There is a caveat though. The ´nothing´ is not correct; it is all what you want to achieve. Even a small lip behind the rear deck has a quite neat effect on the round butt of the Spyder. No it does not do ánything concerning downforce but aero is as much about drag as it is about downforce. Just 2-3 cm high rubber lip will give a tear off ridge for the flow and will decrease the detached wake; the turbulence behind the car considerably, redúcing drag, reducing perceived instability.
So, even a 2-3 cm low lip will do ´something´, just not downforce.
Again; it is just what you want to achieve.
Our Spyder has pretty bad aero and quite marginal changes will have worthwhile effect. Hence the disproportionate effect of a rear lip per example.
The minute OEM ´dams´ in front of the front wheels are a notable example as well. Those insignificant dams reduce instability because they create an ever so small lower pressure in the wheel wells.
Add the optional chin lip and it will reduce lift under the front; less air gets pushed under the car.
Add even small vents in the bonnet (US: hood) and ditto; some air passing through the rads will now pass over instead of under the car, counting double to reduce lift.
Either will make the car notably less ´instable´ at speed.
And then rake. Lower the front even just 5 mm more than the rear and the effect is HÚGE. Even a marginal pressure difference working on the whole surface of the car times twice (instead of pushing upwards, pushing downwards) becomes several kilos.
Still not convinced? Ever noticed what that little flap behind the seats does? I you have not then ok, aero will do nothing for you. I you have ... well see how ´turbulence´ is affected even by a silly little flap behind the wake of the seats ?! Should be easy to grasp what a lip on the rear or under the chin does
Also the car is relatively light meaning that a larger, higher rear wing wíll have wortwhile effect.
With a silly big wing hovering above it, the lip will have an éxtra effect; it will speed up the air under the wing, enhancing the (inverted) airfoil effect.
A rear wing derives downforce from twó factors; the airflow over the car directly pushing it down ánd the iverted airfoil effect; just from the air speed differential above/under, it gets pushed down.
And indeed, in both it is pressure times surface are so the larger the surface the larger the total force.
Again, any force will have correspondingly more effect on a lighter vehicle than on a heavier vehicle. Thís is where the urban myth about wings not working on road cars comes from. ´Road´ car being a 2400 kg SUV or a 900 kilo roadster? See?!
@Node even if you low wing does not do downforce it still does drag, has effect on the wake and wíll affect stability at speed like fletches on an arrow. Those slow down an arrow considerably but also are vítal to make it work towards its function.