I like reading Quora because some really smart people can be answering the questions. Case in point: somebody asks about "spoilers" and how much force they produce, and Ryan Carlyle, a subsea hydraulics engineer, sets the record straight. The following is reposted with his permission and will hopefully educate more of you about how automotive aerodynamics work.

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To be blunt, most people putting body kits on consumer automobiles are scientifically-illiterate and have no idea what they're doing. The typical internet "collective wisdom" is shockingly stupid about this. While answering this question, I had serious trouble finding illustrations on Google image search that weren't wrong.

THIS IS WRONG:

Here's the crux of the issue: there is a HUGE difference between a "spoiler" and an "airfoil" or wing. They have different shapes and do different things.

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An automotive airfoil is shaped like an upside-down airplane wing — it deflects airflow upward to generate down-force on the rear of the vehicle. This does NOT particularly improve aerodynamics. In fact, an aggressive airfoil adds a substantial amount of drag, in exchange for more traction at high speeds. You see these on Formula 1 racecars, where unbanked racetracks make grip on corners critical to success:

Lots of airfoils working together in the rear wing:

Typical F1 down-force distribution:

Let me repeat it so it's clear: airfoils add drag, reducing your top speed and top-end acceleration. But at high speeds, they push the car down and add traction so you can turn faster. This is a trade-off.

Redbull gives you WINGS, not spoilers:

AIRFOILS ARE NOT SPOILERS AND SPOILERS ARE NOT AIRFOILS. Ok? Don't confuse the two, or you will look dumb on the internet.

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So what the heck is a spoiler? It's an obstruction to localized airflow that improves the overall airflow around a vehicle. Basically you're adding a barrier to a region of undesirable air behavior so the air will flow somewhere else.

The worst airflow a car sees is at its rear edge, where the shape of the vehicle pulls air downward (causing dangerous lift) and generates turbulent, low-pressure air pockets behind the vehicle (contributing to drag). Spoilers change that airflow.

When you look at NASCAR spoilers, you don't see a wing or a graceful ramp, you see a frickin' air-blocking wall at the tail of the car:

This air-blocking flap creates a relatively stagnant pocket of air (shown in green) between the rear window and spoiler:

This is where people's intuitive grasp of aerodynamics is wrong. Most folks think airflow exactly follows the surface contour. Even many auto-designers used to think that, until computational fluid dynamics and rigorous smoke-trace wind tunnel testing improved our understanding of airstream behavior. This is why so many older vehicles had idiotically-ineffective spoilers or wings. Like this dumb beauty:

The Plymouth Superbird —because cars that look like rocket ships are awesome.

But in reality, fast-moving air doesn't like to enter blind pockets. Why should high-speed air follow an elaborate contour when it doesn't have to? The main airstream over a vehicle will flow around the obstruction without entering the blind pocket. Meaning the bulk of the airflow doesn't hit the spoiler so much as avoid it. Path of least resistance.

By preventing airflow from entering a region with an unfavorable body shape, the flow streams around the entire vehicle can be improved. Laminar airflow will avoid the obstruction, modifying the effective body shape, so a properly-designed spoiler can improve the drag coefficient of the vehicle even though it looks like a wall. That reduces drag and improves efficiency.

Naturally, this only works well if you do it right. As in, perform some CFD analysis and then prove up the design in a wind tunnel. Internet companies selling ridiculous body kits to "ricers" generally aren't doing a lot of that stuff.

Just to make it extra clear what I'm saying here: spoilers that stick up above the roof of the car are blisteringly stupid. They make the car's slipstream significantly larger and add a ton of drag. Don't be the guy with the idiotically-large spoiler. No good comes from that.

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Here's more CFD, comparing a NASCAR spoiler (top) to a NASCAR wing (bottom). These were both designed by competent people. Blue is turbulence. You can see how the red laminar airstream avoids the spoiler, but hugs the wing:

Wings are designed to interact with a lot of air. Whereas spoilers are all about redirecting airflow away from the region where the spoiler sits. The bulk airflow isn't even supposed to hit them.

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More CFD, showing the stagnant pocket (dark blue) in front of a spoiler (rear car) vs a wing (front car):

Of course, CFD is only moderately reliable. (Fluid dynamics are hard.) The proof is in the pudding. Here's a wind tunnel smoke trace over a well-designed spoiler on a Porsche:

You can see how the airflow is cleanly redirected from a downward direction (which generates undesirable lift) to a horizontal direction (no lift). Very little airflow goes under the spoiler. It doesn't need to be shaped like a wing or deflect a lot of air. It just creates a little bit of a stagnant air pocket to deflect the main bulk of airflow.

Now compare to a typical sedan tail end:

See how the airflow expands and flows downwards as it exits the rear of the vehicle? This is producing lift as well as some extra drag. (The smoke expands as it slows down.) The faster the car goes, the more the rear end will try to lift off the ground. A good spoiler (or wing) reduces that up-lift.

To summarize:

  • Both wings and spoilers reduce up-lift at the tail of the vehicle, but use different mechanisms.
  • Wings are airfoils designed to directly deflect air upwards and thus push the rear of the vehicle down. They generally add quite a bit of drag.
  • Spoilers are barricades to undesirable flows, and thus are able to reshape airflow streams around the vehicle. This can help keep the rear of the vehicle down and decrease drag by changing the effective vehicle shape.
  • You need computational fluid dynamics and/or wind tunnel testing to quantify spoiler/wing performance.
  • Neither have any positive impact whatsoever on straight-line low-speed acceleration. Both are primarily intended to improve stability and cornering at high speeds.

Got it? Good. I'm tired of the internet being so consistently wrong about this.

All images via Google Image Search.

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Reposted by permission from Ryan Carlyle on Quora.
Original post can be found here.

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