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An SAE paper is available for sale online for $16. There's lots more said in the paper that I won't post.
Morelli's newer paper 2000-01-0491: a new aerodynamic approach to advanced automobile basic shapes. There's so much information, that I have to break it up into several posts.
"tests in scaled down models gave, in the best cases: Cx ~ 0.15.
Taking into account that the frontal area of a wheel Sw, is approx. one twentieth that of the body, and assuming Cxw ~ 0.5 the drag coefficient of an isolated wheel, the interference coefficient, CxI, results:
CxI = Cx - CxB - Cxw (Sw/S)*(4/3)
= 0.15 - 0.07 0.5*(1/20)*(4/3) = 0.047
The factor 4/3 stays for the number of wheels (4) multiplied by the area ratio of the wheel not covered by the body (1/3)."
Morelli doesn't define CxB, so I'm unsure of its relevance. My main interest is that Cxw can estimate drag due to wheels. Instead of the 1/20 assumption, it should generally be Afw/Af (the ratio between wheel frontal area and total frontal area). the 1/3 term should be Afw/Af. Since the tire area is width times ground clearance; hG*tire width/Afw.
i.e. 0.5*Afw/Af*hG*tire width/Afw
this simplifies to 0.5*hG*tire width/Af.
To minimize wheel drag, you need to have minimum ground clearance, minimum tire width. Although a larger frontal area minimizes the wheel drag contribution, it hurts overall drag. Wheel fairings hide the tires, without increasing overall frontal area. Lowering ground clearance also affects drag in mysterious ways.
Morelli also cites a paper which analyzed centrifugal fans on the rear wheels. They saw a drag reduction of 18-20% - substantially higher than simple wheel fairings.
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