Adnectere

Was curious about front spoilers so I re-read a little of Katz' aerodynamics book. If anyone knows this please chip in.

It sounds like the purpose of a front spoiler/air dam is to block flow under the car and re-route it around the car. Usually there isn't a flat pan under the car so there is some drag because of the driveline components and this re-routing lowers the drag. Supposedly there is also a slight benefit to front downforce (his graph shows about ~-0.18 for the Clf @ 50 mm front dam height). Pressure behind the dam is also lower than the pressure in front of it.

I can kind of work this out in my mind. I can understand the re-routing air and the idea of drag b/c of the components under the car. I can also understand that the air will build up somewhat in front of the spoiler as opposed to behind it (where air is also flowing through the radiator). So I understand the high pressure in front, low pressure behind.

But what I dont get is how this helps -lift. Is it literally as simple as having low pressure behind the air dam? I know the concept getting rid of lift is as simple as having higher pressure above the vehicle and lower underneath it. But in my mind at any rate, and I think Katz mentions it elsewhere, is that you need more airflow velocity under the car (so lower pressure) and less airflow velocity over the car (so higher pressure) which will push the car down. But the front dam blocks air from moving under the car and pushes it around the vehicle so less air flows under the car and you'd wind up with higher pressures than if you had air flowing under the car. So I don't get how blocking the air via a dam and lessening air flow under the car will get you lower pressures there. I would think less air flow and less air velocity would increase under-car pressure.

May Spin

Exactly.... Just like an airplane wing, which needs lift. So design allows for a faster flow over the top of the wing surface (lower pressure), and slower (high pressure) below the wing. This is often described as Bernoulli’s principle. Now... At high speeds, a car would benefit from the opposite of this. In this case you would want the air to move faster as it flowed under the car, to create a lot of low pressure.

A front air dam can divert air around the side of the vehicle, an attempt to create less air under the car (low pressure). A perfect design would want to crerate a vacume under the car. The remaining air that does get under the car would need to flow faster than on top. Factory engineers know this only to well. Climb under a Corvette C5/6, G35, even my old 93 MR2 Turbo to name a few and you'll see the same thing, plates of either sheet metal or plastic lining the floor pan designed to offer liitle resistance to air flow.

Scott.

Adnectere

In my mind, if you limit flow under the car, like in 'vacuum-cleaner car' the prevailing pressure under the car will be set. With an old say chaparral vacuum car, you'd shoot for getting the fan to give you a vacuum of as much as possible. With a side-skirt/front dam combo, you'd wind up with the base pressure near the rear being the prevailing pressure. But if you have no front dam/side skirt/vacuum design, what is the prevailing pressure? I would assume it would go more towards just an ambient pressure since there is less air involved. (And speeds in that area are lower?)

I completely agree about the low drag flat underpan. But if we stick that pan on a car, it's not that it creates downforce, it just gets rid of drag. Other stuff like diffusers/venturis can create downforce, but that's on another discussion. If you wind up with a flat underbody, the topside of the car has the same front-to-rear length 'as the crow flies', but its actual body distance is greater than under the car to account for the passenger cabin, rising hood, descending truck, etc. So since the air covers more distance up top, it must be going faster, which would mean it's lower pressure and you get lift.

May Spin

Ok.... I see what you're saying, we're still wing if we are to rely on Bernoulli's ideas. Lift is always a major issue with all high speed vehicles, especially those that want to corner at speed (140mph+). F1 and Indy cars use huge spoilers to create downforce on the front and rear tires. Their sidepods use venturi's to lower pressure under the car.

The average passenger car won't use any of these features. So minimizing the lift created by low presure over the top of the car can be obtained by raking the car so the rear sits higher at the rear. As air flows under the car the widening gap in theory will create a vacume (low pressure) effect. Front air dams are obviously designed to limit air flow under the car. The dam would create alot of low pressure directly behind it (good for the front tires). You will always create lift, in a passenger car I feel your're just polishing a turd as far as combating lift. I wonder at what speeds would an F-body benefit from a front dam, let alone the Formula's rear spoiler or the Trans Am's wing.