ah, you see, aerodynamics dont really come into play until ablove 70mph. And the hatch back design is one that is effectively more aerodynamic than the tradytional saloon style design, which is why volvo entered its estate car in BTCC first, but it was too heavy. The air comming off a hatch back basically justflows over the roof, then oops, into empty space if ya follow, whereas on a saloon, it has to go along the rear window, boot then off the back. Traditionally, it was thought that if air comes off smoothly, itll create less drag, but the shapes employed to creates this smooth runoff of air, ended up like a aeroplane wind cross section, creating more lift. The advantage of a hatch back, is air just enters open space, and swirls behind the car on the way off, creating the area of low pressure behind the car, not on top.
What can most easily be explained as a gurney flap on the clio cup cars, is as you say to decrease lift at high speeds, but the air wouldnt follow the tailgate anyway. The upside down wing as you say, works, and it was the primary design on the F1 lotuses in the 60s. However, the road spoilers you can buy, create more drag than they do downforce as they are effectively a J profile mirrored on the horisontal plane, the rotated 90 deg clockwise (sorry, bad explanation i know), and these priciples are to redirect the flow of air so that it pushes down on the car, not creating an area of lower pressure as spoilers should do. So, they create huge amounts of drag, and you will go slower, but who really goes 100mm though corners, and the ones you do go through that speed, arent sharp enough to warrent a spoiler.
maybe their called spoilers for a very obvious reason
