Spoilers and Wings - the Difference

Discussion in 'General Tech' started by mcglsr2, Mar 18, 2015.

  1. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Alright, inspired by a recent thread I posted in, I decided to create a new thread on wings and spoilers, the difference and what they do. There will be a lot of words here, so if you don't like reading, sorry :( Also, following is my understanding of "How Things Work" based on my reading, research and studying. I may be wrong.


    First thing to understand: Wings and Spoilers are two different things, totally different in concept and how they work. Unfortunately, a lot of people talk about them like they are the same thing, which makes things really confusing. So just always keep in mind: Wings are not Spoilers, and Spoilers are not Wings. If someone talks about them like they are, they are about to give you incorrect information.

    Now, with that in mind:

    Wings
    Wings, or airfoils, are used with the express purpose of generating downforce. They are essentially upside down airplane wings. Where an airplane wing is designed to generate lift (so they can fly), a race car wing is designed to generate negative lift. Wings (which may be a single airfoil or consist of multiple airfoils) force fast moving laminar air flow upwards, which pushes the wing (and whatever it is attached to) downwards.

    So, the good: they generate downforce which pushes our cars to the road. This is great, especially for fast cornering. The bad: they also generate drag. This relationship is proportional: typically the more downforce the wing provides, the more drag it must therefore also create. There is no free lunch - with more downforce comes more drag. In racing, this trade-off is acceptable, as the cars typically are traction limited and NOT power limited (see end of post for a description of this). So, even though the cars give up some top-end speed (because of drag) and some acceleration (because of drag), the speed they are able to carry through the corners due to the greater downforce more than outweigh the negative of the drag.

    Wings need to be mounted high enough so they encounter the faster laminar air flow - on a production car this is typically a bit below the roof line. On a purpose-built car like an F1, the wing can be lower as the body is designed to channel air to it - however, one will see that if the engine air intake is thought to be the "roof line", the wings are usually just a bit below this, or thereabouts. For production cars, wings that extend above/beyond the roof line tend to have diminishing returns, because at this point the wing is drastically altering the shape/slipstream of the car, and producing A TON of drag/downforce. If the car is severely traction limited and has an overabundance of power, this may be necessary. However it is still extreme.

    A real-world example of this is F1 cars. They have a wing on both the front of the car and the rear. This creates drag, however the cars have more than enough power and they can go faster through corners. This is due to them running on (essentially) flat tracks. The downforce keeps them on the track through high-speed cornering. However, the cars have to be traveling fast enough for the wings to do their job. Going through a corner slowly is actually more dangerous/harder than going through the corner at a much higher speed. Because: downforce. There isn't enough at the slower speed, thus the car has no grip. At higher speeds, there is downforce, and magically there is grip when you thought there were none.


    Spoilers
    Spoilers are a different animal than wings - they are placed in the same spots (in the front, also called an "air dam" and in the rear). The job of a spoiler is different than a wing. To understand, one needs to realize that on a production car, the worst flow area is the back of the car/trunk area. The air, as it leaves the trunk, does a bad thing. It angles to the ground as it flows off. At slow, legal speeds it doesn't really matter. At higher speeds, this air flow can start to generate lift on the rear end. It could make your rear-end start to feel loose. Porsche had this problem, thus the iconic "whale tail" on their 911's. Ideally, the air coming off the trunk needs to be encouraged to go elsewhere - just not down. Enter the spoiler. The spoiler creates a pocket of stagnant air between the rear window and the spoiler itself. This dead zone encourages the faster flowing laminar air to travel over it. Air, like fluid, goes to where ever it's easiest to go. If it's too hard for air to travel into a spot (like the dead zone created by the spoiler), it will instead go around (or over) it. In doing this, the spoiler actually encourages air to slide off the rear car easier than normal, and results in a car that cuts a "slicker" path through the air. It actually reduces drag - unlike a wing. Of course, it doesn't specifically generate downforce either. However, what it does do is eliminate the rear end lift cars can experience. So it doesn't really create downforce, but rather cancels out the lift produced off the rear of production car rear ends.

    Nascar uses spoilers - that big wall looking thing on the rear. Initially, one might think that it creates a lot of drag. However, it does not. It does the opposite. If you think about it, Nascar's have those huge banked turns - the car riding up on this turn generates the downforce for the car - the car itself does not need to. So, Nascar runs a spoiler up front and a spoiler in the rear, to make the car slicker through the air stream. (Note there is still downforce generated via the smooth underside of the car). I suspect if a Nascar were to stay on flat tracks like F1, they would be using wings rather than spoilers.

    Also, most production cars use spoilers - though a lot are incorrectly referenced as "wings." Our stock SN95's fall into this category. The stock wing on the Mustang is actually a spoiler, not a wing. Most production cars have spoilers back there - even if there are gaps under the "wing" and trunk lid, the air won't travel under there - it's too hard and remember air doesn't like to go where it's too hard. Instead, these act like spoilers. Which manufacture's like - reduced drag typically results in better MPG, the potential cancellation of lift on the rear results in better handling (think sports models of production cars), and they don't really care about downforce (thus no wing) because one would need to exceed the legal speed limit to benefit from them. There are few exceptions, like the STI and EVO that have big wings - but the manufacturers position these cars for track use (whether or not the respective owners actually use them on the track).


    Summary, and Which Do I Want, If Either?
    So, quick recap: wings make downforce (good) and drag (bad). Spoilers negate rear-end lift (good), make the car smoother through the air (good) and don't make downforce (meh). It would seem that spoilers here are better, as there aren't really any negatives (maybe other than styling?). However, it all depends on what you do with your car. If you have a production car or drive on the street exclusively, then spoiler all the way (even little stock "wings"). If you race the car, and you need more downforce in the rear, then you pretty much have to go with a wing. How aggressive of wing (referred to as "angle of attack") depends on how much downforce you need and how much resulting drag you are willing to live with.

    If your race car is low on power, you would be better served by a spoiler. It won't hurt your run up to top speed on the straights. You should be able to carry enough speed through the corners - your car will be able to generate only so much speed. Think Spec Miata's. The vast majority do *not* have wings. They are power limited.

    If, however, your car makes gobs of power, the straights won't be a problem. Carrying that fast speed through the corner will be. You will be traction limited. If you find your rear end getting loose a lot on the corners, you may want to run a wing to provide some downforce. Depending on how aggressive you go with the wing, your power will offset the generated drag. Cars that tend to be very, very quick (like maximum attack/time attack type cars) - which also tend to have an overabundance of power - will run larger wings as they can use the greater downforce on the corners. And the power of the engine will overcome the drag. They will ultimately lose some top-end and acceleration, but with that much power, they have much more to gain in the corners then in the straights.


    So - in terms of our SN95's:
    If you are at stock power (like me, at 185 rwhp), run a spoiler. Having a spoiler is better than having nothing at all - because of the rear-end lift that can be generated. Though, it depends on how fast you get up to, you may not even notice it getting squirrely - in this case no spoiler would be fine. However, the benefits are there, so better off running one regardless.

    If you have decent power - you have a decision to make: are you carrying the speed through the corners that you want? If not, is it because your front gets loose or your rear? If you find that you are over-steering through the corner, you could tinker with your anti-roll bars, OR maybe you need some downforce in the rear. If so, a fairly non-aggressive wing might give you the bite you need without too much drag.

    If you have gobs of power, you probably want a wing. Possibly with an aggressive attack. The amounts of drag created will be compensated for by your power, and the added downforce will help you keep your speed up through the corners.

    This is just a guide - aero has to be tuned in conjunction with your suspension, anti-sway stiffness, tire pressures, etc.


    Further Reading:

    Traction Limited vs Power Limited
    On a race track, your car will accelerate, and go through corners. What the tires do here dictates whether your car is Traction Limited or Power Limited. Which one your car is dictates how you drive it.

    If you smooth into the throttle and get on it, if the rear tires spin - you are traction limited. In other words, you have enough power to break the tires loose and more power will have no benefit. The speed at which the car accelerates is based on the grip, or traction, of the tires.

    If, instead the car accelerates with no complaints or slipping from the tires, you are probably power limited. In other words, the car doesn't have enough power to break the tires loose. In a corner, this has an effect on how early one get on the throttle. Cars with lots of power have to smooth in the gas pedal or else the rear tires will spin and you find yourself maybe going sideways. On lower power cars, like Miata's, they can pretty much stand on the gas pedal almost right away (and in some cases, depending on the corner, they don't even lift) - there isn't enough power to break the tires loose.

    Keep in mind that traction/power limited changes with suspension and tires. For example, a lower power car with really, really crappy tires can seem traction limited (think of stock Mustang going around a track on spare tire donuts). Totally traction limited. However, put on a grippy set of R-compound tires and now all of a sudden that same car is power limited.


    Is the Rear End Design Really That Important?
    In a word, yes. The design/shape dictates how the air flows off and what behaviors are exhibited by the car. For production cars, it almost always never matters because 99% of them are driven at or below legal speeds. Race cars are different. As a point of example, take a look at a 1964/65 Shelby Daytona. Look at the rear, and that crazy slant it has going on. Peter Brock, when designing the car, specifically made it that shape so it would cut through the air better - the angle helped to separate air off the rear of the car and reduce drag. This was necessary to hit the higher top speeds to compete against (and beat) Ferrari. Believe it or not, if that rear end was vertical, or slanted in the other direction, it would have disastrous effects on the handling/slipstream of the car. Since, on our production cars, we can't realistically change the shape of the rear end, we instead use wings and spoilers to accomplish the effects we are looking for.


    Splitters and Diffusers
    Works similarly as wings and spoilers, but with a different focus. While spoilers and wings deal with air that travels over the top of the car, splitters and diffuesers deal with the air that travels *under* the car (although splitters can also generate downforce on the front of the car - and do; the amount of downforce is related to the size of the splitter). For them to be really effective, the bottom of the car should be smooth. Our Mustangs (and most older generation cars) are anything but smooth. Look under your car. See all that suspension stuff, engine and transmission bits, exhaust pipes, frames and stuff under there? Very bumpy. Not smooth at all. So splitters and diffusers on our cars are not super effective. They help, I'm sure, but the full benefit is not seen. One would have to install a full under-body tray to smooth it out. If one were to look at newer cars underneath (like an Audi A4), you will start to see a lot of plastic trays under there - it will look a lot smoother. On these types of cars, it's done to increase the efficiency of the car moving through the air, in the quest of the ever increasing MPG. They tend to not use splitters and diffusers though, or at least not aggressive ones like race cars.

    The splitter's job is to encourage air to travel under the car and speed it along. The under-body tray's job is to encourage this fast moving air to keep moving fast (a bumpy, not-smooth underside slows the air down, and starts to create lift, which is bad). The diffuser at the rear's job is to help the faster moving air re-integrate smoothly with the slower, low pressure air zone at the back of the car. These things working in concert can generate considerable downforce - in fact, roughly half of the downforce on a F1 car is made via this method (the other roughly half is made by the combination of the front and rear wings). If the air makes it through faster under the car then the air traveling over it, the whole car essentially turns into an upside-down airplane wing, thus downforce (and drag!!).

    So if you really want to get the full effect of a splitter and diffuser, you would do well to try to smooth out the underside of car. The danger, here, is to encourage the air to move quickly under the car, just to have it slowed down by the bumpy underside, and now all this air the splitter is sending under there starts to stack up and stagnate, creating a low pressure zone - and if the air over the car travels faster, you have now turned the whole car into an airplane wing - you will get lift. And this is worse than if you didn't run a splitter at all.
     
  2. Michael Plummer

    Michael Plummer Active Member

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    Great write up but I disagree with your statement above. A wing is separated from the body except where it's mounted so air flows both above and below it. A spoiler is a lip on the edge of the trunk lid.
     
  3. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    I hear what you are saying but I think that's a simplistic approach. I would amend what you said to say that a wing is separated from the body such that air can and will pass between it and the body. In the case of our cars, and a lot of other production "wings" on other cars (such as Porche's), the gap between the "wing" and body is so small that air will not go there. Keep in mind air looks for the easiest path. Going through the tiny gap on our stock wings is very hard. Air won't do it. So they are really spoilers, even though they look like wings.
     
  4. CC'S95GT

    CC'S95GT Legend

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    Nascar uses spoilers - that big wall looking thing on the rear. Initially, one might think that it creates a lot of drag. However, it does not.

    THIS IS WRONG AS WELL. yes, it does create down force. but it DOES create lots of drag too. It's a trade off, More spoiler creates more down force and it creates more drag too. NASCAR has played around with the height of the spoiler for years by either making it taller to slow the cars down or making it shorted which makes the cars faster but it also makes the cars dance around more in the corners.
     
  5. DavidBoren

    DavidBoren Active Member

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    Thank you for the write up.

    If you disagree with what has been said, please support your argument with as much pertinent information as possible, so we can all learn something from this discussion.

    From elementary knowledge of this subject, everything stated in the OP's write up is spot on.

    Leaving the tailgate up on a pickup truck increases gas mileage. You can have lightweight trash in the bed that doesn't blow away at 80mph. Same truck, same lightweight trash goes everywhere with the tailgate down. Ask me how I know.

    Simply put, the tailgate is a huge wall, but it actually reduces turbulence. However, even on freeway roads, wirh mild corners, you can feel the ass end of the S-10 start to float. No downforce is created by the tailgate. Drag is reduced, as evident by gas mileage and the lightweight trash windtunnel test, and by windtunnel I of course mean freeway.
     
  6. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Nascar may play around with the height and angle of the spoiler for various reasons and desired handling characteristics. However that doesn't change the point that a spoiler, when set up properly, reduces drag. Spoilers by themselves do not typically create downforce - that's not the point of them. I don't see how it's possible, actually, as there is no air traveling under the spoiler to encourage the anti-lift. I suppose a big enough spoiler *will* start to generate drag - but why would one want to create drag when they don't have to? What they are doing is playing with how much rear-end lift they are willing to accept to get higher top speeds. This is not playing with downforce per se but rather negating the inherent tendency for rear-end lift. Thus them getting looser in the rear with small spoilers.

    I remember reading a page with some really good fluid dynamics diagrams - including ones for NASCAR. I will find it and post it. Update: link posted below in next post.

    Yup this is it - truck tailgate acts as a spoiler. Trucks are faster/get better MPG with the tailgates UP. Because spoiler. However, they do not have massive amounts of downforce on the bed because of the spoiler - because spoilers don't create downforce.
     
  7. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Aha! Found the link: here. Lots of awesome CFD diagrams! This is one of the places I had found with good info. Others include books and such on race car tuning.

    Also, for those of you thinking that a spoiler cannot have a gap between itself and the trunk/body, here are some spoilers pretending to be wings.

    My 87 CSX. Note the rear "wing." I say wing because, according to the definition of having a gap - it does. Albeit very small. You can just barely make out gaps. There are 3 of them. And they are true gaps, meaning I can see through them. Despite this, that is totally a spoiler - no way is air going to flow through those tiny holes:

    [​IMG]


    And of course, the stock GT spoiler off the Mustang - that's a huge gap, look at that thing! Well, actually it's not really a huge gap at all. Again, tiny. Air will avoid it. Spoiler. (Image "borrowed" from American Muscle.)

    [​IMG]


    Surely you must think I am full of it. That is a gap and a half. Surely that is a wing. Well, from the site I linked above, here's a Porsche in an air tunnel with smoke trace going over it. What's the smoke doing at the rear? Going under and over the "wing?" Nope. Just over. Spoiler. And, I daresay, the gap on that spoiler is larger than the one on the Mustang.

    [​IMG]


    This is a wing, without a doubt (lulz):

    [​IMG]


    So where it gets hard for manufactures and racers is when does a spoiler with a gap between it and the body of the car stop being a spoiler and start being a wing? Good question, and tough to answer. One will need to do wind tunnel testing and CFD rendering to find the answer. I suspect the answer depends on the shape of the car and the shape and size of the spoiler/wing. Probably somewhere between the Porsche spoiler and the STI wing I would think (how's that for accurate) :p
     
  8. ReplicaR

    ReplicaR Well-Known Member

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    So, I've been speaking to someone who knows a lot about aerodynamics, and he explained it to me that in a nutshell, drag is actually vacuum that gets formed behind the the object and pulls it back. I guess I must have had a different understanding what drag meant. So having said that, and referring to what you said earlier about the deadzone (or vacuum?), spoiler should generate drag as well, correct?
     
  9. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    This is sort of right regarding a vacuum that is created; however I like to think of it as Smooth Flow vs Turbulent Flow as this is really more accurate. In situations where flow is smooth - there is no aerodynamic drag. In situations where airflow is turbulent - aerodynamic drag is created. The amount of drag created is related to how much turbulence there is.

    Okay, so, thinking about the rear of a car (akin to the vacuum example), you have a bunch of smooth flowing air encounter the front of the car. This air travels over the body and under it, doing it's best to find a smooth path around. When it finally reaches the end of the car (assuming no wing or spoiler), it encounters a cliff if you will. All of a sudden, the body it was flowing over, and all the little things like side mirrors and roofs and underbody bits and front bumpers that made the air move around is gone. However, because the air was moving around, it does not seamlessly come back together after the car. Rather, turbulence is created (all the stuff the air had to move around). So the air sort of rolls and tumbles together (turbulence) after the car until it eventually smooths back out.

    Okay. This turbulence at the rear of the car creates a low pressure zone. This is where the "vacuum" in your example comes from. Since this low pressure zone is moving much more slowly compared to the other air around it, it creates a negative force, or drag. The key here, though, is that it is *moving.* Save that tidbit for later.

    The amount of turbulence at the end of the car is related to the shape of the car and body panels (think F1 design versus a production car - very different). A minivan, for example, will have a huge low pressure zone behind it because it's essentially a brick pushing through the air. Lots of drag. A Veyron will have a lot less drag, due it's more efficient design.

    Alright, so drag is directly related to the turbulence produced by an object moving through the air - the smoother and "skinnier" the object, the less the air is disturbed passing around it, the less turbulence it creates, the less drag it will therefore have.

    I'm getting there - I promise. I know, long answer. So, to your question about the dead zone in front of a spoiler creating drag. The important thing here to keep in mind is it's not just the low pressure zone (which is why I don't like to think of it as vacuum), but rather how the flowing air interacts with it. If the air *did* enter the space between a spoiler and the rear window, this would mean the air travels down the rear window, over the trunk, hits the spoiler, and then rockets off in a different direction, likely upwards. If this were the case, it would create drag because of the really turbulent path (and would in fact make this spoiler a wing, and generate downforce). However! Air doesn't work like that. Rather, what happens is that as the faster moving air travels down the rear window, it encounters a pocket of stagnant air. The stagnant air just sort of sits there, "held in place" by the spoiler. When the faster moving air encounters this, the stagnant air creates a sort of buffer if you will, and the faster moving air just goes over it. It's almost like you extended the surface of the trunk up to the height of the spoiler. This encourages the faster moving air to just sort of pass right over the rear part of the car, it does not introduce a lot of turbulence, and thus does not create drag. In fact, it helps to smooth out the air at the rear of the car, *reducing* the low pressure zone and the tendency for rear-end lift.

    But what about the air that's just sitting in that pocket between the window and spoiler? It's a low pressure zone, thus vacuum, thus drag, right? No, not really. This is why I don't like the vacuum analogy. Just because you have a low pressure zone does NOT mean you have drag. Drag is a result of turbulence. And if that air is stuck there, just hanging out in that pocket, it isn't really going anywhere, it isn't really flowing, thus it is not creating turbulence, thus it is not producing drag.

    Make sense? Sorry for the long answer lol. "You: What time is it?" "Me: This is how you build a clock..." lulz :D


    Edit: the above, and pretty much all my discussions on spoilers, assumes a properly designed and implemented spoiler. If one does it wrong, then everything I've said could go right out the window (along with any of the good benefits of a spoiler).
     
  10. ttocs

    ttocs Legend

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    [video=youtube;q_eMQvDoDWk]https://www.youtube.com/watch?v=q_eMQvDoDWk[/video]

    and watch this one when the spoiler flips up

    [video=youtube;B29ScK8tIwY]https://www.youtube.com/watch?v=B29ScK8tIwY[/video]
     
  11. Orange 94

    Orange 94 Moderator Staff SN95 Supporter

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    +1


    Good write up and discussion so far. Stickying thread.
     
  12. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    ^ Nice vids. They do a good job of showing turbulence; on the car one, pay attention to the stream closest to the car and what it does as it leaves the rear end before the spoiler is raised. It's more turbulent *and* it angles downward (producing lift). Once the spoiler is raised, it lifts up the smoke, negating the rear-end lift and encourages more seamless integration back into the other air streams - reducing drag.
     
  13. white95

    white95 Apex Junky Admin

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    Great write up!! Wonder exactly how effective my S-351 spoiler is?
     
  14. badsheep5.0

    badsheep5.0 Member

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    Thank you sir very informational I am not all the way through but my break is over will continue reading later
     
  15. DavidBoren

    DavidBoren Active Member

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    The S281 spoiler gives the air coming off the back of the car more time to level out, and, in theory, reduces size of the vacuum/drag/flow detachment directly behind the trunk/rear bumper. It's not there to deflect air, or change air speed (like an airfoil), it is just there to smooth out the transition of the air coming off the back of the car. Or at least that is my understanding of those types of spoilers.

    If anything, the S281 "spoiler" is actually a diffuser.
     
  16. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Yah not sure. It's the flat-ish one, that extends past the trunk lid, right? If so, it's certainly helping I would say. It would seem that any spoiler would help in reducing the rear-end lift - which one you choose tends to be a matter of preference/style.
     
  17. DavidBoren

    DavidBoren Active Member

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    Dammit, I can't read. You said S351, not S281. Ok. I think that the S351 is another classic example of a "wing" with too small of a gap under it... making it not a wing at all, but just another spoiler. I could be wrong, the way the S351 ends extend out and down may catch air coming off the sides of the car. I don't know.

    I need to pay better attention.
     
  18. ttocs

    ttocs Legend

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    how do you figure raising the spoiler reduced drag?
     
  19. ttocs

    ttocs Legend

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    [video=youtube;jYaIXWNOa_A]https://www.youtube.com/watch?v=jYaIXWNOa_A[/video]

    This one goes into a better definition of all the terms discussed.
     
  20. mcglsr2

    mcglsr2 Well-Known Member SN95 Supporter

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    Pause the Mercedes video at 1:16. Look at the the smoke trail closest to the body of the car. When it reaches the end of the car it turns turbulent and angles downwards. This creates a turbulent area, which is a low pressure zone, which creates lift (and drag). Also, look at that smoke trail in relation to the others. There's quite a bit of separation, which means turbulent air (harder to merge the air back together).

    Okay, now pause the video at 1:19. The spoiler is up. Look again at the smoke trail closest to the body of the car. It is now practically level, no longer angling downwards. This means the airflow reintegrates better in the air, less turbulence, not so much of a low pressure zone, which reduces lift (and drag). Again look at the smoke trail in relation to the others. Much closer together, less turbulence, easier to merge back together.

    With a properly designed spoiler, rear-end lift can be eliminated completely. And the drag coefficient of the car is greatly improved. Again, that's the point of the spoiler - to reduce/eliminate rear-end lift (and thus drag). Think of the pickup truck example again - with the tailgate up, the truck gets better MPG. How is that possible if the spoiler (tailgate) creates drag? The answer: because it doesn't create drag, it in fact reduces drag. <-- That is the whole point of spoilers.

    If you want downforce, then you are talking wings. And you get drag as a negative by-product of the design of the airfoil. Drag is not a by-product of downforce. It is a by-product of turbulent air (which a wing creates in order to generate downforce). And a spoiler's job is to smooth the air exiting off the vehicle so it's not as turbulent. Which happens to reduce drag.

    Awesome vid on wings, thanks :) It's amazing the engineering that goes into those things...