Quote:
Originally Posted by jacob300zx
I think there is something to be said of a car thats easier to drive at the limit? ^^
Speaking of NA aero, what aero would be free or lowest drag? Splitter, diffuser, under tray, mirrors, front tire smoothers,...
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The simple answer is that it's complicated
A more complicated, airplane-geek answer:
Other than sealing the wheel wells or other serious body work, the splitter/undertray/diffuser route is probably going to reduce drag the most.
Air reaching the front of the car has to choose between "up and over", "out and around" and "down and under". On the stock car that decision point (called the stagnation point) is pretty high, so a lot of air takes the "under" route. The under route is fairly rough so the velocity is slow and the pressure and drag are high. The over the hood route is long and relatively smooth, so high velocity and low pressure, leading to lift.
The splitter causes the air to decide to split at the height of the front of the splitter rather than bumper center. This leads to much less flow under the car and lower pressure, so lower drag and less lift. The area on top of the splitter is subject to the stagnation pressure of the essentially "stopped" air in front of the bumper. At 125 mph you could see 1/4 psi on top which adds up. At 62 mph it's only 1/16th psi which is unlikely to generate much force on all but the craziest of aero packages.
Undertray stuff keeps the velocity high, so even lower pressure and less drag.
Air likes to speed up. As it accelerates the pressure is reduced - the air flows from higher pressure to lower pressure and it likes that, which means it resists separation from whatever surface it is running along. Air is much pickier about slowing down - the unfavorable pressure gradient (progressively higher along the flow path) causes the air to separate with almost no provocation. It needs to be carefully herded along, so that the energy in the high velocity, low pressure air can be recovered into high pressure, low velocity aft of the car.
The diffuser helps with that. It provides a progressive increase in cross section between the skinny slot of fast air under the car and the big open volume aft of the rear bumper.
Other aero doodads (wings, canards, ricey stickers) are probably going to just trade downforce for drag. Not an improvement for a stock power car.
Tire drag is a tough nut. Bonneville-style bodywork is impractical. Most efforts I have seen are big, flat plates used to deflect air from something that act like... a big, flat plate. It is probably more productive to try and prevent the static, high pressure air in the wheel wells from getting sucked into the carefully arranged stream created by the splitter/undertray/diffuser. LMP1/2 cars vent the wheel wells to low pressure air on top. Who will be the first to go louver happy?
Mirror drag is probably 1-2% of total car drag. You can probably reduce it by half, so look somewhere else unless you like carbon and hate spare cash.