A wind tunnel is an apparatus used to determine the complex interactions between a high-speed, velocity-controlled stream of air and the forces exerted on a solid object. The movement of air around an object, whether it is an airplane, bicycle, automobile, or person, is considered aerodynamic flow. When wind tunnel tests are performed on bicycles, they help to optimize the cyclist's position on the bicycle and to improve the aerodynamic design of bicycles, cycling equipment (such as bottles, wheels, helmets, and handlebars), and clothing. Thus, wind tunnels are important tools for maximizing performance in cyclists competing in such international bicycle races as the Tour de France.
The wind tunnel's structure consists of a large chamber where giant motor-driven blades are positioned at one end in order to simulate air flows comparable to actual wind conditions. The floor consists of a huge aerodynamic balance, where the smallest movement imposed on the test object can be measured, recorded, and analyzed by instruments and computers. Such equipment provides a relative measure of aerodynamic efficiency. The path of the air stream around the bicycle and/or cyclist can be studied by generating smoke streams to make the airflow visible, attaching materials (such as ribbons) that flow with the wind's direction, or using optical devices.
Generally, a bicycle is secured on the floor of a wind tunnel with two struts—the front strut is attached to the hub of the front wheel and the rear strut is attached to the bicycle's frame. During the test, the wind speed and direction are changed to provide an assortment of force and moment of inertia data.
Results taken from wind tunnel tests have shown that only a minority of drag (about 30%) comes from the bike itself. The majority of drag (up to 70%) is due to the cyclist, primarily the position of the cyclist; the positioning of the cyclist's hands, forearms, and shoulders; and the amount of time spent to reach the best aerodynamic position. Specifically, tests have shown that for best aerodynamic position the cyclist's thumbs should be placed above the handlebar extensions and parallel with the road; forearms positioned parallel with the road; elbows in front of the knees; shoulders positioned low; and head positioned up and stationary.
Wind tunnel tests also show how to aerodynamically design clothing and equipment such as shoe covers and helmets in order to minimize drag. Such tests show that for the least amount of drag, the air should remain attached to a rider as much as possible, especially on the trailing side of the hips. Based on wind tunnel results, manufacturers use various orientations and types of textures to control airflow over rider wearing skin-tight clothing (commonly called skinsuits). Technology has advanced to the point that many skinsuit fabrics are considered better than shaved skin for aerodynamic flow.