The vehicle frontal area is used to determine the total aerodynamic drag on the vehicle. The frontal area is the true area of the vehicle's silhouette or projection. This can be approximated by measuring the width and height of the vehicle and multiplying to get the total area. Below is a list of several vehicles and their frontal areas for comparison:
Vehicle Frontal Area
GM 1-Ton full size van 3.35 m2
Ford Minivan 3.25 m2
Chrysler Minivan 2.97 m2
Typical U.S. Sedan 2.30 m2
5 passenger Volvo ECC 2.01 m2
1991 Honda Civic DX 1.80 m2
GM Experimental Ultralite 1.71 m2
Renault VESTA II 1.64 m2
The drag coefficient is a dimensionless number related to the drag of the vehicle based on its shape and details. For example, the shape and placement of the outside rear view mirror as well as the angle of the rear glass and deck all affect the drag coefficient. One of the largest, and most forgotten, areas of impact on the coefficient is the bottom of the car. Newer automobiles tend toward lower drag coefficients as illustrated in the list below:
Drag Coefficients Cd for a range of Vehicles
Average 1975 U.S. Estate Wagon 0.60
Average 1979 U.S. Car 0.48
GM 1-Ton full size van 0.47
Average 1987 U.S. Car 0.37
Average 1992 U.S. Car 0.33
Chrysler T-115 1990 Minivan 0.32
1986 Subaru XT Coupe 0.31
Best 1993 U.S. Sedan 0.29
General Motors Electric Impact 0.19
GM Sunraycer Solar Car 0.13
The aerodynamic drag is a function of the frontal area, drag coefficient, air density and vehicle speed.
FA = ½p•A•Cd•v2
where: FA = Air drag in Newtons, (force)
p = Density of air = 1.23 kg/m3
A = Frontal area in square meters
Cd = Drag coefficient
v = velocity in meters/second
As an example, the drag force of the GM impact will be determined for about 60 mph or 100 kph;
A = 1.58 m2 Cd = .19 v = 28 m/s p = 1.23 kg/m3
FA = ½•(1.23 kg/m3)•(1.58 m2)•(.19)•(28 m/s) 2
FA = 144.7 kg•m/s2
= 144.7 N or 32.5 lbf
Notice that the aerodynamic drag goes up with the square of the velocity. Using this formula, a plot can be made of the drag verses velocity for the GM Impact. Use velocity as the independent variable and drag as the dependent variable as shown.