| When I raced a derby car, I was told that adding
a fender for the front side of the rolling wheels
would deflect the oncoming air (from the car's
point of view) allowing the car to go faster.
However, when I recently saw a formula for drag, I
noticed a "speed of the media" component.
Thinking through the speed of the wheel from the
body point of view, the forward surface of the
wheel above the axle is rotating into the oncoming
air at faster than the forward linear motion of
the car body, and the forward surface of the wheel
below the axle is traveling forward at a slower
rate than the body of the car (the tread touches
the track). This leads me to ask: Are fenders
only aerodynamically effective for shielding the
forward surface of the upper half of the rolling
wheel, and counterproductive for shielding the
front surface of the lower half of the rolling wheel?|
Fenders prevent air flow in and out underneath
the car. The drag happens because the air is
interacting with (and exerting lift on) the body
of the car itself. You are correct in that the
top of the wheel is moving faster (exactly twice
as fast) as the car at large, and the bottom of
the wheel is not actually moving at all. Fenders
prevent any air from dragging on the car
underneath, however, so in all they help the
car resist drag.
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