Most of them are cosmetic only, and don't really add any downforce to the car. Some, however, such as the Greenwood pieces, and the Guldstrand one, do actually work as they are designed. Both of these types have braces that go from the bottom side of the rear deck(under the spoiler/wing) down to the frame. This is what makes them really work, adding downforce to the chassis, not just flexing the body. I'd have to say this type would be for the hard-core, solo racer/autocross guys.
For just cosmetics, the bolt-on kind is fine.
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Joel Adams
C3VR Lifetime Member #56
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"Money can't buy happiness -- but somehow it's more comforting to cry in a CORVETTE than in a Kia"