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Why doesn't a moving bicycle fall?

  1. Sep 24, 2012 #1
    I have always had this question and I have received various answers from angular momentum conservation to gyroscopic effect. But I couldn't really understand the principle. Also, why it becomes easier to balance the cycle when you move the handle?
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  3. Sep 24, 2012 #2


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    If you steer, you can counter any deviation to one side by moving the handle: You force the front wheel towards one direction, and the bike leans towards the other direction.
    However, bicycles can "steer on their own" too, and it is quite complicated how that works.
  4. Sep 24, 2012 #3


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    The basic idea is "conservation of angular momentum". If a spinning wheel oriented along the y-axis of an xy-coordinate system, rotating clockwise as seen from the positive x-axis, tilts "to the right" (a line pointing up the z-axis tilts so it is over the positive x-axis) then angular momentum causes the wheel to "turn to the right" (a line pointing up the positive y-axis turns si it is over the positive z-axis) which causes the wheel to "right itself".
  5. Sep 24, 2012 #4


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    Last edited: Sep 24, 2012
  6. Sep 26, 2012 #5
    I used to play disc golf, and was always amazed at how little spin it takes to stabilize a disk. Disks can stay upright when rolling very slowly.
  7. Sep 26, 2012 #6
    Gyroscopic reactions alone can't stabilize a byclicle, or a rolling disk. What must happen is that if the bycicle/disk starts falling over to the right, it will turn to the right, so the contact point with the ground will move under the center of gravity again to stop the falling.

    For a rolling disk, this can only be done with gyroscopic forces, but for a bicycle with cyclist, the trail and the steering inputs of the drivier are more important.
  8. Sep 26, 2012 #7
    I can say from empirical observation that a bicycle has an incredible ability to stay upright. I was getting a ride home from a coworker one evening, when a female college student from the University of Maryland attempted to ride with the flow of traffic on a highway with an exit ramp. For some incomprehensible reason, the driver did not notice her. He actually hit her, but she remained upright. I could tell by the skid marks that angular momentum had kept her upright.

    Strange the applications of physics that are apparent to the learned manual laborer.
  9. Sep 27, 2012 #8
    I tend to think in this way:
    Because the wheel has an angular momentum perpendicular to its surface. If the bicycle leans, that is, the direction of the vector of angular momentum changes(assuming the turning speed remains constant), hence the difference in vectors, which is, under small angles, approximately dL = L dθ. Considering dL/dt is the torque, we can see the torque required to bring about such leaning is proportional to the angular momentum.
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