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Speed and stability

  1. Sep 17, 2006 #1
    I was just wondering how speed can a bike more stable.
    What I mean is at lower speeds it the bike will tend to fall aside if you don't keep your balance, but as the speed increases the more stable it will become. Is this because of the steering, shape of the wheels or is a body actually harder to rotate at higher speeds? I just cannot find an explanation.
  2. jcsd
  3. Sep 17, 2006 #2
    Are you familiar with concepts like torque, angular momentum?
  4. Sep 17, 2006 #3
    It's to do with the inertia of the wheels. The rotation of a massive object, like a wheel, is specific to its spatial orientation - so 'spinning' while oriented north-south is not the 'same' motion as spinning east-west. In essence, a spinning wheel resists changes in its orientation as its rotational momentum is an analogous property to 'linear' momentum. So, the faster a bike travels, the faster its wheels are turning, the more angular momentum and thus rotational inertia they have, so the harder it is to turn them from their current orientation.

    I'm aware the terminology is a little ropey, but that's basically it.

    Historical note - there was once experimentation into cars powered by massive flywheels. The inventor clearly hadn't done his homework, as the prototypes proved extraordinarily difficult to steer due to the massive inertia of the flywheel in their chassis.
  5. Sep 17, 2006 #4
    It's a bit like a spinning top, peter. It won't stand up if it isn't spinning. When the bicycle is moving its wheels are spinning.

    There's also the core skill of learning to ride a bike: if you start to fall over you turn in that direction.
  6. Sep 17, 2006 #5
    Thanks for the explanation Sojourner01. I just didn't know the angular momentum is dependant of the orientation.
  7. Sep 17, 2006 #6


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    Have you read the issue of Physics Today of this month?. There is a great article talking about this.
  8. Sep 17, 2006 #7

    The general answer is "because of inertia".
    Inertia is that property of matter that makes it difficult to change its motion.

    Here, the wheels have a fast rotation.
    The parts of the wheels don't fly apart because of the internal forces that make it a solid.
    These forces are really responsible for the rotational motion of each part of the wheel.
    Without these "solid forces" the parts of the wheel would really fly apart.

    But there are no other forces to counter-act on inertia.
    Therefore, to modify this rotational motion, additional forces are needed.
    This is to say that the wheels make the bike more stable.
    Indeed, additional forces are needed to destabilise it.

    Translate this to analytical mechanics and you get a nice part of physics with famous applications.

    Last edited: Sep 17, 2006
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