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Originally Posted by serialk11r
Gets complicated. Say you have a straw. The straw is really narrow, but does that mean air actually wants to go through it and suck the walls of the straw in? Not unless you have enough of a pressure difference to make flow happen.
If you want to force air under the car, it's really easy. Just channel the air below the car. I am fairly sure the only job of the diffuser is to slow the air down as it leaves the rear and recover most of the energy that went into accelerating the air as it got under the car. Of course, the air "rubs" the ground and slows down and loses energy, so there's a cost to downforce produced in this manner, but it's not nearly as bad as the high angle of attack wings.
No, if you put an airplane wing to the ground it'll get sucked to the ground. The ground is a tricky thing.
The reason cars generate positive lift is because typically cars have a giant low pressure zone behind them (called the "wake") and a far-from-streamlined shape, so that this low pressure zone influences (and sometimes extends directly over) regions of air sitting over the top of the car. The bottom sides of a car are usually very "dirty" and so not much air tends to flow through and you don't get much negative lift from the bottom side of the car.
I guess another way to put it is that the usual car has close to zero attention paid to aerodynamics. Softened edges and tire spats have been around for a while, but they're only starting to actually put effort into aero for everyday cars.
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This has been demonstrated quite dramatically a few times on LeMans' Mulsanne straight. At least Porsche and Mercedes (don't know if others) got their closed roof prototypes airborne there when the speed and bumpy surface pitched their noses up just a tiny bit too much. The Porsche does an actual flip. Mercedes a bit more.
[ame="www.youtube.com/watch?v=rQbgSe9S54I"]Mercedes CLR-GTR Lemans Flip - YouTube[/ame]
So even the high downforce cars are pretty much wings if you take them away from the ground.