Car Aerodynamics

Car aerodynamics deals with all processes which are observed as air flows through and around a vehicle.

Air resistance W = c_w . A v². Q/2

where c_w = drag coefficient,
A cross-sectional area,
v driving speed
Q air density.

The factors to be influenced by the car manufacturer are the drag coefficient c_w as a measure of car aerodynamics quality of the vehicle shape, and the projected vehicle cross-sectional area A.

Table 1. c_w values for various vehicles

Attempts to reduce the c_w value by adding spoilers, underbody panels, etc. are not practical, since their effect depends upon the aerodynamic characteristics of the basic vehicle body (see Table 2 for effects of individual modifications on drag coefficient). These types of modifications in optimized form are standard features of modern car design. This means that the fitting of supplementary devices to further improve the c_w value and the axle lifting-force coefficient can only result in very minor improvements. Often, such modifications result in the impairment of the vehicle's day-to-day capabilities due to reduced overhang angle or reduced ground clearance. In other words, such add-on parts serve more for improving the vehicle's appearance than for improving its performance.

The c_w value can be influenced by individual aerodynamic or design measures. Airflow through the vehicle as well as roof-mounted fixtures will always increase the c_w value. Examples (- = better, + = worse). Examples are given in Table 2 on the next page.

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