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Question: stability of bicycle

  1. Dec 9, 2007 #1
    I'm doing a big physics lab for my extended essay (IB diploma).
    I'm exploring two things, where my question is
    what affects the stability of a bicycle more, velocity or the weight on the bicycle?
    So for the first variable velocity,
    i pushed the bicycle in different strengths to alter velocity and observe how long they go.
    For the second variable, i tried to keep the velocity the same as possible and added weight to observe the stability.

    So here is my first question:
    For both of these variables, what should my hypothesis (expected outcomes) to be?
    for velocity, as velocity increases stability would increase too (my prediction),
    but for weight, i'm not so sure since it could be also directly proportional due to gravity but also it could be the opposite due to the increase in torque to fall down sideways.
    What do you predictwill happen?

    here is my second question:
    what are the calculations i can use for these to support / analyse the results?
    i can see that i can use torque to see how the bicycle falls,
    but i need more calculations to do.. some ppl suggested angular momentum, law of conservation etc.. could anyone explain this a bit more specifically please?

    i know i'm asking a lot of things, but it would be really, really helpful if you could spend some time helping me, this is really important for me.
    I promise i'll give ur help back sometime later when you need it:-)
    thank you very much again and have a nice day!!
  2. jcsd
  3. Dec 9, 2007 #2
    Have you thought on modifying the angle of the fork?
  4. Dec 9, 2007 #3
    what do you mean exactly?
    if your'e talking about investigating another variable, i cant do that now.. its too late :-(
  5. Dec 9, 2007 #4
    Well, as far as I know, the angle of the fork (I mean, it is not a vertical fork but has a forward angel) is what makes cycling easy enough. When the running bicycle tends to fall, the steering turns into the falling direction, creating a centrifugal force (that of the curve the cycle is following) that will push the bicycle upwards.
    As you pointed, more speed makes the cycle more stable as it is needed less curve to create enough centrifugal force.

    I was pointing you to the angle as more weight will favour the steering to turn, but at the same time you will need more speed to increase the centrifugal force needed to keep more weigth on track. I don't know the answer, but the fork angle is something to take into account (mountain bikes are more unstable that road bikes, for example).
  6. Dec 9, 2007 #5


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    Homework Helper

    The steering geometry of a bicycle is setup so that if the line of the pivot axis were extended to the pavement, this intercept point would be in front of the contact patch of the front tire, and the distance from the pivot axis intercept point back to the center of the contact patch is called trail. When a stationary bicycle is leaned, gravity pulls down on the bicycle, and the pavement pushes up at the contact patch, and because the contact patch is "behind" the pivot axis, it cause the front wheel to steer in the direction of lean.

    The amount of trail determines the minimum speed that the bicycle requires to be vertically stable.

    The amount of weight doesn't matter, but a low center of mass will increase the time of recovery, and may also require a higher minimum speed.

    At higher speeds, gyroscopic forces resist changes in lean angle, and there will be a transition from vertical stability to lean angle stability.
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