: 32,599
    Anyone can post links, but probably this ^^^^^^^^^^^^^^
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

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  • bornsinner
    bornsinner Posts: 3
    Thanks for the replies.
    I did go out today and bought a new crank arm but it came off after a few miles easy riding.
  • The Rookie
    The Rookie Posts: 27,812
    Then you aren't doing the retaining nut/ bolt tight enough, it's done up pretty much as hard as you can using a 12" arm such as a quarter drive ratchet.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • bykeshed
    bykeshed Posts: 2
    I agree with The Rookie above ^^^ with both comments. It is worth also worth checking the tightness of the drive side too. I think most crank sets need tightening up a bit at a time alternating sides, until they are both tightened fully. There are a few crank sets that even say in their manuals that failing to tighten alternating sides will cause the left crank to fall off. I've never understood why though...

    It's common on most new (cheaper) bikes for components to come loose in the first few rides. Most manuals do state to check everything before each ride for the first x amount of miles.
  • cooldad
    cooldad Posts: 32,599
    Nonsense - the ends of a square taper axle are independent of each other.
    I don't do smileys.

    There is no secret ingredient - Kung Fu Panda

    London Calling on Facebook

    Parktools

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