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Bicycle Physics

  1. Sep 2, 2012 #1
    Hi guys,

    Why does a Bicycle stay upright ?
    (It's not because of the gyroscopic forces applied by the wheels as shown by http://ruina.tam.cornell.edu/resear...ics/stablebicycle/StableBicyclev34Revised.pdf)

    In short this bicycle has wheels of the same size rotating in the opposite direction thus countering the gyroscopic force but it still manages to stay upright.
  2. jcsd
  3. Sep 3, 2012 #2


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    There is more than one way this can be accomplished, but what the methods have in common is to steer the front wheel into the direction of lean sufficiently to correct the lean and return the bicycle back to vertical orientation (although the direction of the bike will have changed).

    For a normal bicycle, this is done with "trail", the point of contact with the ground is behind the point where the pivot (steering) axis of the front wheel intercepts the ground. This results an inwards steering torque due to the upwards force from the pavement being applied "behind" the extended pivot axis.

    Some research type bicycles don't use any "trail", and instead use a mass mounted above and in front of the front tire, so that the center of mass is in front of the pivot axis and above the rotational axis, which again results in the front tire steering into the direction of lean, but I don't know the details involved with this type of design.
  4. Sep 3, 2012 #3
    I'm sorry if I wasn't clear, I already have a similar understanding of how it turns, my question is how it tends to stay upright when it's travelling in a straight line ( all the other sites say it's because of the gyroscopic effect which has now been proven wrong by http://ruina.tam.cornell.edu/resear...ics/stablebicycle/StableBicyclev34Revised.pdf )

  5. Sep 3, 2012 #4


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    To stay upright it makes slight turns.

    Note that the gyroscopic effect is not supposed to keep the bike itself upright. Is just steers the wheel into the direction the bike falls over, so the bike straightens up again. The trail effect does the same. This wasn't proven wrong for normal bikes. It was just shown that the same steering effect can be achieved by other means, without trail and without gyroscopic effect.

    Check out this thread where Andy Ruina, one of the authors, commented himself:
    Last edited: Sep 3, 2012
  6. Sep 7, 2012 #5
    Thanks, that video was really helpful.
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