A Not So Perfect Belly Pan.

[Christ]

So if you're going to extend the air dam, shouldn't you relocate the belly pan to the new lower edge, to keep the flow under the car attached as well (instead of creating a micro-wake behind the new valance), or is this insignificant?

Intuitively, based on what I know about fluid dynamics, you'd want to lower the leading edge of the belly pan from where it is now, and connect it to the low point of the new front air dam, leading the angle from the front to about 10* if possible before meeting up with the rest of the body's under side.

[Bicycle Bob]

If it is tricky to seal the front of a "scoop" you can eliminate almost all of the associated drag by sealing the air exits so nothing flows through.

[Frank Lee]

That's exactly what I keep saying about headlight buckets on older rigs.

[Bicycle Bob]

Well, there's a reason an anemometer spins in the direction it does, so a concave front end is not harmless. It can contain a turbulent zone, and thicken the boundary layer.

[Frank Lee]

[Deezler]

Basically my pictures suck, so its hard to fully see the mounting environment for this panel. The front edge is hidden behind the body's underside where it curves around the rear edge of the foot room for the rear passengers. Its only open a little bit in the middle where I mounted it below the exhaust, but even that is higher off the ground than the trailing edge of the front skid plate. So its hard to say how much airflow that part is even seeing. Sealing that part would be tough.

Christ, I get what you are saying, but I already have a front skid plate / belly pan. When I jacked the car up to put this new panel on, I hadn't yet decided whether to mount it directly behind the front skid plate and continue that panel, or go back to the rear axle like I did. I chose the rear because it appeared a much more turbulent zone.

If I throw a new front valence/lip on there, I will definitely seal it back to the front skid plate for full smoothness. My thought was that allowing less air under the car would lessen the drag of any wake or turbulence effects still occurring under there.

Bob, why does it spin the direction it does? Rotation direction seems pretty arbitrary to me. Factoid; anemometer sizes are measured in units of "crow".

[Bicycle Bob]

The anemometer presents identical cups to the wind, facing opposite directions. The concave surface catches more wind and has a larger effective frontal area, so it spins with the wind, while the convex side spins toward the wind. IIRC a parachute has 35% more drag than an equivalent flat plate.

[Frank Lee]

Oh...

but you see, anemometer cups and parachutes don't have 100x larger AFTBODIES.

[Bicycle Bob]

We could set up a balance test with tails on the cups, even though spinning would not work with them. I'm sure the results would be similar. It would be nice if air would just stagnate and pile up as if an ice cream cone had been given one scoop, but the air in a cup or any similar leading shape is unstable, and keeps spilling out one way or another, pushing the streamlines out farther than they would go with a conventional rounded nose. On a belly pan, a quick squirt of caulking foam might help many situations.

[Frank Lee]

Yah, a bucket probably isn't 100% as good as a hemispherical leading shape, but I don't think it's the huge draggy parachute air brake it's frequently made out to be either.