Speed wobble

mike ives
mike ives Posts: 319
edited May 2008 in Workshop
Everytime I go over about 35mph going downhill teh bike tends to start wobbling. It is very un-nerving and not a little scary. Can anyone tell me what causes this and is it related to a particular part of the bike. What do I need to do to stop this happening?

Comments

  • LangerDan
    LangerDan Posts: 6,132
    There is a wide variety of potential causes - lack of frame stiffness, incompatible fork / frame geometry, wheel type, rider position, jupiter being in conjunction with mars etc.

    It can happen with any level of bike - back in the late '80s a friend of mine had a ex-TdF team bike. It was fine for the original rider (a Tour stage winner!) but suffered dreadfully from speed wobbles under my mate.

    Unfortunately there is no easy fix, though as a starter you might try what another poster here did. Take off the front wheel magnet and let the wheel rotate until its heaviest point settles at the bottom. then replace the magnet at 180 degrees to the low point. If that doesn't work, you might try swapping the front wheel.
    'This week I 'ave been mostly been climbing like Basso - Shirley Basso.'
  • heavymental
    heavymental Posts: 2,091
    Search the forum for wobble or shimmy and do likewise on Google and you'll get alot of answers. This question comes up here quite alot.

    Good luck with it, its scary when its happens.
  • aarw
    aarw Posts: 448
    you may not need to change anything on the bike itself. try holding your knees in against the top tube and don't grip the bars so tight. try to poosen your posture gererally. the first thing that will give you speed wobble is being stiff on the bike.

    worth trying this before spending money on the bike only to find out it wasn't the bike at fault.

    apologies if you're already doing this or likewise...
  • comsense
    comsense Posts: 245
    My suggestions
    1. Can you get another rider to try the bike? Preferably a good descender.
    If it wobbles with another rider the fault is definitely the bike.

    2. If its the bike check the wheels, tyres ( substitute the wheels one at a time if possible)headset etc - I have had wobbles on and off over years and as I knew I was the problem and not the bike.1 time I did not check the bike and you guessed it - that 1 time it was the headset.

    If its you then you need to narrow it down.
    Are you locking your arms and "sitting up" as you reach a certain speed ? Some people (I am one) do this when they are going faster than they are comfortable at,this will put you into a wobble. Think about it - lock your inside arm as you turn a bike and the bike goes in that direction very sharply - lock both arms and the bike wants to turn both ways!
    Relax your arms and stay low over the top tube.
    Sit up and your trunk catches the wind - pushing even more weight over the back wheel.
    BTW - did you ever notice yourself descending better when you are tired? This is a sure sign that its nerves as you tend to be a bit more relaxed ( or couldn't care less whether you lived or died :D ) after a hard spin.
    Weight distibution can also be the source.
    Put your bikes front wheel on a bathroom scales and the back wheel on a phone book. Sit up on it and get down in the drops, trunk low to the top tube, classic racing style - check the weight on the front.
    Weigh yourself and the bike by picking it up and standing on scales. Divide the weight on the front by the combined weight to get the precentage of weight over the front wheel - this should be ideally 42 -45% , if its less this may be your problem as too little weight over the front wheel can cause shimmy.

    Good luck!
  • aarw
    aarw Posts: 448
    i like the bathroom scales idea, think i'll have to try it out...
  • over-the-hill
    over-the-hill Posts: 144
    This is what it says in myWilier bike owners manual (roughly):
    Heavier riders on larger bikes may experience a shimmy or harmonic oscillation or frame vibration at certain speeds
    Experts disagree about causes of a shimmy but some believe:
    a loose headset
    improper spoke tension or frame alignment
    riding with no hands
    front wheel impact

    I had never heard of it before.
  • theoldbiker
    theoldbiker Posts: 9
    I agree - its the scariest thing - I've suffered it on £1000 bikes and old ones that had just had a wheel change - but I don't get it on an old (very old) Raleigh mountain bike. There's a good article here http://www.sheldonbrown.com/brandt/shimmy.html but once it starts and you've applied all the tricks to mitigate the problem I have found that steering a very elongated 'S' course will control things until you get comfortable again. Good strong cycle clips around the bottom of your shorts/trousers also helps!!!
  • mike ives
    mike ives Posts: 319
    Thanks to everyone for the information. I am going to try the scales excercise and the wheel magnet postion etc, and then take it from there, bit by bit.

    All the information is really helpful. Thanks.
  • andy_wrx
    andy_wrx Posts: 3,396
    over-the-hill wrote:
    This is what it says in myWilier bike owners manual
    ...
    riding with no hands
    ...


    So you get a speed wobble if you descend at high speed with no hands ?

    Jesus, who thought to try that one ? And how did they live long enough to report it ?

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