Does Area Under Car Count in Frontal Area?
 

Looking at Phil Knox seminars, particularly #2, I am left with a question or two. We obviously want to lower the "drag index" of a car in order to achieve the most aerodynamic vehicle. (Drag index being the product of Cd and physical frontal area.)

In this context, is the area under the car and between the wheels usually counted in the physical frontal area? The question has some importance if one considers lowering a car to achieve a lower drag index.

I think I have heard that there is a theoretical "ideal" height of a car's underbelly to achieve minimum drag. Can't find a ref though. Anyone have a ref or info?

In order. . .no, yes and sort of.

The area under the car doesn't count towards frontal area. The tires should count, but I wouldn't put it past manufacturers not to, and I might have useful info but most likely not.

The problem with any object is its amount of surface area. A long car will have a higher Cd than a short car of the exact same shape. Drag happens not only as it comes across the front of the car but as its pushed aside and "drag"ged along with the car. That part is what I call concurrent drag, or the drag running along the side, literally.

So if you can decrease the amount of air you have to accelerate(in a vacuum you travel faster) it costs less. If you lower the car it makes it more difficult for the air to get under the car. If there is less air underneath the car then each molecule of air the car does accelerate is not immediately decelerated by as many surrounding molecules.

Lowering the car is like creating a greater vacuum under the car. less air less friction.

Skinny tires provide less frontal area.

An example if you have ever ridden on a train and had your head by the window as you and another train cross paths it hurts your ears. I traveled on the Eurostar and when we passed another train headed the opposite direction only inches away the pressure change would hurt your ears until it passed.

Frontal area is, as its name implies, any area you can see from the front. As illustrated in Hucho's book, if you were to park your car facing away from a wall with a single light source at infinity, the area covered by the shadow is your frontal area.

So this obviously includes objects situated under the vehicle like control arms, axle beams, or even mufflers in some cases.

Right I Failed to mention this, although if you lower it it will reduce the effective drag on these components as well because the airspeed around the part will be smaller. Even better than that though would be to go snag a big sheet of aluminum and drill/weld it under your car being careful to not get any components too hot via lack of ventillation.

Exactly, if you know your underside is aero dirty, with lots of parts sticking out in the flow and plenty of interference drag, you want as little air flowing there as possible. That is why air dams work too, trading frontal area for Cd.