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Old 02-14-2008, 06:24 PM   #25 (permalink)
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Your usual street car spoiler should have an impact of roughly +-0.01 on the CD depending on how it's designed. The error in a coast down test is bigger than that so there's no way you'll really know.

Spoilers work by increasing the pressure between the back of the roof and the spoiler itself, so there is less of a low pressure zone at the bottom of the winshield. If the slope from the back of the roof to the back edge of the trunk is more than 15 degrees, you can benefit from a properly designed spoiler as long as the slope from the back of the roof to the top of the spoiler is around 12.5 degrees, or as close to it as possible. Also, if the air has someplace to go below the spoiler the aerodynamic purpose is somewhat defeated.

In a way it works a bit like a tailgate on a truck.

Here's a good example of a working spoiler
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Last edited by tasdrouille; 02-14-2008 at 06:41 PM..
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