Aerodynamic drag

The force which opposes the forward movement of an automobile is aerodynamic drag, in which air rubs against the exterior vehicle surfaces and forms disturbances about the body, thereby retarding forward movement. Aerodynamic drag increases with speed; thus if the speed of a vehicle is doubled, the corresponding engine power must be increased by eight times. Engineers


express the magnitude of aerodynamic drag using the drag coefficient Cd. The coefficient expresses the aerodynamic efficiency of the vehicle: the smaller the value of the coefficient, the smaller the aerodynamic drag.
Figure 1.18 illustrates the improvements in aero-dynamic drag coefficient achieved by alterations to the shape of vehicles. Over the years, the value of Cd has been reduced roughly as follows:

1910 0.95 1960 0.40
1920 0.82 1970 0.36
1930 0.56 1980 0.30
1940 0.45 1990 0.22
1950 0.42 1993 0.20

During the wind tunnel test all four wheels of the car rest on floating scales connected to a floor balance, which has a concrete foundation below the main floor area. The vehicle is then subjected to an air stream of up to 112 mile/h; the sensitive balances register the effect of the headwind on the vehicle as it is either pressed down or lifted up from the floor, pushed to the left or right, or rotated about its longitudinal axis. The manner in which the forces affect the vehicle body and the location at which the forces are exerted depends upon the body shape, underbody contours and projecting parts. The fewer disturbances which occur as air moves past the vehicle, the lower its drag. Threads on the vehicle exterior as well as smoke streams indicate the air flow, and enable test engineers to see where disturbance exists and where air flows are interrupted or redirected, and therefore where reshaping of the body is necessary in order to produce better aerodynamics .Car Repair Talk's forum.