Two further thoughts, both very different, so I'll deal with them in different posts:
1) In another thread...
got me thinking. Someone there asked about the effect of a "tea tray" spoiler.
Roger wrote:
Brookwood wrote:
I don't understand big tea trays bolted on top of boot lids. At what speed to they serve a purpose?
It varies from car to car. However, typically they come into their own at illegally high speeds (in this country) and actually make matters worse at 50mph. By careful design and typically muchj smaller "tea trays", they can be made to not do harm at lower speeds, cut in and do a minute amount of good at higher speeds. however, in almost all cases, they serve as decoration. I think. anyone got a different take?
I reckon that you're spot-on.
As aerodynamic "aids" they can work, but not really at legal speeds in this country. The odd small spoiler on the roof of some hatchbacks can help to keep the back window and tail lights clean but doesn't contribute anything to "downforce". I'm fairly sure that none of the decorative spoilers fitted to production cars ever have a positive effect on downforce, regardless of the speed, but may act to neutralise lift at high speed. The only figures that I have to hand at the moment are for the Porsche 911 in its "pre and post-Carrera" incarnations and they're quite strange because of the near-aerofoil shape of the early cars... At 150mph, without spoilers the car generates a shade over 400lbs of lift - ie almost 20% of the weight of the car - whereas with spoilers, lift is reduced to 37lbs. They also reduce drag somewhat so allow a higher top speed and/or better fuel consumption.
F1-type downforce (which is what I suppose is being implied by the marketing and styling departments of "adorned" road cars) is only achieved at the expense of
massive aerodynamic drag, to the extent that at about 160-170mph, putting the car into neutral produces retardation of better than 1g without even touching the brakes!
Can this be exploited and an appropriate "tea tray" be deployed, if necessary enhanced with air, to give this effect without the resultant people-to-brick-wall-blower?