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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?
Answer 1:

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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