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The diagram
illustrates a rigid wheel rolling on soft ground. A depression is made
in the ground of width, b, and the effective point of the wheel that is at rest
with respect to the ground is a distance of about (b/8) in front of the centerline
of the wheel. This position is the "Instantaneous center of rotation,"
and the vertical reaction force between the ground and the wheel,
W, acts at this point. This force produces a torque of magnitude (Wb/8)
about the wheel center in a counterclockwise direction opposing the wheel rotation.
This is the source of the second component of rolling resistance. It
is relatively unimportant on a normal road but may dominate the rolling losses
under off-road conditions.
Under most circumstances the two sources
discussed dominate the rolling friction. At higher wheel speeds, aerodynamic
forces may also start to contribute. |
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