The Wall of Air

A cyclist at 40 km/h pushes through air that resists with a force proportional to the square of their speed. Double your speed and air resistance quadruples. At racing velocities, roughly 90% of a rider's power output goes to overcoming aerodynamic drag.

The drag force depends on three things: air density, the rider's frontal cross-section, and the drag coefficient. A rider can control two of those — body position and equipment. Crouching low on the handlebars reduces frontal area by up to 30 percent.