Broken bars

fjellrunner
fjellrunner Posts: 64
edited December 2009 in The workshop
abcd0005cn.jpg

How often does this happen? Never heard of it myself.
Happened on friday with half a mile of a 20 mile commute left. Lucky really.

Comments

  • cjcp
    cjcp Posts: 13,345
    Looks like fatigue failure. How old are they? Also, they seem to have snapped at the clamp. Did you overtighten the clamp?
    FCN 2-4.

    "What happens when the hammer goes down, kids?"
    "It stays down, Daddy."
    "Exactly."
  • N0bodyOfTheGoat
    N0bodyOfTheGoat Posts: 6,065
    Happened to the Profile bars on my old Principia RSL... I was just thankful that I managed to keep hold of the sheared bit (to stop it fighting with the spokes), keep my balance and stop safely!

    I suspect my shear was due to age, they were ~7 years old by then.
    ================
    2020 Voodoo Marasa
    2017 Cube Attain GTC Pro Disc 2016
    2016 Voodoo Wazoo
  • will3
    will3 Posts: 2,173
    Did you remember that the RH brake still worked?
  • fjellrunner
    fjellrunner Posts: 64
    cjcp wrote:
    Looks like fatigue failure. How old are they? Also, they seem to have snapped at the clamp. Did you overtighten the clamp?

    They're only 3 years old so it crossed my mind too that I'd overtightened them.
    How do you tell? Any advice on how tight to go without a torque wrench?
  • fjellrunner
    fjellrunner Posts: 64
    will3 wrote:
    Did you remember that the RH brake still worked?

    Fortunately I was pretty much stopped at lights so just used left one. I wasn't thinking straight enough to know what was happening!
  • cjcp
    cjcp Posts: 13,345
    fjellrunner wrote:
    cjcp wrote:
    Looks like fatigue failure. How old are they? Also, they seem to have snapped at the clamp. Did you overtighten the clamp?

    They're only 3 years old so it crossed my mind too that I'd overtightened them.
    How do you tell? Any advice on how tight to go without a torque wrench?

    Check the other side for grooves/indents in the bar metal.

    OTOH, I know people who change them regularly (one chap, every three years!) because of the risk of failure.
    FCN 2-4.

    "What happens when the hammer goes down, kids?"
    "It stays down, Daddy."
    "Exactly."
  • mmacavity
    mmacavity Posts: 781
    The fatigue life of aluminium alloys is governed primarily by crack initiation, which is accelerated by the presence of microporosity in the alloy structure.
    Or put another way if you can see a scratch in the bars then scrap them. Possible cause of scratch, is from any sharp edges on H/bar stem. Use a file to remove the sharp edges on the stem.
    http://www.truetemper.com/Performance_T ... ctions.pdf
  • mmacavity
    mmacavity Posts: 781
    http://www.thenew3t.com/contest.aspx
  • fjellrunner
    fjellrunner Posts: 64
    Cheers mmacavity. I've entered with a link to this thread and a picture of my bike. Fingers crossed!
  • always_tyred
    always_tyred Posts: 4,965
    cjcp wrote:
    Looks like fatigue failure. How old are they? Also, they seem to have snapped at the clamp. Did you overtighten the clamp?
    I've seen this happen before.

    My guess is that if you scratch the bars at some stage - for example if they slip around in the jaws of the stem when too loose - they will fatigue over time.

    The scratch will act as a stress point and eventually propogate.

    Its much more of a problem with open face place stems than the old quill type , since you have the opportunity to misalign the top and bottom of the face plate by having one screw too loose and getting the pressure from the other one.
  • always_tyred
    always_tyred Posts: 4,965
    mmacavity wrote:
    The fatigue life of aluminium alloys is governed primarily by crack initiation, which is accelerated by the presence of microporosity in the alloy structure.
    Or put another way if you can see a scratch in the bars then scrap them. Possible cause of scratch, is from any sharp edges on H/bar stem. Use a file to remove the sharp edges on the stem.
    http://www.truetemper.com/Performance_T ... ctions.pdf

    You mean propogation, I think. In competition with defect migration. Titanium is actually very similar to Al, but the relative rates of the two processes is the other way around. So whereas aluminium fatigues due to crack propogation, the defect migration in Titanium is rapid enough to truncate the crack propogation processes, such that ti actually gets stronger over time.

    I'm not aware that aluminium, doped, alloyed or otherwise, is porous (other than the surface). You might be thinking of microcrystalline. All metals are made up of little crystallites packed together. Aluminium is comparatively brittle and brittle fracture would be between the crystallites. However, brittleness is probably due to a number of other mechanical properties and I can't think of a particularly strong argument as to why the bonds between crystallites would be any weaker in Al than any other material.

    I liked metallurgy. Pity that the above represents pretty much the sum of my knowledge. :cry:

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