Auto Racing: basic concepts for rear spoiler, rear fender flares, rear spoiler

Question
I have a rear spoiler mounted on my race car, set at about 14 degrees.  The height of the spoiler is about even with the roof of the car, width is even over the rear fender flares, depth of the spoiler is 8". Increasing the angle and/or adding a rear edge lip does the same thing correct-- at what point does the benefit of adding lip height stop? Are there individual benefits of doing one over the other?  Higher spoiler angle helps or hurts me in straights or turns?  Thanks

Answer
A rear spoiler's main purpose is to provide downforce on the rear of the vehicle. maximum achieved speed has a major role in the attack angle of the spoiler. You must find the balance between traction and aerodynamic drag. If the angle is too steep you will increase the drag and use more horsepower to move the vehicle through the air.
Without wind tunnel development its basically trial and error. For road racing I would use a front air dam set close to the track surface and a rear spoiler that can be adjusted for angle. Make sure the rar spoiler has vertical spill plates on each end to prevent air from rolling over the sides of the spoiler. A rear mounted camera with some strategically placed pieces of ribbon can help you determine the effectiveness of the angle.

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