My skirts are made by myself in fiber paint (a sort of 6 cm angle plastic), ultra light but very rigid. It can very easily removable in 20 seconds, to lift the car very quickly with 5 Torx !
This 2 skirts are very interesting for aérodynamics effects in the endplates to limit the free air ! Lotus started ground-effect revolution in 1977 with a car that carried stubby side wings, the enplates of which were sealed to track for maximum effect. The Lotus 78's wing-section pods were shaped with the ground in mind but would also have worked infree air. Similiray, the opération of front wing can be enhanced through ground-effect; an underwing works purely through interaction with the ground and consequently would not fonction in free air.
The underbody shaped and the two Shirts achieve negative pressure effect below the car. If flow accelerates, it's pressure reduces; this is the essence of the "venturi effect" principle.
The rear diffuser is ultimately responsible for the attraction of a low-pressure area underneath the car. It draws in air to fill its volume - nature abhors a vacuum.
The general flow pattern around the moving car ensures that diffuser fills from the front and discharges beneath the wake region... Of course, the negative pressure in the diffuser would tail off rapidly as the air decelerated back towards road speed.
The two skirts reduces wake pressure, increasing base drag but at the same time making the diffuser work better
(Hum ! I hope that my english are not so bad, for explain this technical part !!!!)
Franck / Valmont