Regarding lift, you can think of it like this.
1. We want the car to
not be a lifting body, as this causes induced drag.
2. Cars develop lift over their upper curved surfaces, and the longer that flow stays attached, the greater that overall lift. (Refer to the pressure measurements shown earlier in this thread to see this in action.)
3. But we have two surfaces we can work with, and if they each develop low pressures, they pull in opposite directions.
4. Therefore, if we develop sufficiently low pressures
under the car over a sufficiently large area, the car ceases to be a lifting body.
5. This is most easily achieved with a flat underfloor and simple rear diffuser.
6. Such a flat undercar body is also good for reducing drag (not just induced drag but also pressure drag).
I think maybe some of the misconceptions here about this come from:
- The idea that downforce = induced drag (but of course, an equal measure of
lift will cause an equal measure of induced drag)
- Bodies with attached flow don't develop lift. This is of course completely wrong, as picturing the lift being generated by an aircraft wing will soon show you.
The Tesla Model S is a good example of all this.
Attached flow on upper surfaces from nose to tail:
But offset by a truly excellent under car flat floor and diffuser:
Resulting in low lift figures:
CL (front: 0.1
CL (rear): 0.1