I found this at Wikipedia:
Spoilers on the front of a vehicle are often called air dams, because in addition to directing air flow they also reduce the amount of air flowing underneath the vehicle which generally reduces aerodynamic lift and drag.
Spoiler (automotive) - Wikipedia, the free encyclopedia
I see three alternatives for how to treat the wall of air a car in motion have to struggle:
1: Make something of a 3D teardrop body that is teoretically the best in open air. Close to ground this shape will force extra air under the belly of the car in an already narrow space. This will increase lift and add drag. From the little I know about aerodynamics I see a possibility that a slight increase of airflow under the car can be better than having a sharp edge in the front that will cause turbulent flow under the car. I guess this is when a horizontal splitter may be a solution.
2: Cut the perfect teardop in halves and put wheels under the flat side. Even if the air under the vehicle goes undisturbed such a body will create a lot of lift as it is actually an airplane wing. Some of the air under the vehickle will ofcourse be stirred up by the wheels in some degree.
3: Build an air dam in the front that will showel most of the air up over the vehicle and to the sides. But will it not also create a lot of turbulence under the entire floorpan when air is accelerating through the narrow slot between the air dam and the ground? And will not the lower pressure under the car not only create good negative lift but also increase drag a lot? For a competition car where efficiency is not the primary goal there´s no disadvantage but when the goal is to save fuel we can´t afford luxury such as improved road grip.
If we manage to leave the air under the car undisturbed it will rub against the vehicle, if we try to trap it and make it follow the car it will rub the road. If we change the air pressure under the car the air have to flow faster (but the direction depends on the modifications)
So, what is the best way to treat air UNDER the car? As I see it, handling the rest of the air is only a question of smooth shapes and small angles in a teardrop shape. (with or without the Kamm-back)