The Cx coefficient is more and more often provided by designers of new cars, to emphasize how streamlined they are. Cx is dimensionless, its value in new, aerodynamic cars is below 0,30.
It must be emphasized, that air resistance has a decisive influence on the kinematics of the car, it also affects fuel consumption, to the level of noise generated while driving the car. How much air resistance is there when the car is moving?, can be determined by sticking an open hand through the opened glass. We will feel a lot of resistance then. And the frontal area of a passenger car is approximately two square meters. So the air resistance is very high, it takes a lot of energy to overcome it. Therefore, designers try to give car bodies such shapes, that the Cx coefficient is as low as possible.
– Whether Cx depends solely on the shape of the body?
– Not only. The shape of the car is decisive, but Cx is also dependent on the square of the velocity and on the air – its density, viscosity. But the shape of the car's body has a major influence.
– How Cx is calculated?
– The pattern is very complicated, it would take too much space to cover it in detail. So it is enough to say, that air resistance, the Cx coefficient is tested in wind tunnels. We are dealing with a reversal of the situation there: the car is stationary, and the blown air flows over him. Often, to show the shape of the stream visually, it is colored. Then you can see, like the air flows around the car, where turbulence arises, etc..
The German designers of the new Mercedes E-Class model have come up with an original method of determining the aerodynamic shape of the body. They did not test in the tunnel, but in a special pool of water. The water flowed around the scale model of a Mercedes 1:5.