I looked in a book by Fred Puhn: How To Make Your Car Handle (Pro methods for improved handling, safety and performance. How to set up chassis for highway, road course, drag or oval track. Practicle data, formulas and instructions).
On page 173 of the book, he talks about a rear spoiler (the spoiler is usually mounted low on the body behind the rear window). The air at this point is totally separted from the body surface and is extremely turbulent. A spoiler requires reasonably smooth air flow to work effectively.
Many rear spoilers on production cars are for looks only. If you want a rear spoiler to work on the road it must be quite large, and even then it won't do much for you at legal speeds!
Doug
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Doug
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