Most wind tunnels are built as a rectangular hallway in which air blows through a flow straightener and nozzle from the front of the car, the air will be driven to the rear wall of the tunnel. The speed of the air flow in wind tunnels can reach 300 mph.
To carry out the measurement process, the car is kept on a rotating platform to rotate the car and put the nozzle in the right way with the aim of simulating side wind. Gieratoria platform in a treadmill underneath the car starts to simulate the movement of the wheels and the driving surface. Furthermore, a scale is installed on the floor of the measurement point. It measures all forces and impulses experienced by the car because of the wind, such as the elevation of the induced drag and the lateral and longitudinal forces. Using these values the Cd (value representing the force of friction on a car caused by air and consists of different forces) value is calculated.