How come you have more stability riding a bike than standing still with a bike?

  1. Femme_physics

    Femme_physics 2,555
    Gold Member

    My friend asked me that this morning but I couldn't think of the answer. I know there's probably some major physical law behind it, but so many to choose from! Can you guys help me clarify this issue for myself and my friend?


    (when I say "riding a bike" - I mean riding on a bike. When I say "standing still with a bike", I mean sitting still on a bike)...
     
  2. jcsd
  3. K^2

    K^2 2,470
    Science Advisor

    Two effects. Main one is the fact that the fork is angled, so the front wheel turns in the direction you lean in. That makes you go along a curve, and that causes centrifugal force to push you straight again.

    Second one is gyroscopic effect on the wheel. This one is significantly stronger on a motorcycle, but it contributes to the bicycle's stability as well.
     
  4. K^2

    K^2 2,470
    Science Advisor

    Keep in mind that in order to lean, you usually use a counter-turn first. Especially on a motorcycle. That's not always explained very well.
     
  5. Cleonis

    Cleonis 691
    Gold Member

    To be in balance the center of gravity of you and the bicycle combined must be straight above the contact patches of the wheels.

    If you are standing still, and you start to fall over, then you are forced to put your foot on the ground.

    When you are moving, and you feel you are in danger of falling over to the left, you steer ever so slightly to the left, thus moving the contact patches of the wheels underneath you again. (In actual bicycle riding the contact patches are only on average right underneath the center of gravity.)

    As K^2 pointed out, in order to corner to the right you initiate the cornering by turning the handle bars slightly to the left - just for a fraction of a second. That moves the contact patches away from right underneath you. As the bicycle start to lean you follow that with the handlebars.

    A well manufactured bicycle has a natural tendency to turn in the direction in which it is leaning. That is what makes bicycle riding feel natural and intuitive


    By the way:
    I think this countersteering is a striking example of subconscious learning. To corner means you have to countersteer. (Well, in the case of a very wide corner the countersteering is so subtle that it drowns in the random wriggling of cycling.)
    Most people are unaware they are using countersteering. The skill has been acquired through trial and error, with no conscious perception of what is going on.
     
  6. Try a bike with opposite steering, i.e. one of those clown bikes. They are difficult I can tell you!

    Even worse when you have to pedal backwards to get forward...

    [​IMG]
     
  7. Femme_physics

    Femme_physics 2,555
    Gold Member

    So, how come when I'm standing and I feel the danger of falling down, I can't just steer to the opposite direction and stay still? It still doesn't explain how motion is different. I supposed though from what I understand that momentum reduces weight...
     
  8. K^2

    K^2 2,470
    Science Advisor

    That's because when you steer, even when you are standing in place, it results in shifting the frame, and therefore, the position of CoM relative to contact patches. This effect is fairly small, however, so you have to be almost perfectly balanced, and only make small corrections. If you actually start to fall over, you cannot stop it without moving forward or using your foot.

    Don't need a clown bike for that. Just grab handle bars on ordinary bike cross-hand and try not to hurt yourself when you fall.
     
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