[mike leech]

Originally Posted by Tom Vaught

Good question.

1) You would like to keep the air out of the underneath of the car. Not having the air bang around on all of the suspension components and drive line stuff. The only way to do that is to minimize the air that gets under the car.

2) Air also can get under the car from the wheel openings. Most good front air dams keep the air from entering the wheel houses from the side.

3) So now if you limit the amount of air coming thru the grill to the minimum required for cooling (more road race vs drag race) you can reduce the air flow under the car quite a bit.

4) Air, by the time it gets to the rear edge of the door is starting to get dirty so having small deflectors at the front of the rear wheel openings might help on a street car or car with a very shallow tire/wheel depth. A Drag car has the tire sunk into the body pretty well so minimal rear tire impact contacting the air from the sides really.

5) Transitions from things like the windshield or doors to the body can make a difference in how smooth the car is going thru the air.

So after saying all of that, if the rear of the car has more of a rake vs the front then, yes, so dirty air can eventually get under the car but the big deal is not allowing the air to get under the front of the car.

Where does the cooling air from the radiator go, typically under the car, unless you have a Trans Am type side extractor set-up (which actually worked pretty well). Less air getting into the engine compartment, less air banging around on the firewall trying to act like a parachute that slows the car down.

So the front of the vehicle is where the measurements need to be taken. Measure at the lowest point of the spoiler to the ground (if it has one). You can document the rake angle, (which we do) but the effect is minimal in normal testing. I built screw jackers to raise and the rear of the vehicle quickly, when it went into the wind tunnel. 4" difference in high rake vs low rake showed an effect but a small one vs the front height change of the vehicle.

So a) measure from the spoiler to the ground and that area you subtract from the area calculated in the measurement from the ground to the top of the windshield. Also add the tire frontal area (footprint of the tire when looking at the tire at the spoiler height). Then do the math in the formula.


Hope that helps with your question.

Tom V.

I know for a fact that removing the factory wheel wells from a early camaro/firebird will cause a low 12 car to lose 2 mph. et was about the same, a touch slower. air hitting the fire wall is a bad deal and care should be taken to limit introduction as well as relieving any built up pressure under the hood. these would have an effect on how the car moves through the air as well