Also known as wind resistance, aerodynamic drag is the retarding force exerted by the atmosphere on the bicycle and rider. This is the aerodynamic resistance which must be overcome using the rider’s power (watts).
The aerodynamic drag force, FD, is modeled using the general formula for fluid dynamic forces, where ρ is the air density and vw is the apparent wind speed relative to the cyclist. The remaining parameter, CDA, is universally referred to as the drag area. This term reflects the aerodynamic efficiency of the cyclist and his equipment, which is dependent upon riding position and wind direction.
FD = 0,5 ⋅ ρ ⋅ CDA⋅ (v + vw)2
Note that the drag force increases with the square of the apparent wind speed and that the power needed to overcome aerodynamic drag, Paero, is equal to the drag force multiplied by cycling speed. Therefore, in conditions of zero wind, the watts expended by the cyclist to overcome aerodynamic drag increase with the cube of riding speed! Thus positioning and equipment optimized for a low CDA are critical for economizing the power needed to sustain high cycling speeds.