Air Drag Coefficients and Frontal Area Calculation

The use of coefficient of drag and frontal area are common when determining horsepower requirements for vehicles in motion. One source of these figures are past issues of Car and Driver or Road and Track magazines. These values can also be determined using the following (obtained from "The ACCELERATOR Slide Rule"):

Coefficient of Drag

The aerodynamic "features" of a vehicle in motion are reflected in its drag coefficient values. Low coefficients indicate low air resistance. The following chart list some ranges for various vehicles, which will suffice when actual measured values are not available: 

     Vehicle           Drag Coefficient (cd)
     Description       Low     Medium   High
     ----------------------------------------
     Experimental      0.17    0.21     0.23
     Sports            0.27    0.31     0.38
     Performance       0.32    0.34     0.38
     60's Muscle       0.38    0.44     0.50
     Sedan             0.34    0.39     0.50
     Motorcycle        0.50    0.90     1.00
     Truck             0.60    0.90     1.00
     Tractor-Trailer   0.60    0.77     1.20

Calculation of Frontal Area

Frontal area (farea) represents the front projection area of the vehicle. If one takes a picture of a vehicle, it is the area included in the outline. Use the following to calculate:

1. Calculate the area of a rectangle which would encompass the front of the vehicle (multiply width by the height). For motorcycles, use the handlebar width (to a maximum width of 30 inches) and a height consisting of seat height plus an estimated "seat to helmet" height.

2. Adjust the figure obtained above for areas not included, such as top rounded corners, etc. Typical adjusting values are 85 percent for cars, 70 percent for motorcycles, and 100 percent for trucks.

Bruce Bowling