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It broke while as I started pedaling when a redlight turned green.

In the pics it is possible to see that basically the metal broke! Was it just a poorly constructed product or are there some common causes that explain why this happened (like i should have unmounted it and cleaned the dirt out of the screwing lines sometimes or else..).

I wouldn't want this to happen to my other pedal.

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Original pictures: https://m.imgur.com/a/awW3NgK

Thx

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  • Most likely a manufacturing defect. Hope it did not cause you to crash.
    – Argenti Apparatus
    Commented Jun 15, 2018 at 11:44
  • I would suspect that the crank has been subjected to rough treatment in the past, either being dropped on its side a lot, or being struck by a car in your garage or whatever. (I'm guessing the bike is 30 years old, and has quite a history to it.) Prior to this failure, did the crank arm seem to be bent inward?
    – Daniel R Hicks
    Commented Jun 15, 2018 at 12:02
  • @Daniel R Hicks, the crank arm didnt't seem to have any defect. Actually the arm is good it's only the axis that broke. However i bought the bike second hand, and it's indeed an old bike
    – incas
    Commented Jun 15, 2018 at 16:21
  • You need a better photo to look for fatigue or defects, two most likely scenarios . At least 2 X close up , 5 X if you can do it. From what I can see ,a fairly peculiar fracture face.
    – blacksmith37
    Commented Jun 15, 2018 at 19:16
  • @incas - It's hard to tell for sure, but in your first photo the arm appears slightly bent.
    – Daniel R Hicks
    Commented Jun 15, 2018 at 20:02

1 Answer 1

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The "bolt" that broke is not a part of the crank. Your bike has a so-called square-taper bottom bracket and one side of the axle broke off. This axle is a solid metal part that runs from one crank arm through the bottom bracket shell to the other crank arm. It cannot be serviced and should not break. The most likely reason is metal fatigue.

You'll have to buy a new bottom bracket if it is a cartridge type or at least a new axle if is the cup-and-cone type.

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  • 1
    Is it just me, or does the metal axle look like it is made of cast iron? The surface of the break looks very rough, too rough for steel. Cast iron is not a material should be used for axles...
    – MadMarky
    Commented Jun 15, 2018 at 10:08
  • @MadMarky You are right, it does look a bit like cast iron. However, I have no idea how bad that would be.
    – StefanS
    Commented Jun 15, 2018 at 10:50
  • Square taper axles can be bought separately in different lengths, so there may be no need for a whole new bottom bracket. But then the broken bit may also be hard to get out of the crank-arm.
    – Carel
    Commented Jun 15, 2018 at 11:22
  • 2
    @Carel you are right, if it's a cup and cone type, the axle can be bought separately (and at least the crank looks old enough for that to have been likely). I don't think the broken bit should be that hard to remove though. If the crank bolt can be removed (threads unbroken), one can fixate the crank arm in a vise and hammer out the axle remnant with a punch.
    – StefanS
    Commented Jun 15, 2018 at 11:32
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    @MadMarky - The axle (even if cheap) is made from hardened steel and, in a fatigue scenario, will develop a fracture surface like that.
    – Daniel R Hicks
    Commented Jun 15, 2018 at 12:04

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