I always seem to be banging my head on a brick wall over this, but what the hell, I'll give it another go.
Cd, the coefficent of drag, is intended to indicate the relative efficiency of a given shape and is a massively abused term. It's derived by measuring the force exerted on an object at a given air speed and dividing by the reference area times air density times half the air velocity squared. Notice that I said 'reference' area and not 'frontal area'. Whilst frontal area is almost universally used for automotive applications 'plan' or 'skin' area is more likely to be used when comparing aerofoil shapes. When used for simple shapes (flat plate, shpere, brick, aerofoil, etc.) in free air it's a term that can be trusted providing you know which reference area was being used. The area of all these shapes is going to be trivial to calculate. When applied to a vehicle operating within inches of the ground it should be treated with a great deal of suspicion. How was the frontal area measured? Working back from published Cd, top speeds, and quoted power for cars, it would appear that gross height by gross width (ignoring mirrors) has been used. In particular this ignores the airflow under the vehicle. Remember that Cd is used largely a marketing tool and that exagerating the frontal area will give a lower Cd figure. Do you really think that a car with all it's intakes, grills, rough underside, gaping wheel arches, door gaps, etc, etc, is as aerodynamic as a bullet (both approx. Cd 0.3)? If you used outline frontal area (quite a bit smaller) then the Cd wouldn't look nearly so good. A rotating wheel causes extra drag and a rotating wheel inside a wheel arch is even worse. Very few vehicles are tested in a moving floor wind tunnel. There is a boundry layer on the floor of the tunnel that should be removed via a slot bofore the air passes over the vehicle. The old UK MIRA tunnel had a boundry layer about a foot thick, so the bottom of the vehicle contributed little towards the drag figure. Manufacturers loved it! Now if you're looking at a motorcycle, it's obvious that using gross width and gross height is going to give a huge error in frontal area and outline area is more likely to have been used. Final Thoughts When we are trying to work out how much power is going to be needed to move a vehice at a given speed, the figure we want is the drag force measured in the wind tunnel before all the calculations were applied to turn it into Cd. If you knew that all vehicles were tested in the same wind tunnel, used outline frontal area, and were tested in their true roadgoing configuration (ride height for example) then Cd would be of some use. The shape of a car isn't that important, what really makes the difference is radiator ducting, flush fitting glass, gutterless roofs, etc. I suggest that you buy or borrow a copy of 'The Leading Edge' which focuses on ultra streamlined vehicles, mostly LSR bicycles and Solar vehicles. Paul Compton www.evguru.co.uk www.batteryvehiclesociety.org.uk www.sciroccoev.co.uk www.morini-mania.co.uk www.compton.vispa.com/the_named
