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Old 11-27-2013, 11:41 PM   #78 (permalink)
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Originally Posted by twinair View Post
My statement isn't about streamlining, it's about the effect of the distance between road surface and bottom of a parallel surface on pure shear forces (and that these shear forces wouldn't be substantial, since it is not practical to lower the car to a point, where these shear forces would start to play a significant role).

Maybe there are other detrimental effects than increased shear forces which may start to play a role when lowering a car.
Can you show any data that shows that lowering a passenger car has increased aerodynamic resistance?
Consider Sighard Hoerner's Fluid Dynamic Drag book, specifically his section on external stores (i.e., bombs and drop tanks) and the interference drag created when two otherwise streamlined shapes operate in close proximity. I think this may be instructive re. your problem regarding shear forces. Basically, the flow past the wing gets tangled up with the flow around the drop tank, increasing drag well beyond the arithmetic sum of the respective bodies. Further, absent an undertray and wheel fairings, the bottom of the car is not nearly as smooth as the underside of a wing or shape of a drop tank, so the car bottom traps air and floats with extra lift. Not good.

FWIW, I suspect the HPV streamliner bikes at Battle Mountain are virtually all running too close to the surface--the air under them has no place to go, so generates more drag than it otherwise would. They too should consider Hoerner, per above.

Back to cars: Surely the most bang for the buck is a $7 Home Depot garden edging air dam, smoothing the airflow laterally past the wheels, which are normally in deep yaw and act like buckets or sea anchors. Push much or most of that air past the wheels in barge-like fashion and get most of the benefit of undertray, wheel fairings, etc. at a tiny fraction the cost of materials and effort, then call it a day.
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