For the most part, spoilers are an aesthetic add-on that can make a car look sportier. On sports cars, spoilers are, in fact, functional because they add stability at high speeds. Spoilers use air moving over the car to put push down on the front or rear. This forces the tires to the road and improves traction and stability.
Spoilers can also be used to improve a car’s aerodynamics and gas mileage. On the 2009 Ford Escape, Ford reports there’s a .75 mpg improvement at 70 mph with a new front spoiler and rear tire spoiler.
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That depends heavily on the individual spoiler. The older Audi TT's for example, have spoilers that are functional at rather low speeds, since the car is shaped like a wing otherwise. Also, if you look at footage of the first spoiler introduced in NASCAR (I think this was in the 60's or 70's) the difference is dramatic. The speeds weren't even all that high. I would think that the larger the spoiler, the more profound the difference, and the more difference it would make at lower speeds. I know of ultralights that can take off at 35 mph, though, so If that is enough for their airfoils to become effective, it stands to reason that even at very low speeds (25-35 MPH) you'd still see a difference; although, you usually aren't driving close to the limit at those speeds, so you wouldn't be actually taking advantage of the increased downforce.
blake,
any idea what speed would make a difference for a wrx? picture of the rear spoiler is below
http://www.flickr.com/photos/ironhide/210621036/
thanks
How about the spoilers on front drive cars? Do they take off traction from the front wheels?
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Joe,
You wrote "On sports cars, spoilers are, in fact, functional because they add stability at high speeds."
For the rear spoiler, what is a high speed that would have any appreciable effect? 60mph? 90mph?