Aerodynamic drag is the major factor affecting a bike and rider - it can account for up to 90% of the overall resistance that a rider must overcome. There are several types of drag relevant for us. First, pressure drag. As a body (in this case a bike and rider) passes through the air, it forces the air molecules to move out of the way in order to pass through them. These molecules push back on the body, creating pressure. The component of this pressure that faces aft (to the rear of the body) is called pressure drag.

Secondly, there is friction drag. Air, like all fluids, has viscosity (or "thickness"). The air molecules that come into contact with the body stick to its surface and stay stationary in relation to the body. As the body continues through the air, other air molecules pass by the stuck molecules as they flow around the body in layers, following parallel paths. This is the laminar boundary layer. The viscous nature of air creates a shear force, or friction drag.

At some point on nearly all bodies, laminar flow cannot be maintained and the air molecules tumble and mix instead of flowing smoothly. The transition point is where this turbulent boundary layer begins. This behaviour of flow is related to a parameter called the Reynolds number, which is determined by several physical characteristics of the flow. The laminar flow regime exists up to Reynolds numbers of around 10,000. Beyond Reynolds numbers of 10,000, the flow transitions to “turbulent” flow, as shown in the figure below.