Wings and spoilers function differently but the end result is the same. First a little background.

The shape of the Crossfire mimics a wing and as a result lift is generated. In order to have lift you must first have a high pressure zone and a low pressure zone. Obviously in the case of the Crossfire the high pressure zone is under the car and the low zone is above. Both zones are pushing against the car but the high pressure zone pushes with more force and lifts the car. The low pressure zone is not strong enough to counteract the high pressure one to push the car back onto the road. So how are these zones created? It's done by air being moved faster over the top of the car than the bottom. Look at the Crossfire in profile and mentally draw a line from the grill, along the hood, over the top and down the back. Now do the same except go underneath the car. Which line is longer? Since air molecules like to "hang together" those that go over the top of the car must move faster (since they have a greater distance to travel compared to those going under the car) so they can meet up with their respective drinking buddy molecules at the rear of the car. This accelerated mass of air has less "push" against the car compared to the air underneath. Another way of looking at it (and maybe easier to visualize) is that you have the same number of air molecules above and below the Crossfire. Since the molecules above have a greater area to cover (remember our mental line) they are spread out more and have less weight or push per square meter, foot or whatever the measurement of your choice.

What a spoiler does is to disrupt or kill the airflow over the top of the wing or Crossfire in our case. No airflow means that no low pressure zone is created.

Now I know that many members here already understand all of the above but I thought that those members not as knowledgable about aerodynamics would appreciate a basic primer.

How Chrysler determined the amount of downward force generated by the spoiler is beyond me. Part of the problem may be that it actually is a wing and is just misnamed. But from what I've seen of it I would call it a spoiler.

The gizmo on the back of the SRT-6 is a wing. It is shaped like a wing and is designed to allow air to flow over and under it.

Now for another basic primer on the SRT-6 wing. Imagine the Crossfire in profile in all it's lift producing glory. Now iimagine another profiled Crossfire except this one is little. Flip the little Crossfire over and stick it on the back of the big Crossfire. There's your wing. Just as air is pushing at the bottom of the big Crossfire, it also pushes at the bottom of the little Crossfire. And the push of the little Crossfire is what keeps the rear of the big one planted.

And why a wing on a SRT-6 and not a spoiler? I'll echo the thoughts of others that it is more effective for higher speeds and acceleration, and that it makes the SRT-6 standout more from the standard Crossfire. But I'll add that I think Chrysler went with it to keep costs down.

 

Bernoulli's Principle. For those who don't know, Google Daniel Bernoulli, Swiss physicist circa 18th century I think? Venturi effect. This is how fuel is atomized in a carb, pitot tube sounds a stall warning in a Cessna (or other GA aircraft), or the reason you can walk down a street with a light breeze on you and then you pass by the opening of an alley and almost get blown over. Neat stuff.

As the velocity of a fluid increases (in this case air; fluid- matter that takes the shape of ts container) it's pressure decreases and vice versa.

 

Thought about mentioning Bernoulli but didn't want to try to spell his name.

One minor clarification: a pitot tube lets in ram air and compares it against static air to give an air speed indication. A stall warning device is what activates a stall warning indicator in GA (general aviation - Cessnas, Pipers and Beechcrafts).

As for alleyway breezes, I've been told that a contractor who wants to put up a building in Chicago has to do a study on the effects the building will have on the local winds. There's a reason Chicago is known as the windy city.

 

This is something that will blow your mind. I saw a show on PBS I think it was Nova, that dealt with the handling of Indy cars and the wings that they use on the front and back. They must be adjustable to get just the right amount of balanced down pressure. Too much or too little on the front leads to serious steering problems, understeer and oversteer.

Here's the really cool part. (Is "cool" a scientific term. Ha!)

The Indy car, at track speed, over 200 MPH, has so much down force on the car from the wings, that if physically possible you could turn the track upside down and the car would stay on the track.

Thanks to all of the above responses I now understand the differences between a spoiler "spoiling" the airflow to eliminate lift and a wing creating down pressure.

Maybe this Nova program is available on DVD.

 

Excellent discussion and information on wings, spoilers and downforce guys, thanks.
Here's a little visual demonstration on how effective a spoiler, or in this case a wing, can be when working properly or not.
http://www.youtube.com/watch?v=53ALA2hLMZQ

 

All really good bits of information and all seem to understand the concept and design intent... The OEM provided some stability in the rearend when at lower speeds, but was really inadequate keeping the rearend planted at anything over 110 mph. So, here was my answer... I found that a 9 degree angle produced the best results to stick the rear and not scrub speed from excessive drag.

 

If I may gently pick a nit - a spoiler has many purposes to the aerodynamicist, and in some cases disturbing of laminar flow can produce downforce.

(By the way, the wikipedia articles on spoilers and car aerodynamics in general are completely wrong. I read the articles and they have comments like "Spoilers are almost always ignored in racing applications" (I am not kidding) which makes me wonder if the author of the Wikipedia article has ever watched a lemans, dtm, or even NASCAR race, let alone any other kind of motorsport.)

It's common on aircraft to view spoilers as a way to disturb laminar flow (and hence cause boundary layer separation and therefore reduce lift) on wings. However, cars are significantly different from aircraft. Why? Because of two reasons:

1. Automobiles want downforce, not lift.
2. Automobiles experience "ground effect". Normal aircraft do not.

Spoilers on automobiles have a vastly different function on cars than on aircraft due to "ground effect". More specifically, if it's possible to create pressure differentials on a car, then it's possible to create downforce.

Ground effect makes it possible to reduce underbody air pressure by using a splitter and relatively flat undertrays, along with undertray angle and venturis. However, the downforce produced can be multiplied by using a front and rear spoiler. In this case, front spoilers are used to create downforce by rapidly decreasing the airflow directly above the front splitter. As the boundary layer is destroyed, so the air velocity decreases significantly. Higher pressure on top with lower pressure on the bottom means a force is exerted - hence downforce.

 

The downforce generated is fairly easy with a wind tunnel and load cells. The weight of the car will change with different velocity across the car.

As for the other functions of the gizmo on an SRT 6, I suspect it was related to the higher top speed creating more lift, thus the bigger wing/spoiler/gizmo

The other thing it does quite well IMO is help the look of the car. I'm not hating the base/limited shape just think the SRT wing really cleans up the back of the car style wise.

 

it keeps out Butt down.... that is the best way I can say it!

 

This thread is fun!

Actually aircraft do experience ground effect. When an airplane (small, single engine general aviation aircraft here) gets within a wingspan of the ground the air under the wings is compressed. This thicker air provides more lift so instead of settling down onto the surface the airplane tends to float above the surface before landing. So flying within a wingspan of the ground is considered to be in ground effect.

Of course what is considered "ground effect" in automotive terms may be completely different in aviation terms.

Sonoronos, your explanations are good so could you please explain a little more what ground effect is for cars?

 

Agreed. At first I wasn't thrilled about the wing but like many others it has grown on me. I think the profile of the Crossfire rear looks kind of chunky and to me the wing breaks up that chunky appearance quite nicely.

 

Brent,

Thanks you make a good point. I always thought that airplanes had a different term "Wing In Ground Effect" but I must be mistaken. Also you're flattering me a bit here, I just know what I know and try to keep things correct in my head. Thanks for the correction.

As for cars, ground effect simply means that the presence of the moving ground is taken into account when attempting to reduce drag and produce aerodynamic downforce.

 

so not to be a heretic, but.....

if all the above discussion is true, then why not put the srt "wing" on the roadster and replace the little airfoil?

breaking up the contour of the "tubby" rear end of the roadster with the wing sounds like a good plan to me. perhaps someone has already done this? or if not, maybe someone could photoshop it.....

i do realize that the roadster probably should not/could not go as fast as the srt6 coupe. does the srt6 roadster have a wing or the little pop up spoiler?(i have yet to actually see an srt roadster).

thanks.......lakeside

 

The SRT6 roadsters have the big fixed wing.

 

It's been done by some in the forum. check the galleries.

 

A Formula 1 car can also theoretically be driven on a ceiling due to downforce created by it's wings.

The wings on F1 cars, etc are highly adjustable and set to different angles depending on the track. Tracks with very long, high speed straights (Monza) are considered low downforce circuits so the wings are set flatter to reduce drag and increase speeds on the long straights. Tighter tracks (Monaco) are considered high downforce tracks so the wings are set at such an angle as to maximize downforce. Top speeds are generally much lower so there is no speed penalty for such a setting. Watch a Formula 1 race and you can tell by the angle of the rear wing whether they are on a high or low downforce circuit - on a high downforce circuit the wing angle is so steep you can easily read the sponsor logo on the wing when viewed from in front of the car. On a low downforce circuit the wing is so flat you need be above the car to read the sponsor logo!

A couple of the greatest examples of aerodynamic wizardry were the Chaparral sports cars and Brabham F1 cars that made use of a large fan to suck air out from under the car and greatly increase downforce. The Brabham F1 cars ran a single race (and won it) before being banned. I've heard that sitting in the pits in neutral, if the driver revved the engine so that the fan spooled up, the car would drop 3/4" just sitting there!! Imagine a mechanical failure of that fan halfway through a highspeed corner!!

 

Thanks Mike, very interesting.

 

As for airplanes, "Wing in Ground Effect" sounds like the official description but I've only heard of it being referred to as plain old ground effect.

 

Like I said, I think you're right and I was wrong. I'm not very familiar with aircraft.

 

I'm not an engineer but I stayed at a Holiday Inn Express last night. After reading several of the posts on this thread I think I'll wash down 4 Excedrin with a glass of Black Jack and leave my spoiler alone!