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Aero Drag on Motorcycles


Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

Imagine air impinging perpendicular to a flat plate at velocity “V.” It doesn’t matter whether it’s the air moving or the plate, but rather the relative velocity of the two. As the oncoming air stops against the flat surface, the kinetic energy it has is converted into pressure energy. That pressure is called “Q”—the dynamic pressure. It is, as you’d expect, proportional to the density of the air (engineers call that “rho” because using Greek letters makes their calculations look cool). It’s also proportional to the square of the velocity, V. So Q is equal to 1/2 rho times V squared.

I save time by looking it up on a table I keep handy. If a MotoGP bike in the recent Sepang preseason test moves at 335 kph (that’s about 208 mph) that’s 305 feet per second. Here on my table it says Q will be equal to 0.052 times the local still air pressure: roughly 5 percent. Sea level pressure is nominally 14.7 pounds per square inch, so Q acts on each square inch as an extra 14.7 x 0.052 = 0.764 lb./sq. in. Multiplying times the 144 square inches…

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