Quote:
Originally Posted by Piwoslaw
As much as 30% of drag comes from the wheels and wheelwells - this the critical area, still not understood well. One of the funny things is that the drag coefficient of a car is about 0.01 smaller when the wheels are spinning. The basic principle is to leave as little room as possible between the wheel and wheelwell.
. . . . in today's cars the air no longer exits around the transmission but through the wheelwells, where there is a lowpressure zone that sucks the air out.
So what does the future hold for aerodynamics? More research into wheelwells, which cause a lot of turbulence.
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This was very illuminating. So not only are wheels and wheelwells the single greatest aero factor besides the overall body shape, but the aerodynamics involved are so complicated that not even the manufacturer aerodynamicists completely understand.
Obviously within the wells there will be low pressure. But then the wheels themselves must create high pressure on their front/inner surfaces, and low pressure behind them. Now the mfrs, at least the ones that spend a lot of r&d on aero design, are using the low pressure in and behind the wheelwells to extract hot air from the engine bay. On my observations I see 3 ways they are doing this: (1) Front underbody deflectors in front of the well openings and front-facing tire surfaces create high pressure in front so as to (a) trip the attached underbody airflow past the wells so it doesn't get sucked inside and (b) open up a low pressure zone to extract engine bay (radiator) air. These also extend to somewhat reduce the drag created by front tire surfaces, but I would argue that this is a less significant benefit. (2) Closing up the wheelwell gaps, especially at the fenders and the front underbody, while opening up and radiusing at the rear underbody. This also helps keep attached airflow from being sucked into the wells and allows air to be extracted from them. (3) Stragegically placed vents from the engine bay into the wheelwells. These can often bee seen as louvered vents in the wells directly opposite the front-facing tire surfaces, where the pressure is likely the lowest. I wonder, though, how much air is drawn through since these are at the front of the engine bay, often even in front of the radiator.
Has anyone designed his own underbody mods with these considerations in mind? Has anyone cut the fender lining to extract air there so the underbody can be closed up more?
I've been thinking for so long that short of a kammback the best thing I could do is underbody trays, but it would seem from this interview that the wheelwells are the most significant aspect to focus on.