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Bag-o-Dimples

Aftermarket car dimpling kit
 
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It’s widely believed that adding dimples or bumps to certain parts of a car will decrease aerodynamic drag. That may not be true, but it’s a neat theory, and it would look interesting – especially on your souped-up Honda.

Instead of taking a ball peen to your beloved vehicle, I recommend using an aftermarket car dimpling kit, available at the same place where you buy racing stripes and window decals. They’re not really dimples; they’re hemispherical bumps of clear plastic, one centimeter in diameter, slightly flexible, with a strong adhesive on the flat side. You simply stick them on your car: on the trailing edges, at the front of the hood, and other “problem spots” where you’d like to “reduce drag.”

AO, Dec 12 2003

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       Similar to a golf ball, it might be better if they were concave instead of convex. I'm not sure how that would work exactly.
kevinthenerd, May 31 2006
  

       these are also refered to as "vortex generators" the idea is that by creating drag right before abrupt curves, the air is slowed to better fill the vacume created by the curves, thus reducing vacume drag. so its best to put these lumps or dimples (doesnt matter since both create drag) about 2-3 inches before an abrupt curve, the most popular place to put vortex generators is about an inch before the back window drops off, and near the tail lights. the vorticees created by dragg amazingly cling to the surface of a car. filling even the toughest of vacume bubbles.
chickenninja, Aug 29 2007
  

       sp. vacuum, vortices
pertinax, Aug 30 2007
  

       The dimples on a golf ball are to create turbulence so the ball does not 'fly' (although it still will with enough back spin).   

       On a car, you need the opposite (ish), you want to direct air uniformly around the body. The coanda effect will help you out by keeping a moving airflow attached to the body. The 2 major areas you can effect low pressure drag are above and below the back of the car. In these areas, air will naturally fall off the sides as the body pushes the air up and down. You can decrease this by putting small fins on the back of the roof and under the rear bumper.
marklar, Aug 30 2007
  

       [marklar] Wouldn't that also move the centre of pressure forward, making the car less stable in a side wind?
Cosh i Pi, Aug 30 2007
  

       I hadn't considered that but I think it would increase the momentum of the body + trailing air in the direction of travel, increasing inertia against sideways movement. So only a slight decrease in cornering / handbrake turns.   

       Examples of this in use: Top - Mitsubishi Evo IX (I think); Bottom - Lumina SS, Ferrari 360 (with venturi); Both - Airbus A330.
marklar, Aug 30 2007
  
      
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