14 Vehicle body aerodynamics

The constant need for better fuel economy, greater vehicle performance, reduction in wind noise level and improved road holding and stability for a vehicle on the move, has prompted vehicle manufacturers to investigate the nature of air resistance or drag for different body shapes under various operating conditions. Aerodynamics is the study of a solid body moving through the atmosphere and the interaction which takes place between the body surfaces and the surrounding air with varying relative speeds and wind direction. Aerodynamic drag is usually insignificant at low vehicle speed but the magnitude of air resistance becomes considerable with rising speed. This can be seen in Fig. 14.1 which compares the aerodynamic drag forces of a poorly streamlined, and a very highly streamlined medium sized car against its constant rolling resistance over a typical speed range. A vehicle with a high drag resistance tends only marginally to hinder its acceleration but it does inhibit its maximum speed and increases the fuel consumption with increasing speed.

Body styling has to accommodate passengers and luggage space, the functional power train, steering, suspension and wheels etc. thus vehicle design will conflict with minimizing the body surface drag so that the body shape finally accepted is nearly always a compromise.

An appreciation of the fundamentals of aerodynamics and the methods used to counteract high air resistance for both cars and commercial vehicles will now be explained.

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