Creaking and flexing aluminium frame? Fatigue?

trek_dan
trek_dan Posts: 1,366
edited March 2017 in Workshop
My approx 6 year old Focus Mares cross bike has developed a rather hideous groan when putting any power down through the pedals over the last few months. I just put it down to the 'usual suspects' at first so replaced the bottom bracket, replaced headset, made sure there was plentiful grease on the stem and seatpost interactions and it was still no better. The groan when I put any power down through the pedals has continued to get worse. Last night I noticed when I put any weight down through a pedal in isolation the whole frame around the BB area appears to flex significantly - it may be an optical illusion but certainly seems like that is whats happening. There doesn't appear to be any cracks etc so Is it metal fatigue? Has the frame best its sell by date so to speak? I would never have thought an alu frame would fatigue in this way.

Comments

  • sungod
    sungod Posts: 17,150
    aluminium fatigues

    for the frame to flex badly, there'd need to be a crack, could be in a tube itself and/or at a weld

    you should be able to see such flex by standing next to the bike and putting weight on one pedal

    or it might by the bb or crankset, in which case replacement is straightforward

    isolate it before riding more, a crack can lead to catastrophic failure, which could be nasty
    my bike - faster than god's and twice as shiny
  • term1te
    term1te Posts: 1,462
    If you can't see a crack, then I suspect it is OK, but look very carefully. I had a creaking sound when I was out of the saddle, after removing and cleaning virtually every component, it turned out to be the front skewer not being tight enough.

    I have had some aluminium handlebars give up on me, there was a creaking sound for about 30 minutes, then one side slowly bent downwards. The crack was just inside the stem clamp, so I couldn't see it initially. I'd probably over tightened the clamp, rather than it naturally fatiguing, and I happily still ride a 1990 aluminium frame.
    The Master
    The Moser
    The VN
  • trek_dan
    trek_dan Posts: 1,366
    I've got an even older aluminium Trek CX frame which is still in regular use, which is why I'm surprised by this one seemingly failing. Will have a really thorough look for a crack tonight.
  • sungod
    sungod Posts: 17,150
    al doesn't behave the same as steel, roughly speaking...

    as long as stress is below a certain level, steel can undergo an indefinite number of load/unload cycles without failure

    but with al, failure rate increases with number of cycles, eventually it fails, failure under load can be sudden/catastrophic

    it's really luck of the draw, design, usage, alloy, forming, heat treatment, welding, stress risers, corrosion etc. all affect it
    my bike - faster than god's and twice as shiny
  • dennisn
    dennisn Posts: 10,601
    trek_dan wrote:
    My approx 6 year old Focus Mares cross bike has developed a rather hideous groan when putting any power down through the pedals over the last few months. I just put it down to the 'usual suspects' at first so replaced the bottom bracket, replaced headset, made sure there was plentiful grease on the stem and seatpost interactions and it was still no better. The groan when I put any power down through the pedals has continued to get worse. Last night I noticed when I put any weight down through a pedal in isolation the whole frame around the BB area appears to flex significantly - it may be an optical illusion but certainly seems like that is whats happening. There doesn't appear to be any cracks etc so Is it metal fatigue? Has the frame best its sell by date so to speak? I would never have thought an alu frame would fatigue in this way.
    Me? I think you're assuming it's the frame. If you don't trust it buy a new one.
  • trekvet
    trekvet Posts: 223
    The last Trek I had creaked. I took out seatpost and could see where frame tubes fit through each other they have vent holes. So I put some (tsp = 5ml.?) oil down it. Sealed top of frame and rotated frame slowly around and around. Refitted seatpost. Rode, and no creak. Check oil doesn't then drip on carpet. Easiest, cheapest fix I've every done.
    The Wife complained for months about the empty pot of bike oil on the hall stand; so I replaced it with a full one.
  • onionmk
    onionmk Posts: 101
    Do you use clipless pedals? FWIW, I built up a steel frame recently and noticed a creaking sound when powering down which Got me really REALLY annoyed. Tightened the chainring bolts, cleaned, greased and refitted the BB which got me more frustrated as it led me to believe the frame was faulty. Ultimately found it was my shoes/ cleats which were the culprit. Couple drops of wet lube on each and it was sorted.

    With regards to frame flex, try and get a second opinion from someone preferably experienced so you can be sure? Even the stiffest frames move side to side when under heavy stress so it's difficult to judge what's normal and what's not

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