Standing by the track, we hear the changes in sound frequency resulting from the Doppler effect. As the bike comes toward us, its sounds take less and less time to reach us, giving each successive pulse a head start over the previous, making the pitch rise. As the bike passes, its pitch drops. The rider hears no such thing, and sound and vibration join into a single experience. Once on a straight and near top speed, the engine sound holds almost constant, like a test on an absorption dynamometer. That is two major forces in balance: the force of aerodynamic drag against the force of the engine, multiplied through the primary gears, the transmission, and the sprockets and chain, then through the spokes and rim to where the tire pushes back against the whole earth. And there is sound.