n How is wind resistance calculated, for instance for vehicles?
Page 63
If you've noticed an error in this article please click here to report it so we can fix it.
& In theory wind resistance increases as the square of the road speed of the vehicle ind "other things being equal" is proportional o the frontal area of the vehicle. In practice, iowever, wind resistance at higher speeds is function of the streamline characteristic of he main structure of the vehicle to a very naterial extent and—if there are protuberances —of the components that project into the air tream.
The partial vacuum created at the rear of he structure and at the rear of each mojection lay, in total, create a higher retarding force han the pressure on the frontal area, and at given speed this varies with the length of le structure or component. Thus, the higher le air speed the greater is the length of the ideal" body.
A platform body with a high headboard may ffer a greater resistance to the wind force then unloaded than a van with the same .ontal area, because the van body provides degree of streamlining.
Making an accurate assessment of wind Isistance is therefore impossible, by calculaon, in the case of a commercial vehicle. Tests I a wind tunnel provide the only means of Ding so and the Motor Industry Research ssociation has done just this in a series of tsts on smaller vehicles.
Manufacturers base their calculations on npirical formulae, and because the shape of ,e vehicle is largely determined by the neccesty to provide optimum box dimensions or atform area there is little they can do to reduce wind resistance. Obviously, however, the overall top-gear ratio should match the shape of the vehicle as well as the average running conditions. A ratio that is suitable, for example, for a trunking vehicle of the platform type may be too high for a large van based on the same chassis and carrying the same load.