Adding a SPOILER! Will it help or hurt?

[CiVTEC]

It is about time someone explains whether a spoiler will work or not. I guess I will take a crack at it.

Question: Will adding a spoiler work?

Answer: Yes and No.

A spoiler is supposed to work by generating a downward force on the car as the car moves. The force generated by a spoiler is a result of the difference in air pressure under the wing and over the wing. Think of a spoiler as an inverted wing. For a spoiler, assuming we have non turbulent air flowing through it, there is a lower pressure (higher velocity) on the bottom side of the spoiler. The top side has a higher pressure (lower velocity) which causes a downward force to be generated.
A simple analogy would be to hold a flat piece of plywood while the car is moving. Now invert the plywood downwards in the direction your car is moving and you will feel it pushing you down.
If you want to look at it mathematically, then here it is:

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

where
Cl=lift coefficient
L=Lift
r=row=density of air
V=velocity
A=wing area (chord length * wing span)

Assume lift coefficient to be 1, then, L=((1/2)r(V^2)A)

This is very general but it works.

Of course the angle of attack makes a big difference in the lift force generated. The larger the angle of attack, the more force generated. However, it is at the expense of drag.

Now here is the problem. Given that we all know a spoiler will generate a lift force (I am using lift as the term for the downforce) if air flows smoothly through it, then the spoiler works. However, in the case of Hondas we drive, the air flow immediately after the roof is turbulent. The air isn't flowing in a streamline to the spoiler. Flow is now looked at in terms of large Reynolds numbers and it becomes almost unpredictable. It is possible for the air to flow backwards since there is a wake created from the stream of air separating at the end of the roof. So in this sense (for most of us), the spoiler is absolutely useless and creates a large drag on the car which SLOWS you down!

But don't despair, to get the spoiler working, we have to have it mounted above the boundary layer of flow and into a region where the air is smooth. So for all the people who have high (ricey) spoilers, they are actually using it to their benefit.

Of course there is another problem created: DRAG! In race applications where aerodynamics is key, such as Indy cars, the spoilers actually create a huge amount of drag on the vehicle. However, this drag force is negligible because of the benefits of having the car solid on the ground. In our case, the cars we drive rarely go fast enough where the negative lift force generated outweighs the drag force. With a large wing in the back, we are actually slowing our cars down (at least making it less efficient anyway). So in this sense, the spoiler is useless.

However, a spoiler adds aesthetic quality to the car (in my opinion) so it is still helpful. I have the tall stock spoiler on my civic and I can tell you it's probably useless but i'm planning on running some tests to see if it's non turbulent flow.

[CiVTEC]

More info:

Look at the Ford Focus RS, that wing does work!
http://www.fantasycars.com/1/News/Fo...fordfocus.html


A front lip (airdam) is actually more helpful since it prevents a lift force being generated from beneath the car.

Actually, I'm working on designing a simple solution to creating nonturbulent flow to the rear spoilers on any car. I will have it patented and it will sell like hotcakes in the aftermarket departments...make me rich!

[fritz_269]

Ever thought about the possibility that small, production car style spoilers and wings are not so much intended to increase downforce, but are actually mostly intended to reduce aerodynamic drag?

It's true. The idea is to cleanly slice the air away from the rear bumper so that it can mesh smoothly (more laminar) with the air from under the car. Without it, more of the air will follow the contour of the trunk and rear bumper (corbis effect) causing large vorticies, thus high velocities, thus a low pressure area directly behind the car (bernoulli), thus drag!

If you take a peek at almost any modern car, even if there is no sizeable spoiler or wing, there is a dimple or crease in the trailing edge of the trunk, and a sharp angle from the horizontal to the vertical plane of the trunk lid. In essence, it's a mini-spoiler; but it's for drag, not downforce.


Here's another question - at what speed do you have to be moving for a rear wing to provide enough downforce to increase your lateral-g cornering ability? What if you're in a FWD car that exibits understeer characteristics (like most Hondas)?
:smoker2:

[sparq]

Originally posted by fritz_269
Here's another question - at what speed do you have to be moving for a rear wing to provide enough downforce to increase your lateral-g cornering ability? What if you're in a FWD car that exibits understeer characteristics (like most Hondas)?
:smoker2:

Depends on the car, the wing, etc... but a good shot in the dark is around 80-90... and if you are taking a corner at this speed in a honda guess what?

[texan]

Aerodynamics start to take effect around 45 mph, wings and spoilers start to really show their effects at around 70 mph (depending upon design). And to clarify the difference between a wing and a spoiler, a wing is any airfoil with airflow over two sides, a spoiler only has airflow over one.

[LjasonL]

theres also the fact that 99.9% of spoilers, wings, or whatever u wanna call them were only made for looks and very little regard was given to whether or not they actually do anything good to your car. theres a joke i heard about roughly 95% of the spoilers out there produce maybe 2 pounds of downforce at 180 miles per hour. of course thats a big exaggeration but it illustrates my point.

[texan]

Originally posted by ldelaysionl
theres also the fact that 99.9% of spoilers, wings, or whatever u wanna call them were only made for looks and very little regard was given to whether or not they actually do anything good to your car. theres a joke i heard about roughly 95% of the spoilers out there produce maybe 2 pounds of downforce at 180 miles per hour. of course thats a big exaggeration but it illustrates my point.

But that's an opinion, not a fact. Last time I checked, automotive egineers still earned a living, which is why the state of automotive art continues to move forward little by little, even after 100+ years of r&d. Aerodynamics in particular is no different... form follws function and Cd levels, among other things, fall a little more every model year. It might look good, but that doesn't mean it won't work.

[LjasonL]

i wasnt exactly referring to stock spoilers when i made that comment i was referring to the ricer spoilers that are getting way too common. u cant tell me if u look in an advertisment for spoilers and u see theyre all more of a round bar than a flat or curved plane, and have this wierd bend in the middle, and then another one on top of that but that ones straight so the 2 planes are less than an inch apart in the middle but 3 or 4 inches apart on the sides, u cant say u think that actually does anything do u? of course i have no proof.

[texan]

Originally posted by ldelaysionl
i wasnt exactly referring to stock spoilers when i made that comment i was referring to the ricer spoilers that are getting way too common. u cant tell me if u look in an advertisment for spoilers and u see theyre all more of a round bar than a flat or curved plane, and have this wierd bend in the middle, and then another one on top of that but that ones straight so the 2 planes are less than an inch apart in the middle but 3 or 4 inches apart on the sides, u cant say u think that actually does anything do u? of course i have no proof.

Sure I'll agree with you, but your description is hardly 99.9% of the designs out there, more like .1%. The fact is most spoilers and wings are either factory made parts or modeled after factory made parts, and you'd be surprised at how many were designed for a function beyond their looks.

[Someguy]

The other "real" question is how many cars would see any overall benifit from a after market spoiler or wing. Very few.

[kane2k]

Simple fact is that a rear wing by itself is not a benefit to a front wheel drive car. The good news is that you would need a wing far beyond the standard ricer wing to get any real downforce benefit. The bad news is that any downforce produced exclusively by a rear wing serves to destabilize an FWD car. The reason for this is pretty simple physics. If you need a visualization take a Hot Wheels car and press down on its rear deck. How does this effect help a car that depends on its front wheels for the vast majority of all power inputs ( accel, braking, steering). Rear wings can be useful at high speeds and with an aerodynamically tuned car. First off to go back to the math lift is dependant on V^2.. drag follows the exact same equation but with Cd instead of Cl ( subscript would be really helpful right now ). Unfortunately Cl and Cd are also dependant on each other in a polynomial relation of angle of attack for a given wing. This means that if you want Cl to be high ( i.e. downforce at low speed ) Cd will also be high for any wing. So assuming you can get up to speeds where this is useful then you hit the next important concept.. centre of pressure. In a standard passenger car the centre of pressure is located in the approximate middle of the car.. which is perfect.. unfortunately that standard centre of pressure is where the lift acts.. not downforce. Adding a rear wing to this will cause the hot wheels effect discussed earlier. But then you ask, why do FWD race cars have rear wings. The answer is that the rear wing is the sticker on the side of your car that says "aerodynamic" instead of Greddy. Sure anybody can put the sticker on.. but you cant see whether or not the car actually is unless you look under the car.. as you would normally look under the hood for a turbo. An FWD racecar is quite an achievement because the front end and the bottom of the car have to be completely reshaped to turn that car's aerodynamics upside down. If the front end can cause air to move more quickly under the car and the underside of the car can cause this air to decompress while its under there.. then downforce is born. Once you have this very important effect... which by the way... is not a fibreglass front bumper you can slap on... then you can go ahead and add the rear wing. The rear wing then creates downforce which, by itself, moves the centre of pressure further back.. but also collaborates to strengthen the underbody downforce by reducing the base pressure at the back of the car.. thus speeding up the air under the car and further reducing pressure, and increasing total downforce which is properly centred. Wow that was long winded. Anyway long story short... if you have a wing on your FWD car without having redone the underbody of your car then I see you as a Ricer because you bought the sticker without buying the part.

[Moppie]

Nice Rant, but maybe you should go explain it to VW.
The new FWD Turbo Beattle has a small wing on the back of the hatch that opens at speeds above 60mph to help with high speed stablity. i.e. when doing 100mph+ down the Autobahn. It's sole purpose is to increase down force at the back of the car.

The Audi TT (shares the beattle platform and is most commonly FWD) also has a functional rear wing, in fact with out it the car becomes very dangerous at high open road speeds. (also why they added a great big hunk of lead to the rear bumber)

If your theory is totaly correct then both these cars, which dont make use of extensive areodynamic undertrays would suffer from instablity at high speed. However they both completly contridict what you have claimed.

[CornerCarver]

The problem with the Beetle and TT is that they were designed with so much "style" in mind and so little "function", that areodynamically they are the worst possible shapes you could conceivably make a car. Their shapes were so bad that aerodynamics caused the rear of both of these cars to lift at high speeds, causing instability and uncontrollable oversteer while careening down the autobahn. Thus, the wings and added weight in the back ends.

But I think the bottom line is that street driven FWD cars do not need rear wings. Add one if you like, but don't try to tell me it's for function... :P

[JasonAccord98LX]

perhaps you fellas mentioned this already--i may have missed it--but some newer porches and i think a model of the vw new beetle, have spoilers that come up whenever you reach a certain speed. And in the case of the porches, some of them actually automatically change the angles of their spoilers as you drive. e.g. the faster you go, the greater the spoiler angle, which increases downforce.

[Veetec]

Originally posted by CornerCarver
But I think the bottom line is that street driven FWD cars do not need rear wings. Add one if you like, but don't try to tell me it's for function... :P

I´m not sure about this because I don´t like most of the aftermarket spoilers and wings and personally I would never buy one => I never cared or thought about that! But somebody told me that the wing of an ITR for example will help you while cornering because of the additional pressure on the rear tires! I can see his point but I dunno why a daily driver should be equipped with one because IMO it´s really unnecessary. Like most of the other aftermarket parts.