Do aluminium bikes "flex" more as they get older

JimboPlob
JimboPlob Posts: 397
edited December 2013 in Workshop
Hi

I have a 2nd hand CX bike (Genesis vapour). I have noticed that if I give it some beans, it feels like a wet sponge. It is very flexible.

Do aluminium bikes get more flexy as time goes on?

Comments

  • sungod
    sungod Posts: 17,357
    i've got a vapour for commute, the wheels are certainly flexy, the shimano hubs are pretty grim, frame itself seems ok, just 'dead'
    my bike - faster than god's and twice as shiny
  • Monty Dog
    Monty Dog Posts: 20,614
    No, if anything many aluminium alloys age-harden and therefore would theoretically get stiffer - but I doubt the rider would notice. What you're probably noticing is lack of spoke tension and general wear of the drivetrain components. It's a bike shop myth that alloy frames go 'soft' with age - what typically 'kills' frames is reaching their finite fatigue limit which can result in cracking and structural failure. Most other bike frame materials like steel, titanium and carbon have infinite fatigue limits.
    Make mine an Italian, with Campagnolo on the side..
  • cycleclinic
    cycleclinic Posts: 6,865
    Steel will fatigue it does not have an infinate fatigue limit.Steel will reach a base line strength in time and if the loads are too high failure will occur.

    Allumium does not get more flexible. The bike may have always been this way but as monty say flex in what, stem, handlebars, wheels if the frame it was they way it was made.
    http://www.thecycleclinic.co.uk -wheel building and other stuff.
  • No Sweat
    No Sweat Posts: 103
    Are you sure the frame isn't cracked, perhaps around the bottom bracket, or behind the head-tube? You might not see a 'hairline' fracture unless the frame is loaded.....
  • desweller
    desweller Posts: 5,175
    thecycleclinic wrote:
    Steel will fatigue it does not have an infinate fatigue limit.Steel will reach a base line strength in time and if the loads are too high failure will occur.

    Allumium does not get more flexible. The bike may have always been this way but as monty say flex in what, stem, handlebars, wheels if the frame it was they way it was made.

    What Monty is trying to say is that, if the magnitude of a cyclic stress is low enough, steel can be put through an indefinite number of cycles. Whereas aluminium will suffer fatigue failure at any load if it sustains a sufficient number of cycles.

    I think Monty has got the terminology the wrong way round though. IIRC, because steel has this fatigue stress threshold below which it will not fail, it is actually described as having a finite fatigue limit. Aluminium's stress threshold extends to zero stress, so IIRC it is said to have an infinite fatigue limit. But I may be talking bollox; it's one of those things that I could easily have remembered the wrong way round!

    NB note the difference between load and stress.
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  • robbo2011
    robbo2011 Posts: 1,017
    sungod wrote:
    i've got a vapour for commute, the wheels are certainly flexy, the shimano hubs are pretty grim, frame itself seems ok, just 'dead'

    +1. I have one for commuting/bad weather duties. it serves it's purpose, but 'dead' would be a good word for it. It doesn't seem overly flexy though.
  • Monty Dog
    Monty Dog Posts: 20,614
    Yes, I'm probably missing something in my explanation, but it's a long time ago I studied all this stuff. In terms of alloy frames, to compensate for the material's fatigue properties some frames tend to be over-built, giving the characteristic harsh and 'dead' ride.
    Make mine an Italian, with Campagnolo on the side..
  • JimboPlob
    JimboPlob Posts: 397
    robbo2011 wrote:
    sungod wrote:
    i've got a vapour for commute, the wheels are certainly flexy, the shimano hubs are pretty grim, frame itself seems ok, just 'dead'

    +1. I have one for commuting/bad weather duties. it serves it's purpose, but 'dead' would be a good word for it. It doesn't seem overly flexy though.

    If I stand to the side of the bike, push the brakes on and put my foot on the pedal and push hard at the bottom of the pedal stroke, it is noticeably flexy. Its not really an issue because I ride my commute pretty easy. It sometimes even feels like I have a flat tyre. pretty weird sensation. I have had a ribble audax in the past and that felt like a scaffolding pole in comparison.
  • No Sweat
    No Sweat Posts: 103
    Although your commute may be an easy ride, if the frame has partially failed it may well fail catastrophically and without warning - it would not hurt to get an LBS to give it the once over - without being dramatic, you could get hurt by a frame failure.

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