Today, Magoffin asked another of his thought provoking questions: “I always wonder about the purpose of this small plastic wing on the rear fender. Any idea what it’s there for?”
I was going to simply reply – it has to do with aerodynamics, but I was curious to learn more, as I’ve seen mentioned in other forum questions for example, as to why there aren’t wheel well exhaust vents incorporated in the Abarth’s rear fascia, and decided to write a short blog article instead.
There has been a lot of scientific research involved with wheel well aerodynamics (https://publications.lib.chalmers.se/records/fulltext/250512/250512.pdf is a good example), since straight line wind tunnel testing (called onset flow) reveals that it can account for 25 – 30% of the overall drag.
Normally, there is a lot of turbulent air flow within the wheel well, so ideally you want to vent it as much as possible, which is why you’ll see race cars with over fender vents and / or wheel well exhaust vents, to help relieve pressure and create down-force.
Apparently, for highway speeds greater than around 45 mph, the wheel arch spoilers (like a mini mud flap on the forward bottom edge of the wheel arch), which are much easier to accommodate into car design, do a sufficient job of keeping most of the air from entering the wheel well, hence lowering the resulting drag. Because of the contour of the side of the vehicle, especially in the rear, that additional “wing” spoiler also aids in preventing that last bit of air pressure (as seen in the above illustration) from developing behind the wheels.
I find the science of aerodynamics to be quite fascinating. Apparently German racing driver Volker Wawer, was able to shave 20 seconds off his Nürbürgring lap time, by widening the front fenders to better direct air into the engine air intakes on the side of his Porsche GT4, actually gaining an increase in power from his engine (allowing for wider tires for better grip helped too).