|
I'll try to explain my understanding better (FYI: I have no formal training in this sort of thing).
So to start we have an object of a given size and lets make it a ball. The front of the ball is a fine shape, air moves out of it's way as well if not better than any other shape. The air in front of it is of an increased pressure, which causes resistance, but this in unavoidable for an object of it's size. So lets look at the back. The air is unable to hold the steep curve all the way down (except at slow speeds). So to simplify things lets cut the ball in half horizontally, and place it on the ground. The front still works the same and the back still works the same (not so great). If you gradually extend the back end air flow will (at some point for a given speed) stay "connected". The longer it gets the higher this speed can be (everything changes when the speed of sound reached, so this limits the angles worth looking at) Also an angle that is extremely soft will result in a lot of surface drag, as there will be a lot of surface, but for this discussion, that is not worth thinking about (no car can be 80yards long).
So air gets forced up by the front of our half ball, then it wants to stay in it's new place, but the ball needs to end, so we need to bring it down to it's original place in space. If we do this too abruptly we will make turbulence, if we do it less abruptly we will make a smooth flowing low pressure zone. A low pressure zone makes drag, a from of drag known as lift (rather important for an airplane's wing). This low pressure pulls the object up, but this shouldn't be that dramatic at driving speeds in comparison the the weight of a car. So the softer the slope the slower the air needs to get pulled down, and therefor the low pressure is less dramatic, and therefor the drag is lower.
|