It can also lead to significant side forces if the wind is blowing. The aerodynamic forces on the cyclist and bicycle come primarily from differences in pressure and viscosity. The viscosity of air, the ability of fluid to flow freely, is very small, but it is not insignificant, and is responsible for two types of drag. These types are friction drag, and pressure drag. Frictional drag happens when airflow is in contact and flows over a surface.
Pressure drag depends on the way the rider moves through the air.
The flow left behind in the air is called the wake. If something is streamlined, the air flows smoothly around it. If something has a bluff body, the air breaks away whenever a sharp change in direction occurs. Frictional drag is the largest source of wind resistance for streamlined bodies, and pressure drag is the largest source of wind resistance for bluff bodies. A streamlined body will usually have better aerodynamics than a bluff body.