Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Riding a bike on ice

  1. Dec 21, 2009 #1
    If you're riding a bike on ice with no friction and the wheels are rotating just as fast as they normally would when you're riding, will you fall down? (Since you're riding on frictionless ice you have would no linear momentum)

    I think that, due to conservation of angular momentum, the spinning of the wheels will keep you from falling down.

    However, since the ice is frictionless, the slightest imbalance in your seating will create a torque which tips over the bike. But does conservation of angular momentum counter-act even this??

    Actually, I'm not really sure and I would appreciate some more knowledgeable opinions. Perhaps two cases could be considered:
    1. Everywhere is frictionless
    2. Only the surface directly under the wheels is frictionless, while there is gravel on either side of the wheels.

    thanks :)
  2. jcsd
  3. Dec 21, 2009 #2
    I have tried to give an explanation of what I think will happen in each situation:

    You have cases where a gyro is fastened to a stand or a string, and the angular momentum will keep it from falling even when placed at an angle. I think we can draw a parallel to this situation and claim that the angular momentum of the wheels might provide extra support.

    In situation 1:

    When you are travelling with a velocity (and friction), and tou start to tip over, the front wheel of the bike will try to turn the same way you are tipping, causing the bike to turn and, depending in the speed of the bike and the "tip angle", balance itself. In the absence of frinction and velocity, I think that if you tip over, the wheel will turn, but it can not balance the bike. Instead, conservation of momentum says that the bike will be pushed the opposite way you are tipping, and you will fall to the ground.

    In situation 2:

    As in situation 1, as you start to tip over, the front wheel will turn. Consequently, it will touch the gravel, wheere there is friction. If you are constantly pedaling, this might be just what you need for the bike to balance yourself if you give it enough momentum. If you are not pedaling, and the wheel is not spinning at relativistic speeds (hyperbole), I doubt is has enough angular momentum to provide the acceleration itself, and you will fall to the ground.

    These are my thoughts on the two situations.
  4. Dec 21, 2009 #3


    User Avatar
    Gold Member

    I have ridden a bike on both smooth and rough ice. On smooth ice with out any special tires, say ones with metal studs, you are going to fall. Its real easy.
  5. Dec 21, 2009 #4
    Why are bike tires have so much bumps and textures? The answer is because these tires need to "grip" the ground the same way bumps on a basketball "grips" our hands. When the contact region between the tires and the ground is rough, the rubber "fills" the small holes in the ground, thus, when it rolls, the force between the rotating tires and the small bumps and crevices on the rough surface will move the bike forward.

    In ice, it is not bumped(mostly) so that you will not be able to propel the bike, so theoretically, the wheels will roll even though the bike wont. However, practically, this will not be true since the heat of the rubber will deform the ice enough to make it tractable.
  6. Dec 21, 2009 #5
    I think the torque generated by the wheels is much too small to keep you upright. The wheels are only a small part of the weight of the bicycle. bicycles don't flip over because if you start falling to the right, you steer to the right and to get the contact point with the earth back under the center of mass. This even happens by itself if you don't steer. This won't work if there is 0 friction. Look at what happens on real ice once the wheels start slipping.
    It might work with a loose rolling wheel.

    If you start falling to the right a torque will be produced that rotates the bicycle to the right around a vertical axis. If the rotation was fast enough it would in turn produce a torque that kept the bike upright. Theoretically it might be possible to balance a bicycle in this way by rotating the front wheel to the right by a large amount as soon as the bicycle started to fall over to the right.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook