I know a little about the theory of why an airplane flies, I think

Has to do with the air traveling further over the top of the wing than over the bottom of the wing as it is split in front of the wing, thus creating up vacuum......

Now, let's apply this to a car like a C3.
Same theory says the car gets light at X-speed.

Question:

1)What is the influence of the spoiler at diverting airflow from under the car? As in +for diverting and -for increased resistance to forward motion

2)How much resistance does said spoiler create and at what speed does that resistance become significant to effect handling of the C3

3)How low does the spoiler have to go to be effective in keeping the C3 to the ground?
OR, put differently
How do you figure out the distance from the ground at which above this point it is an absolute waste to install a spoiler?

4)A wing in the back will lift the front and affect steering control at X-speeds, right?

5)Some spoilers work and some are plainly a waste, but when?

4 and 5 are more thoughts than questions and need not be answered.

 

I think the spoiler has two effects. (1) it keeps high velocity air from flowing underneath the car. The underneath is not streamlined and high velocity air would create aero drag. (2) The spoiler forces air to flow either sideways or up over the hood of the car. The airflow over the nose of the hood, particularly for Corvettes with sloped hoods, creates a high pressure area over the hood area in front of the front wheels. This high pressure area helps push the nose down and keeps it from lifting. Also, high velocity air flowing under the nose also tends to push the nose up and once again the spoiler keeps this high velocity air lift effect from occurring.

 

I realize you are thinking about the air flow under and around that huge beautifull spoiler mounted on the front bottom of your car. But, In my understanding other than what was stated in the previous post about the smoothing of the passage of air over the not so smooth undercarrage of the car I think a forward "Air Dam" is more usfull to pull fresh air into your radiator and cooling system.
In the late 70's I was enamored by German engineering and owned several Posche and even a 1959 Karmengia with wooden bow rag top. Realy interesting cars. At any rate I read up on this type of wind tunnel testing and learned that the flip on the back of my 1974 RSR 911 would not even creat enough turbulence to affect the car till 120MPH. So I suppose a lot of it is academic but beautifull.

 

It's actually a lot more complicated than that, but ok. With that statement in mind, I'll take the position that armchair aerodynamics is too unreliable to be useful for anything. A spoiler may or may not do anything, and that depends on its design, the design of the car's body, the front and rear ride heights, etc. Here is some example data that I have posted before:



I think the same book (Katz - Race Car Aerodynamics) has data for front spoilers too, but I'd have to go scan it in.

Anyways, that should give you a general idea of the effects of a rear spoiler with changes in its size and angle. There is no guarantee that it has much to do with C3's though.

 

It's actually a lot more complicated than that, but ok. With that statement in mind, I'll take the position that armchair aerodynamics is too unreliable to be useful for anything. A spoiler may or may not do anything, and that depends on its detailed design, the design of the car's body, the front and rear ride heights, etc. Here is some example data that I have posted before:



I think the same book (Katz - Race Car Aerodynamics) has data for front spoilers too, but I'd have to go scan it in.

Anyways, that should give you a general idea of the changing effects of a rear spoiler when size and angle are varied. There is no guarantee that it has much to do with C3's though.

 

Ok, here's the data for a front spoiler, although it is much simpler than the type you are describing.



Are these types of simple air dams stock on C3's? Mine has something like that made out of (what I was told is) kevlar, but I don't know if it is original or not. I know I've seen it on C5's also.

 

Any road vehicle is low enough that any amount of front spoiler/air dam is going to divert a significant portion of airflow. The undercarriage is a mess, and while an undertry would be nice, it would be way too much of a pain. Undertrays are alot easier on rear/midengined cars where you don't have drivetrain and exhaust to worry about!

Anyway, our best bet is to keep the air from getting in there in the first place. You want the spoiler as far forward and low as possible (e.g. 80-82 custom front ends with the "snow plow" look) Further back creates a high pressure area of lift under the front of the nose. This helps feed the radiator but it would be better to get that air front the front grilles. In short, the lower you can get, the better. A sacrificial plastic/rubber strip down from a pacecar spoiler would be a good idea. Side skirts are also crucial for minimizing undercarriage airflow.

I'm not sure, but I don't think a rear wing will actually lift the front end. What it will do is reduce rear lift, and without an proportional amount of front lift reduction the car will be unbalanced. That would give the feel of a lighter front.

A rear spoiler needs to be tuned in a wind tunnel or a competition environment. The usual bolt on things are a crap shoot at best, a virtual parachute of drag and even added lift at worst!

That's about all I know on the subject, good luck with your project

 

Unless your planning on running down the Mulsanne at LeMans or in the Silver State, it has almost no value in terms of aerodynamics. On a vette, especially one with A/C, it's all about cooling. For this purpose, they are vital and very effective. I couldn't drive mine without one (that's the way I got it), and the stock 1" piece was a joke, so I made a 3" spoiler in the same style as stock out of black conveyor belt material. It works great and looks super. It has a sort of matte, semi-gloss black finish, and I don't have to worry about hitting curbs.

Hans

 

If you intend to exceed 100mph or so, it is worth it. If you're going to have alot of track time, then ground-scraping things become more realistic, as well as a serious rear spoiler. An 80-82 rear bumper on 74-79 cars is an improvement too.

The best thing you can do to prevent lift, front or rear, is keep the air out from under the car.

-Chris

 

Thanks for the replys guys!!

A few more questions:
1)Does the spoiler have influence around turns at all?
or
Only at high speeds (above 100MPH) on a straight surface

2)How much resistance will you have against forward motion
with a spoiler at 150MPH
.......as compared to......
How much front end lift will you have without a spoiler
at the same speed

 

some spoilered add a couple hundred lbs of down-force a speed's as low
as 80 but People call it "Rice" they use it in EURO rising
on everything but it isn't rice there

 

The whole point of a spoiler is to go around turns faster. A car that has too much power for its own good will eventually have so little weight on the ground that it will be unstable even in a straight line, but a spoiler has benefits long before that point is reached.

The more weight you have on your tires, the more cornering force they can produce. Spoilers attempt to keep weight on your tires so you can still turn at high speed. Lift is pretty insignificant at the speeds most of us rush around corners though.

That depends on the spoiler. You can see trends off of the graphs I posted. If you're not familiar with the terminology, CL_R=rear lift coefficient, CL_F=front lift, CD=drag. The graphs are plotting the changes in these coefficients when adding a spoiler. A typical drag coefficient for the entire car might be 0.4, and lift coefficients tend to be around 0.3 for a typical sedan. That lift value is kind of a wild guess though.

 

That's more properly called a wing than a spoiler, and it would add so much drag that the car probably couldn't break 100 mph anymore. High downforce race cars often have drag coefficients of 1-4. And no, I didn't leave out a decimal point there.

 

Just remember a car IS a wing, look at the side view.
The only to keep it from lifting is to keep the air from going under it. If the pressure under the car is less than the pressure above it it won't fly. Of course it wil probably increase the drag too.

Another benifit of the front spoiler is that the kiickups in front of the front wheels create a low presure region in the wheel area sucking more air out of the engine compartment.

 

Then what is this???
How a wing works
I thought downforce on racecars was drawing air out from under the car "sucking" the car to the track & the wing(s) create DOWNforce to push the body of the car down more the faster it went?????

 

Jcswm,
Very interesting link!!

I like the theory about the rear starting vortex that causes wing circulation.

Integrate that with Newtonian physics that says basically that action equals reaction.
If you now look at a wing that not only slopes downward after the hump, but the WHOLE wing itself has a tilt towards the downside from front to back.

What they say is that the redirection of airflow as it flows over the wing toward the downside (where redirection towards the downside is more on the upper surface than the lower surface due to the form of the wing) will cause a reaction of upward pressure (Newton).

If this upward pressure breaks atmospheric pressure due to increased velocity from front to back.....Viola, you have lift.

Interact that with the starting vortex behind the wing, that will induce circulation around the wing in this form....
1)Forward flow on bottom of wing
2)Upward flow at leading edge of wing
3)Rearward flow on top of wing
4)Downwards flow at back of wing

Why?....Higher air pressure below wing (lower velocity and less downward deflection than on top of wing, due to the form of the wing) Air will flow from higher toward lower pressure. Due to increased velocity of the air on top of the wing (bigger surface area and downward angle due to form of the wing) the air will flow forward on the bottom of the wing toward the leading edge and then up and over etc....

The other important thing to remember is that as the air is split at the leading edge and flows backward over the top and bottom of the wing, it does NOT take the same time to reach the back of the wing.

INTERESTING (if I interpret it all half correctly...I am no engineer)

SO, C3 airflow (forget the spoiler for a second).....

is redirected upwards over the car that looks like a raindrop lying on a flat surface. Newton says there should be downward pressure.....but maelstrom vortexes under the car due to not having a smooth surface (my made up logic) will cause inefficient airflow under the car that will trap a bubble of air under the car (like a hovercraft).
SO, downward pressure (Newton) around the trapped bubble will not induce lift, but instability.....now I'm confusing myself

If you put a spoiler on front of the car, it reduces the flow under the car and thus the bubble of vortexes.
The circulation theory says that (my opinion) the air should flow forward on the bottom of the car ((1)higher pressure than above as there is less velocity (2)forward motion increases rearward velocity of air over the top of the C3, so airflow has to go downwards at the back)

The spoiler reduces airflow through the bottom of the C3, so we get closer to a vacuum condition (relatively) and now Newton can do what I want him to do.You can never however get rid of all the airflow under the car as you have to have wheels and therefore the higher the speed the more "bubble" effect you will get under the car.

What I'm saying is that a spoiler only postpones the inevitable effect of instability under the car as speed increases.


Wait, now I have to go and think about it some more as I have just created a monster in my mind....I should stick to my day job :-)

Now, that is some crude logic.....I need some beer

 

Anyone else have some more concrete input on this?

My last post was pretty speculative, so I don't really know what percentage of "correctness" is in that "logic".

 

More???
there is a few good books listed at the bottom of the page.
They have lied to us for years and they knew it!!!
Google search "Wing Theory" for lots more info!!!

 

Summerfun, it's hard to understand what you're trying to say, so I can't really comment on it. As a general warning, though, the aerodynamics of (cruising) airplanes and cars is completely different. The ground plays a huge role in modifying the airflow around a car. Most "intuitive" explanations of how an airplane wing works assume that it is completely isolated. Of course real aeronautical engineers have to worry about ground effects, but they have to work very hard for it.

 

That's why I sucked the "under carriage vortices" and "hover craft" ideas out of my hat.
Was waiting for someone to come along and rip that apart to correct me and put me on the right track!

I'll go searching the net to see what I can come up with.

The way I see it, it is all sweet and good to have the best of suspension and brakes but if you know the ins and outs of the huge forces at play around your car while driving it, it will:

1)Help you to expect when things will go wrong and why
2)Get you to learn how to correct when those things happen and actually understand why you are doing it
3)Modify certain things on and around your car to reduce the force of said effects on your car to create a safer and more stable machine around you.
4)Its nice just to learn new things and understand them.