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Old 12-26-2004, 06:08 AM   #1
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Spoilers, what are they good for.

It seems to me that in the "import community" the bigger your spoiler the badder your car is. While I find that way of thinking to have the utmost stupidity put behind it, I can only lead to one conclusion. Why? Well people see SCCA racers in their M3's, Audi S4's, and Acura RSX's with extremly large spoilers and I guess they believe that adding a spoiler to their Civic, Integra, Eclipse will aslo work in the same fashion as it does on a race car.

Well..... Yes and No. A spoiler works by creating downard force on the car as the vehicle travels forward. This is also known as "donwforce". Downforce is generated by the difference in air pressure over and under thw wing. If we have a spoiler and non turbulent air flowing through it (straight flowing air that has not allready been interrupted by other objects), there is a higher pressure on the top side of the spoiler that flows at a high velocity and a lower pressure on the bottom side of the spoiler that flows at a lower velocity. The higher velocity of the air traveling over a spoiler causes a downward force/pressure to be generated.

Mathematicly, the formula for the fucntioning of a spoiler would look like this:

Cl=L / ((1/2)r(V^2)A)

Cl= Lift Coeficient
L= Lift(Downforce)
r= Density of Air
V= Velocity
A= Wing Area

One big factor of the lift force that will be genmerated is the angle of attack(The angle at wich the wing if facing when the air flow hits it). Generally the larger the angle of attack(without going so large that you're creating a wall that blocks the air) the more downward force that will be created. Again turbulence in the air will factor into this equation.

Now that you know how a spoiler works and what makes it work better you next question may be. Well how can we conculde if we have turbulent air flowing into the spoiler. Lets take a 95 Honda Civic. It has a streamlined flow but after the rear of the roof the air becomes turbulent. If you've ever seen a car in a wind tunnel, white colored air is emmitied at the middle of the car in the front and it flows over the car. If it flows over the entire car without intertuption this would be a engineers dream. For air to flow over an entire car without turbulence. However, this is'nt true in a honda. After the air leaves the hood it becomes turbulent. The flow of the air becomes unpredictable and can even flow backwards from the oposing force of the air.
This make the spoiler useless and infact creates unnesicary drag on the car and slows you down.

One factor that people seem to not accept is that the spoilers mointed on the SCCA race cars are really high in the air. Yes it looks rediculous but if the spoiler is mounted high enough into free flowing non turbulant air it becomes more efficient than if trapped beneath a pocket of turbulent air. This is why airplanes shake in turbulence! So actaully the people you see with taller spoilers( I mean like 2 feet in the air almost) actually have a more efficient spoiler than the ones that are smaller. Is'nt that a bitch?

...Everyone is entitled to their own opinion. It's just that yours is stupid...
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Old 12-26-2004, 06:46 AM   #2
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yeah, it's a bitch, but they didn't get the spoiler for that reason, and I think it's best if we don't tell them, because it gives them ammo.

I suppose the only other thing i can add/ask, is, at what speed do spoilers become effective and is it really necessary at that speed? In what I've heard, spoilers only come into noticable effect at roughly 70-80 mph. And, if you're car is still burning out at 70-80 mph, you've got an insane car, which you need for racing, not everyday driving. so, spoilers being unnecessary.

But, there are some cases in which I see spoilers to be necessary. (Like on some cars, such as the skyline). They just look a lot better. Truthfully, any spoiler over 1 foot high schould be banned. But, take in the cosmetic effect also. Because almost nobody buys a spoiler for the actual aerodynamic effect.

Good use of real-world knowledge!!!!!!!!!!!!!
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Old 12-26-2004, 06:58 AM   #3
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Spoilers are in effect everytime the car moves. So long as air is flowing over the car and the car is moving the spoiler is having some effect on the vehicle. I think what you're asking for is when do spoilers become an advantage and yes that is arround 60+ mph. You'll hardly notice any downward force on your car unless you are traveling at higher speeds than any normal road would allow you.

Will it give you an advantage at 100mph, no not really. Will it make your car 10 miles faster, no not really. However in a competetive event where every millisecond counts it just may give you the upper hand.

...Everyone is entitled to their own opinion. It's just that yours is stupid...
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Old 12-27-2004, 07:15 PM   #4
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Nicely done. Especially the last bit about spoilers working as soon as there is air flow.

Something else to remember (and I'm sure ypu;'d ahve convered it). Ther is a difference between a spoiler and a wing. A spoiler is actually built onto the car, while a wing stands off of it. A spoiler's primary function on the back of a car is to reduce drag and reduce lift. A wing can reduce lift and increase downforce.

How does a spoiler reduce drag and lift? Well, that turbulence you mentioned over teh rear window of a sedan or coupe is the key. As the air flows over the roof, it becomes separated from teh car body, and becomes turbulent. this increases drag, and due to being a low pressure area, increases lift at the rear of the car. Virtually all modern sedans and coupes generate lift at the rear. This is bad, and well see why shortly.

A spoiler reattaches that laminar airflow, moving the turbulence to a point behind teh car (much like the Kamm effect rear body forms). By reattaching the airflow, you reduce drag over the car body AND remove the low pressure zone over the rear window nad truck, thus reducing lift.

A wing can do much the same things. A low wing can act like a spoiler, and reattach the airflow over teh roof, reducing lift and drag, but by virtue of being something else in teh airflow, doens't reduce drag as much as a spoiler. This may not be a problem, as it can go farther than jsut reducing lift, but generate a bit of down force. A tall wing adds drag, but by sitting in cleaner air, generates downforce and overcomes the drag and lift of the turbulence over th erear window. Since it can sit higher, it can generate considerably more downforce by being larger and with more angle of attack. This generates mroe drag, but in most situations, the reduction in top speed is worth the increase in overall speeds.

How much downforce can be generated at low speeds? Remember this: a Piper Cub can fly at 35 mph. Given enough wing to body ratio, you can achieve considerable downforce at very low speeds. This is why A-Mod autocross cars have such HUGE wings.

Now, back to why the lift generated by stock vehicles is bad. Most modern cars have fairly pointy noses and at least rudimentary air dams. This gives no lift to negative lift on the front of the car at speed. More so when lowered slightly. This means that at speed, the rear can get "light." And we're talking speeds easily achievable by street cars (70-90 mph). While this can cause los of control at high speed in a straight line, where it's most noticeable at is in decelerationg to enter a corner, like, say, a freeway offramp, or when avoiding something under deceleration.

Most cars are tuned to understeer. It's safer if a car looses traction at the front first. FWD's moreso, as they have a tendency to tend toward oversteer under deceleration into corners, and spinning out is dangerous for the average driver.(some FWD cars have been know to spin out just letting off the gas at 80-100 mph due to being so lightly loaded in the rear) Of course, this usually means pretty bad understeer in low speed corners. If you tune the suspension to go through low speed corners faster (i.e. increase rear swaybar size, or rear stiffness), then you run teh very real risk of spinning out from instability in higher speed corners (say, above 60 mph). In order to regain stability in higher speed corners, you can dial the suspension back out, but you lose tha ability to go quickly through low speed corners.

Enter the rear wing.

The wing doesn't weigh much, so it adds no appreciable mass to the rear of the car. It also adds little noticeable drag or downforce at speeds below 40-50 mph, so won't affect the suspension tune at those speds. But it CAN keep the rear planted as teh speeds rise. This lets you tune the suspension toward low speed neutrality (or even oversteer) and not be out of control entering corners at higher speed). This means corner entry speeds can be higher, allowing for greater speed THROUGH the corner, and a higher exit speed, needing less power to get up to speed after the corner. thus AVERAGE speeds can be higher. This is how a wing can add speed to a FWD car without adding so much as a single hp AND while adding top end drag (which really doesn't come into play until speeds beyond what you'd see on the street or even beyond what the car is capable of).

A typical fallacy is that wings are needed at high speed, not at low speed, and that the larger the wing, the higher a speed the car needs to go to make it work. In fact, the opposite is true. The larger the wing, the lower the speed it becomes effective at.

Another fallacy is that a large wing will cause enough downforce on the rear that the front wheels will lose traction as the car pivots around the rear axle. This does not happen. The front of the car already pushes down. The rear wing simply pushes down on the rear keeping it from lifting.

The final fallcy is that rear wings are seen as only an aid in accellerative traction, and thus only for RWD cars. There isn't a car on the planet that needs a wing to help it launch, as there are NO wings that can hold a car down when it's launching, and thus not moving very fast. Drag cars use wings for the top end stability and traction (losing traction at 100-150+ mph under heavy accelleration can be very bad, indeed.) Even FWD drag cars can use wings to keep stable at the top end (I've seen fast FWD drag cars spin out when letting off the gas going through the lights...)
I'm not mean. You're just a wuss.
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Old 12-27-2004, 10:35 PM   #5
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Good one ChrisV. I thought about explaining the difference between a spoiler and a raised wing, but I thought that would have made the article alot longer than I was willing to commit in those 20 minutes. Yes, good addition... I think this about wraps up rear end spoilers and wings. However if you touched it breefly but if you'd care to explain a little about front end downforce generated by front bumpers and aftermarket front bumpers.

I think thats why NIRA drag cars have a sloped front end with no openings to allow air to move over the hood of the car thus pushing it down.

...Everyone is entitled to their own opinion. It's just that yours is stupid...
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Old 12-27-2004, 11:26 PM   #6
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although i dont respect your attitude dsmer i do respect your knowledge and i enjoy reading each thread you post, therefore i dont hate you thx for the info
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