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Car Aerodynamics - Hot Rod Magazine
" Aero Stuff That Doesn't Really Matter
In addition to our list of five tricks that almost always work, here are some things A2 customers might want to try that are really a waste of time.
Wax: Contrary to what you may read on your favorite message board, well-waxed, smooth paint is no more aerodynamic than the worst spray-can, flat-black primer job you can imagine.
Golf-ball dimples: They do not work on cars, regardless of the scale of the dimples, unless your car is a 1.68-inch-diameter sphere spinning through the air with no ground plane.
Taping seams: Rarely if ever are body-panel seams so large and misaligned that smoothing them with duct tape will make a measurable difference in Cd. We tried it on our Camaro, and it did nothing.
Smoothing rivets and hood pins: The removal of minor surface burbles, such as rivets (remember the Howard Hughes movie?) and hairpin-type hood pins shows no measurable improvement in Cd.
Dropping the tailgate: On a pickup, lowering the tailgate does not usually reduce drag. If you are racing a truck, know that extended cabs and crew cabs are more aero than regular cabs.
The biggie: windshield rake: According to Eaker, "Here's a myth I can bust. Once the windshield is past 45 degrees of rake-and many stock cars average like 60 degrees-you will not see an improvement from laying it down at an even steeper angle." We proved this on the Camaro, building a hugely sloped "windshield" out of foam core. It did nothing.
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Car Aerodynamics - Hot Rod Magazine