Wheel well aerodynamics

photo courtesy of magoffin

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. 

courtesy of AeroApp

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.

without wheel arch spoiler
and with. Note the higher pressure behind the wheel, that a spoiler would negate. courtesy of ÖMER FARUK CAVUSOGLU.

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.

courtesy of AeroApp

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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).

courtesy of Racecar Engineering

4 thoughts on “Wheel well aerodynamics

  1. Althought not as sophisticated and well described as here, I had the exact same explanation by a Track engineer during a track day. He looked at my [wheel arch] spoilers in the fore and aft of the car and said : take them out, they don’t allow air into the bay and this keeps the pads and discs hotter . I asked why then they were there and he told me : to reduce fuel consumption on motorways, but you’re not here for tourism, are you ? I took them out. The above explanation closes the circle.

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  2. Interesting. I thought that the flaps were there for homologatory reasons.
    I.e. EU regulations mandate that the wheels don’t extend past the body. More precisely an area described by an arc starting at 30° in front and ending at 50° to the rear of the hub that has to extend past the wheel.
    I’d imagine that the Abarth doesn’t need this flaps because the rear bumper is shaped differently and already requires enough coverage.
    A common footnote in 124 and Miata TÜV documents for wheel spacers, which you require here so the vehicle remains road legal, is that you might have to widen fenders even with quite thin spacers.

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    • Tobias, there is a chance you could be the winner here. I haven’t been able to locate a document supporting my claim. For instance, although the spoilers (deflectors) are mentioned and shown in the “Under Cover Aerodynamic Performance” section of the Miata’s Service Manual, it does not specifically mention the little wing.

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