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Thread: Mileage or Make?

  1. #11
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    But the part in contact with the ground MUST be stationary (more or less). All else follows.

  2. #12
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    That cannot be correct, surely?

    The wheel is "rotating" at a certain number of revolutions per minute.

  3. #13
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    The 'wheel' is rotating, but a point on the circumference of the wheel describes a series of semi-circles with twice the diamete of the wheel.
    http://en.wikipedia.org/wiki/Trochoid

  4. #14
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    Quote Originally Posted by Santa View Post
    And the thing that always amazes me when I think about it; the thing that is hard to get your brain around, is that any part of the tread on a tyre, on a car doing 70 mph, is accelerating from zero to 140 mph and back to zero between 800 and 900 times a minute. Even more with a small tyre.
    I believe your above theory is flawed somewhat. What difference does the size of the tyre make? For example, on a tractor with small front wheels and large back tyres, can you explain the difference in "speed" of different areas on the front and back tyres, OR are you as stumped as the rest of us?

  5. #15
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    Quote Originally Posted by Santa View Post
    But the part in contact with the ground MUST be stationary (more or less). All else follows.

    ONLY when the vehicle is stationary, surely?

  6. #16
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    Quote Originally Posted by Dennis W View Post
    I believe your above theory is flawed somewhat. What difference does the size of the tyre make? For example, on a tractor with small front wheels and large back tyres, can you explain the difference in "speed" of different areas on the front and back tyres, OR are you as stumped as the rest of us?
    Small tyres rotate faster than large ones at the same speed. Your tractor wheels would be rotating at different speeds, whatever the speed of the tractor.

    Quote Originally Posted by Dennis W View Post
    ONLY when the vehicle is stationary, surely?
    This is surely self-evident. The road is stationary, so any part of a tyre in contact with it must also be stationary - if that is not the case, then the car is skidding.

    Have you ever put a plank on a roller? The ground remains still, the roller moves at speed n, and the plank moves at speed n x 2. This is primary school physics.
    Last edited by Santa; 17-06-13 at 12:12.

  7. #17
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    Quote Originally Posted by Santa View Post
    Small tyres rotate faster than large ones at the same speed. Your tractor wheels would be rotating at different speeds, whatever the speed of the tractor.


    This is surely self-evident. The road is stationary, so any part of a tyre in contact with it must also be stationary - if that is not the case, then the car is skidding.

    I think that you have confused yourself somehow. IF the wheels are stationary then the vehicle is stationary. But if the wheels are revolving then usually the vehicle is moving.

  8. #18
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    Quote Originally Posted by Santa View Post
    The 'wheel' is rotating, but a point on the circumference of the wheel describes a series of semi-circles with twice the diamete of the wheel.
    http://en.wikipedia.org/wiki/Trochoid
    If you excuse the pun, I think you have gone off on a tangent. I cannot see how "trochoids" relate to vehicle wheels.

  9. #19
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    Oh - one last try:
    Imagine a wheel standing still. It supports a car at the centre via an axle. Nothing is moving. If you made a mark on the outside edge of the wheel and jacked the car up, the mark would move in a circle.

    Now lower the car to the road and move it forwards; what happens? The axle moves forward and the wheel rotates. The mark, however, would not move in a circle; it would move in an arc - like the cycloid in the diagram (http://en.wikipedia.org/wiki/Cycloid). In one full rotation, the axle would move forward by exactly half the circumference of the wheel, but the mark would have moved the full circumference. ie - twice as far.

    Since the point in contact with the road MUST not be moving - it follows that the point opposite, at the top of the wheel MUST move at twice the speed.
    Last edited by Santa; 17-06-13 at 16:06.

  10. #20
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    Quote Originally Posted by Santa View Post
    And the thing that always amazes me when I think about it; the thing that is hard to get your brain around, is that any part of the tread on a tyre, on a car doing 70 mph, is accelerating from zero to 140 mph and back to zero between 800 and 900 times a minute. Even more with a small tyre.
    I think that you have got some of the units of measurement wrong in your example. How about using revolutions per minute instead?

    I believe that all parts on the outer edge of a tyre will be rotating at the same speed at any particular instant. Surely that is self evident?
    Last edited by Dennis W; 17-06-13 at 18:15.

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