Lesson 1/Learning Event 1
When the brakes are applied, the wheel will either roll or skid,
depending on which is greater, the friction between the braking
surfaces or between the wheel and the road. Maximum retardation
(slowing down) is reached when friction between the brake surfaces is
just enough to almost lock the wheel. At this time, friction between
the brake surfaces and wheel and road are almost the same. This is
all the friction that can be used in retarding (slowing down) the
motion of the vehicle. The amount of friction between the road and
the wheel is what limits braking. Should friction between the
braking surfaces go beyond this, the braking surfaces will lock and
the wheels will skid.
When a wheel rolls along a road, there is no movement between
(relative motion) the wheel and road at the point where the wheel
touches the road. This is because the wheel rolls on the road
surface; but, when a wheel skids, it slides over the surface of the
road, and there is relative motion because the wheel is not turning
while moving over the road. When a wheel skids, friction is reduced,
which decreases the braking effect. However, brakes are made so that
the vehicle operator is able to lock the wheels if enough force to
the brake lever or pedal is applied.
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