[blue2000s]

Originally Posted by Lil bastard

Well, I think we're going to have to agree to disagree here.

Think of a car in terms of an airfoil or wing. It splits the air as it passes through it. Some air is forced under the car, while the rest flows over the car. The path the air must take over the car is longer than the air passing beneath and so the air must flow faster over the car than under it. Air flowing faster, drops in pressure.

So, now, you have the air rushing under the car, and because of the lower pressure on top, you actually have a partial vacuum. 'Lift' is actually the difference between these two pressures - the air flowing over and the air flowing under the car. But, more than the car being lifted upwards, it's actually more like it's being sucked upwards - into this partial vacuum.

Downforce is created when the energy of the oncoming air is redirected upward by a Wing. This imparts a force upon the Wing. Newton's 3rd Law (every action has an equal and opposite reaction) forces the Wing (and the car) downward. But, the Wing operates above the laminar flow of air over the body of the car - air passes underneath it, and so the low pressure area is still being created. It is just overcome by the separate and distinct downward energy on the Wing.

A Spoiler does not allow this Laminar Flow to pass underneath it. It is really an Air Dam which allows the onrushing Laminar Flow to pile up in front of it. This slows it down and so it's pressure rises.

Because there is now no difference between the air above and the air below the car, lift has simply been eliminated, not just overcome as in the use of a Wing. More simply, there is no Low pressure, so no 'Lift' and so Downforce is not necessary.

Again, we may have to agree to disagree. And in practical terms, it is just semantics to many. But, there is a distinct difference between the existance of a Low Pressure area above the car, and a counteracting force produced to overcome it.

So again, Downforce, produced by rushing air and a Wing is used to overcome existing Lift. But, Downforce is not necessary when a Spoiler is used, because no lift exists to overcome.

Your description of the airflow and resulting pressure is essentially correct, although your use of the term laminar is a bit inaccurate.

Nevertheless, you're referring to lift as an inherent characteristic of a car, and downforce as a measure to counter it. These terms are not specific to cars, they apply to the forces acting on any body moving through a fluid and therefore need to be understood in a more general sense. If you refer to these terms without forcing them to be applied to a car (which they aren't) you end up with my description.