# Ground Effects Packages



## manic_monkey (Jun 24, 2008)

GeeDub said:


> In all the posts I've read here, I see no mention of aerodynamics as influenced by "ground effects" packages. It seems to me that above 45MPH, a wing (upside down spoiler) would lift the vehicle somewhat and lighten the load the batteries are pushing. Thereby giving a greater range?



the energy required to 'lift' the car up has to come from somewhere. The energy would be taken from the cars momentum and translated into lift. you wouldnt gain anything im afraid.


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## Manntis (May 22, 2008)

GeeDub said:


> In all the posts I've read here, I see no mention of aerodynamics as influenced by "ground effects" packages. It seems to me that above 45MPH, a wing (upside down spoiler) would lift the vehicle somewhat and lighten the load the batteries are pushing. Thereby giving a greater range?


There's no such thing as "ground effects packages". 

Ground Effect is an effect. Various things can cause the effect, such as properly shaped undertrays, air dams, side skirts, and wings.

A wing is not an upside down spoiler. A spoiler is a lip mounted flush to the trunk (no air goes underneath it) that 'spoils' the airflow as it comes off the back edge of the trunk. A wing, in contrast, has air flowing above and below it and is typically similar to an airplane wing in cross section, but inverted so the airflow creates downforce rather than lift. Lift is undesirable as it unloads the nearby wheels, thereby giving less traction and can, at speed, cause a car to spontaneously flip or roll.


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## GeeDub (Jun 25, 2008)

Manntis said:


> There's no such thing as "ground effects packages". Ground Effect is an effect. Various things can cause the effect, such as properly shaped undertrays, air dams, side skirts, and wings.


I knew that... Honestly. I was just trying to find a catch all term that would convey the idea.



Manntis said:


> A wing is not an upside down spoiler. A spoiler is a lip mounted flush to the trunk (no air goes underneath it) that 'spoils' the airflow as it comes off the back edge of the trunk.


 I called it a spoiler 'cause that's what my Bill of sale on my Trans Am said 20 years ago. It was elevated and had space below it. Does this mean it was a totally useless piece of pretty non functioning plastic (marketing hype) or just that they should have called it a wing?



Manntis said:


> Lift is undesirable as it unloads the nearby wheels, thereby giving less traction and can, at speed, cause a car to spontaneously flip or roll.


Actually, that was sort of the idea (not the flipping/ rolling part). Since the batteries add substantial weight over the OEM ICE setup, the extra traction should not be needed. I was thinking relieving the weight means less energy is required to push the vehicle at higher speeds. 
Since the air is being displaced _anyway_, sending it to the side does nothing, sending it *over* the vehicle adds weight (as far as the motor is concerned), so sending it *under*, to lift, should extend the range.

What am I missing?


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## Manntis (May 22, 2008)

GeeDub said:


> I called it a spoiler 'cause that's what my Bill of sale on my Trans Am said 20 years ago. It was elevated and had space below it. Does this mean it was a totally useless piece of pretty non functioning plastic (marketing hype) or just that they should have called it a wing?


marketing morons are quite adept at misusing words. It was a wing, though a mostly cosmetic one.



GeeDub said:


> Actually, that was sort of the idea (not the flipping/ rolling part). Since the batteries add substantial weight over the OEM ICE setup, the extra traction should not be needed. I was thinking relieving the weight means less energy is required to push the vehicle at higher speeds.
> Since the air is being displaced _anyway_, sending it to the side does nothing, sending it *over* the vehicle adds weight (as far as the motor is concerned), so sending it *under*, to lift, should extend the range.
> 
> What am I missing?


it's the air's resistance to the vehicle punching through it that causes drag and therefore the huge increase in power needed to go faster. vehicle weight is mainly an issue getting the car moving from a stop, accelerating, and braking. 

When the Lamborghini Countach was first made available with a wing, it was designed to look cool - but would actually cause lift. It was hastily redesigned to prevent high speed loss of control.

There's really not much you can call 'extra' traction. People who live in icy climates often put heavy objects in the trunk to increase traction, since fuel economy is worth less than a snowball when you're sliding into the ditch. 

Formula 1 cars are made as light as regulations will allow. If lift was desirable they wouldn't spend millions on wings and undertrays that provide downforce, the opposite of lift. When it comes to road vehicles, traction = good, lift = deadly.


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## ww321q (Mar 28, 2008)

*Ground effect* is term applied to a series of aerodynamic effects used in car design, which has been exploited to create downforce , particularly in racing cars . In racing cars, a designer's aim is for increased downforce, allowing greater cornering speeds . The bad thing is , like most things , downforce comes at the cost of increased drag . What you are looking for in an EV is a different effect called "streamlining" and the reduction of drag . J.W.


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## Manntis (May 22, 2008)

Nope. He wasn't asking about reduced drag - he was asking about using lift to make his car lighter.


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## ww321q (Mar 28, 2008)

I was just trying to point him in the right direction ...J.W.


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## ww321q (Mar 28, 2008)

And lift also comes with the same cost *"DRAG" *J.W.


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## Heretic (May 8, 2008)

JW would probably be on the right track. Make the underneath of the car perfectly flat, with something like some sheer pvc or something similar, and forget any wings on top - a friend of mine uses this principle in combination with front to rear tilt via suspension adjustment on his track battling 4wd ICE car. If you want to get more serious, you can use a rear diffuser to channel the air (undercar boot spoiler)


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## raceguy333 (Jun 18, 2008)

ww321q said:


> *Ground effect* is term applied to a series of aerodynamic effects used in car design, which has been exploited to create downforce , particularly in racing cars . In racing cars, a designer's aim is for increased downforce, allowing greater cornering speeds . The bad thing is , like most things , downforce comes at the cost of increased drag . What you are looking for in an EV is a different effect called "streamlining" and the reduction of drag . J.W.


 
Lift (or downforce) doesn't always come at the expense of drag. Race teams also spend much time in the wind tunnel working on l/d numbers(lift/drag... aero efficiency). There may be changes you can make that will increase lift/downforce that have little to no affect on drag as well as changes to reduce drag with little to no affect on downforce. That's not to say that most changes don't add or remove both, but just that l/d is something that race teams look at as well.


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## mattW (Sep 14, 2007)

You could potentially slightly reduce the rolling resistance of the rear wheels with a wing at the back of the car, an efficient aerofoil shouldn't cause too much extra drag but I doubt the lower RR would be significant when the small extra drag was factored in. Its a nice idea but I think you'd do better with streamlining- grill block, belly pan, passenger mirror delete, boat-tailing etc see the wiki for streamlining projects .


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## ww321q (Mar 28, 2008)

raceguy333 said:


> Lift (or downforce) doesn't always come at the expense of drag.


Sorry but you are wrong . You can make changes to a car that increase downforce and reduce drag but that's not what I was talking about . If you reduce the drag to the very minimum and then increase lift , drag will increase to . It's not magic , it's physics . You can take a flat piece of sheet metal and hold it so it cuts straight through the air . That's it's minimum drag reshape it in any way and drag will increase . If the sheet metal piece was square you round the corners but thats about it . J.W.


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## ww321q (Mar 28, 2008)

Also weight on an EV is only really bad when starting to accelerate or going up hill . In both those cases lift would help zero . J.W.


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## Number 6 (Feb 18, 2008)

ww321q said:


> Also weight on an EV is only really bad when starting to accelerate or going up hill . In both those cases lift would help zero . J.W.


This is true. Also, the real culprit is mass, not weight - an aerodynamic device may reduce weight (a.k.a., force of the car on the ground) but not the mass of the car (plus the added mass of the wing) and it is that mass that formulates into Newton's equations of motion. Reducing weight factors in reducing the down force on the tires and therefore reduces the net rolling friction (this would be a very slight reduction due to the ratio of the weight of the car to the very very very small amount of lift created at slower speeds), but the mass is always there and still requires the same force to change its motion, as in accelerating. At higher speeds, when lift may have a greater factor in reducing the friction of the tire/pavement contact, the increase drag of the lift (remember, air resistance is quadratic with regards to speed) has to be factored in as a penalty for the lift, so the overall benefit of such a method is not that certain as far as fuel savings. Just my 2 cents worth.


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## raceguy333 (Jun 18, 2008)

ww321q said:


> Sorry but you are wrong . *You can make changes to a car that increase downforce and reduce drag* but that's not what I was talking about . If you reduce the drag to the very minimum and then increase lift , drag will increase to . It's not magic , it's physics . You can take a flat piece of sheet metal and hold it so it cuts straight through the air . That's it's minimum drag reshape it in any way and drag will increase . If the sheet metal piece was square you round the corners but thats about it . J.W.


 
I believe the statement in bold says that I was right.

We are talking about a real world application here, not a flat plate of metal. Given that the cars people are using for EVs are not already optimized, there may be some changes that give the effect of increased downforce and reduced drag. There will also be a plethora of changes that give one at the expense of the other.

I work for a race team (NASCAR), and I have been to the wind tunnel on occasion. I know that we are almost always working towards maximum downforce on a car, but when evaluating cars, we also look at the L/D number for most race tracks. The only exception to this rule is when we are evaluating cars for tracks in which the horsepower is restricted and then we go for all out reduction in drag even at the expense of downforce.


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## ww321q (Mar 28, 2008)

We are talking in relation to an EV not NASCAR . Unless you are racing the EV Streamlining is what you want and not Downforce . 


raceguy333 said:


> The only exception to this rule is when we are evaluating cars for tracks in which the horsepower is restricted and then we go for all out reduction in drag even at the expense of downforce.


And your own statement makes MY point !! J.W.


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## ww321q (Mar 28, 2008)

Number 6 said:


> This is true. Also, the real culprit is mass, not weight - an aerodynamic device may reduce weight (a.k.a., force of the car on the ground) but not the mass of the car (plus the added mass of the wing) and it is that mass that formulates into Newton's equations of motion. Reducing weight factors in reducing the down force on the tires and therefore reduces the net rolling friction (this would be a very slight reduction due to the ratio of the weight of the car to the very very very small amount of lift created at slower speeds), but the mass is always there and still requires the same force to change its motion, as in accelerating. At higher speeds, when lift may have a greater factor in reducing the friction of the tire/pavement contact, the increase drag of the lift (remember, air resistance is quadratic with regards to speed) has to be factored in as a penalty for the lift, so the overall benefit of such a method is not that certain as far as fuel savings. Just my 2 cents worth.


 thank you ! J.W,


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## raceguy333 (Jun 18, 2008)

ww321q said:


> We are talking in relation to an EV not NASCAR . Unless you are racing the EV Streamlining is what you want and not Downforce .
> 
> And your own statement makes MY point !! J.W.


 
"Even at the expense" is not the same as "always at the expense."

I'm not arguing that you want downforce on an EV. Actually, unless operating at city speeds like most EVs do, you aren't really creating enough downforce/lift to do much if anything...

I was mainly commenting concerning the discussion of adding lift to the car to reduce effective RR (which again I'll say, at those speeds, probably isn't enough of a percentage of RR to matter... especially since drag is the dominant resistance). You CAN add lift (or downforce... negative lift) without adding drag. You can also streamline the car without hurting any existing downforce. I just thought I'd add this to the discussion earlier because of the generalized statement that was made about all lift creating drag--it doesn't necessarily.


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## DVR (Apr 10, 2008)

raceguy333 said:


> You CAN add lift (or downforce... negative lift) without adding drag........ I just thought I'd add this to the discussion earlier because of the generalized statement that was made about all lift creating drag--it doesn't necessarily.


That statement is wrong.

Look at aircraft. They are governed by thrust, lift & drag. Lift creates Drag and drag is overcome by thrust.

Any changes made to drag, (pitch or yaw) affect the amount of thrust needed to sustain lift and vise versa.

Otherwise planes wouldn't need to apply more power to climb. To climb they have to increase lift, the drag increases and more power is needed to overcome the drag or the plane will stall.

Now with a car, make it slippery and it can go fast. No downforce and the car can go faster, but is unstable at high speeds. Add downforce and you can now go around corners but your slower down the straights. WHY? Because you've increased drag.
Racing teams (apparently even NASCAR) are constantly trying to balance the need for speed with the need for control by trying to find the minimum amount of downforce they can run on a given track so as not to waste power.

F1 cars are not as fast as a Veyron. Why?

DRAG from the downforce created by their aero-package..

One comes at the expense of the other.

Use a wing for either for lift or downforce and it will increase drag.


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## raceguy333 (Jun 18, 2008)

DVR said:


> That statement is wrong.
> 
> Look at aircraft. They are governed by thrust, lift & drag. Lift creates Drag and drag is overcome by thrust.
> 
> ...


 
Most changes that are commonly thought of (wing angle, yaw, pitch) will increase both downforce/lift and drag. I do not agree with that.

There are changes that can be made (I repeat) that will help both situations. To give a small example:

NASCAR does not allow teams to run a belly pan. Teams starting experimenting with using the brake duct boxes and the radiator ducting under the nose of the car to basically create a pan from the front wheels forward. This not only reduced drag by smoothing out the surface and not giving the air places to stagnate, but also created downforce just like the belly pans/tunnels on an F1 car are used for. NASCAR caught on to this though and have put in rules to regulate both the size and placement of flat surfaces under the nose now.

If it was a direct relationship ("for this car I can have x drag and x downforce... or I can change something and have y drag and y downforce"), teams would not spend millions of dollars a year (even in NASCAR) going to the wind tunnel and looking at not only downforce, but looking at L/D numbers (aerodynamic efficiency).

About the Veyron--don't forget it's not all about Cd, frontal area plays into the picture as well. I can't find any numbers for frontal area of an F1 car... oh, and 250 extra hp for the Veyron.


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## 3dplane (Feb 27, 2008)

raceguy!
While I have to agree with the others on the subject I do understand what you are trying to say.However you seem to be forgetting something.
Your point of there can be something done to improve both(less drag more downforce) is only true on a car where it was already overdone.Example:
somebody went we need more down force so they increased the angle of the spoiler(or whatever its called on the back of the car) to the point where it's stalled, and the added drag became horrible but they thought they gained more downforce.So if now we take this airbrake and decrease it's angle,it will now have less drag and provide more downforce than before. Whaddaya think? 
DVR! No offence but aeroplanes need to throttle up to climb because they are fighting gravity,not because they changed angle of attack(wich caused drag).
Otherwise they would need to throttle up to get into a dive from level flight.(achieve downforce) Anyways GRAVITY SUCKS!  Barna


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## raceguy333 (Jun 18, 2008)

Look guys, I was merely trying to point out that such a blanket statement as "you cannot increase lift/downforce without increasing drag" is not true 100% of the time. I have been to the wind tunnel... I have seen the numbers on various changes made to a car. I agree that things done with wing angle or when talking about putting a car into yaw are going not going to do that. There are some things that can accomplish increased downforce and decreased drag such as the "partial belly pan" like I brought up earlier.

Again, I've been there and seen it. I just wanted to give you some knowledge from my experience. If you don't believe me, that's fine. I am tired of beating a dead horse.


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## 3dplane (Feb 27, 2008)

Hey! Please don't feel offended.As you know aerodynamics is far more complicated than a person with average interest toward it would think.
I believe we all can learn from a discussion like this,when everyone throws in a little of their own perspective. Barna.


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## DVR (Apr 10, 2008)

raceguy333 said:


> Most changes that are commonly thought of (wing angle, yaw, pitch) will increase both downforce/lift and drag. I do not agree with that.


You must live in a very lonely place then



raceguy333 said:


> There are changes that can be made (I repeat) that will help both situations. To give a small example:
> 
> NASCAR does not allow teams to run a belly pan. Teams starting experimenting with using the brake duct boxes and the radiator ducting under the nose of the car to basically create a pan from the front wheels forward. This not only reduced drag by smoothing out the surface and not giving the air places to stagnate, but also created downforce just like the belly pans/tunnels on an F1 car are used for. NASCAR caught on to this though and have put in rules to regulate both the size and placement of flat surfaces under the nose now.
> 
> If it was a direct relationship ("for this car I can have x drag and x downforce... or I can change something and have y drag and y downforce"), teams would not spend millions of dollars a year (even in NASCAR) going to the wind tunnel and looking at not only downforce, but looking at L/D numbers (aerodynamic efficiency).


If you took a hypothetically perfect car that had a CD of 0, added downforce the CD would no longer be 0
You may be able to make improvements to an imperfect car that would buy you back the drag losses of the downforce but that same car would then be slipperier if you removed the downforce.


raceguy333 said:


> About the Veyron--don't forget it's not all about Cd, frontal area plays into the picture as well. I can't find any numbers for frontal area of an F1 car... oh, and 250 extra hp for the Veyron.


So you think the frontal area of an F1 is larger than a Veyron? Oh boy
YOU think that 250HP is the reason why the Veyron is faster? Take the wings and bellypan OFF a F1 add a crap load of weight to keep it on the ground and it will make a veyron look S-L-O-W.

 " Honda test driver Alan van der Merwe, piloting a race legal Honda 007 along a seven mile-long salt straight, achieved an aggregate speed of 220.571 mph (354.975 km) over two runs of a measured mile."

Imagine what it would do if you took it's wings off

Then put those same wings on a Veyron and see if it will do 220mph

I dont care how often you repeat it, you were wrong in saying
Quote:
Originally Posted by *raceguy333*  
_ You CAN add lift (or downforce... negative lift) without adding drag.

_ It's physics 101!


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## DVR (Apr 10, 2008)

3dplane said:


> DVR! No offence but aeroplanes need to throttle up to climb because they are fighting gravity,not because they changed angle of attack(wich caused drag).
> Otherwise they would need to throttle up to get into a dive from level flight.(achieve downforce) Anyways GRAVITY SUCKS!  Barna


No offence taken
Yes and No

_Here are the official, conventional definitions of the so-called four forces:_ 


 _Lift is the component of aerodynamic force perpendicular to the relative wind. _
_Drag is the component of aerodynamic force parallel to the relative wind. _
_Weight is the force directed downward from the center of mass of the airplane towards the center of the earth. It is proportional to the mass of the airplane times the strength of the gravitational field. _
_Thrust is the force produced by the engine. It is directed forward along the axis of the engine._











At a given speed a wing is generating X amount of lift. This lift has to be = to the weight of the plane. however lift also creates drag because you cant have 1 without the other. Drag is trying to slow the plane. All these forces need to be balanced to achieve stable, controlled sustained flight.
Change 1 and it will effect the others. 
Increase the angle of attack (increasing lift), without increasing power to maintain thrust will cause the drag component will rise. The plane will slow down (just like a car), not because of gravity because the weight is unchanged, but because of the drag. This exasperates the problem until finally the wing stalls.

A plane throttles back to decend. Without the needed thrust the plane will slow, because the thrust no longer balances the drag and you will come down, there's nothin you can do about it (no matter how often someone says it wont). Try pulling the nose up drag goes up and you'll stall! You have to balance everything by trading height for speed using gravity for thrust to overcome the drag.
It's what gliding is all about


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## mattW (Sep 14, 2007)

You guys are just miscommunicating with each other and missing each other's points.

It is true that the minimum drag shape, a tear drop, has no lift and adding any lift would increase drag.

Lift is just force perpendicular to the motion of the vehicle, so it takes some of the energy of the car going forwards and directs it upwards or downwards.

Pragmatically though because our cars are far from ideal there are many mods which change the lift characteristics of the car while decreasing drag. A belly pan is a good example, it increases the flow under the car (reducing drag) and alleviates the high pressure build up from the messy underside (reducing lift/adding downforce). 

Would the car be lower drag without the added lift/downforce? Of course, but sometimes it is hard to do one without the other (i.e. the belly pan will most likely reduce the lift that is already there rather than making negative lift, i.e. not a net force downwards, just less upwards force)

Nevertheless we should always be trying to reach the ideal tear-drop shape with zero lift/down-force.

So you are sort of all right, if you start with zero lift, increasing lift will increase drag. If you start with a net lift, you can reduce both lift and drag with the same mod. Truce?


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## DVR (Apr 10, 2008)

mattW said:


> So you are sort of all right, if you start with zero lift, increasing lift will increase drag. If you start with a net lift, you can reduce both lift and drag with the same mod. Truce?


Awww jeeeez.......... OK MUM


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## raceguy333 (Jun 18, 2008)

raceguy333 said:


> Most changes that are commonly thought of (wing angle, yaw, pitch) will increase both downforce/lift and drag. I do not agree with that.


Oops.. heh. If you check all my other posts I said I did agree. This was a typo.. girlfriend was rushing me off the computer.



DVR said:


> If you took a hypothetically perfect car that had a CD of 0, added downforce the CD would no longer be 0
> You may be able to make improvements to an imperfect car that would buy you back the drag losses of the downforce but that same car would then be slipperier if you removed the downforce.
> 
> So you think the frontal area of an F1 is larger than a Veyron? Oh boy


Now I did not say the frontal area of the Veyron was smaller than the F1. I was saying the opposite. F1 with higher Cd but lower area... not sure the actual numbers, but I'd like to see some drag force numbers for the F1 instead of just talking about Cd... since the two frontal areas are (I would imagine) so different.



DVR said:


> If you took a hypothetically perfect car that had a CD of 0, added downforce the CD would no longer be 0


We are not talking about "perfect" car... we are talking about a real car in the real world...



mattW said:


> Pragmatically though because our cars are far from ideal there are many mods which change the lift characteristics of the car while decreasing drag. A belly pan is a good example, it increases the flow under the car (reducing drag) and alleviates the high pressure build up from the messy underside (reducing lift/adding downforce).


Thanks Matt. The same thing I said I've seen in the wind tunnel with my own eyes (although it did result in a gain of downforce because the car already had several hundred pounds of it without the partial pan.



mattW said:


> So you are sort of all right, if you start with zero lift, increasing lift will increase drag. If you start with a net lift, you can reduce both lift and drag with the same mod. Truce?


Truce if we must.


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## raceguy333 (Jun 18, 2008)

DVR said:


> Imagine what it would do if you took it's wings off


246.908mph 

http://www.topgear.com/content/news/stories/973/


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## Kreb (Apr 30, 2008)

I think that all this theoretical stuff misses the point. If you use aerodynamics to create downforce, it'll take more power to move the car. If you use it to produce lift, you create an ill handling, dangerous car. Let's assume for the sake of arguement that you could create 1000 pounds of lift at 60 MPH. Well what happens if you have to change direction? Slow down? get a nasty crosswind? Your grip on the road is tenuous, you get nasty weight transfer and you run the risk of killing yourself. A lousy price to pay for extended range IMO.


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## ww321q (Mar 28, 2008)

I was talking about streamlining which creates neither lift or down force . Streamlining is to reduce drag . Streamlining is to make a design move through air or water as effortlessly as possible . And as Far as an EV goes lift wouldn't help reduce rolling resistance as much as it would create drag . Your handling problems wouldn't start from lift as much as the little footprint these skinny tires have that EVs use and high pressure . J.W.


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## Bugzuki (Jan 15, 2008)

You need to remember that the shape of a car is by nature a similar shape of a wing - flat on the bottom and round on the top. This shape causes the wind on the top to spread out and apply less pressure per square inch then is applied on the under side. So, by nature every car is going to have lift, more as speed goes up.

Sorry, but contrary to common belief the tear drop shape is not the perfect shape. Just a tear drop is a liquid that is falling so naturally the gravity pulls most of the liquid to the bottom and the drag pulls some of the water up to a point at the top.

The biggest thing to look out for is frontal area. If you need a certain size car there is no way to get around frontal area. So, the only thing you can do is apply the area gradually. An example I will use heavy duty trucks. With a COE (Flat Front) you can have a longer trailer - according to legislation. But, with the engine sticking out front you break up the frontal area so you get better gas mileage. The better gas mileage turns out to be the money maker so that is why here in the USA almost every truck on the road is a conventional design. In Europe the rules are different and the nose takes up too much of the shorter trailer space, so they go for the COE configuration.

A gernie lip is a way of adding some downforce to the back of a car while decreasing lift, but I do not know of a way to increase lift without increaseing drag. Taking away lift is nuetralizing the car so there can be some gains, but trying to add lift to what the car is already trying to do is not going to help.

If you want to do a difusser you would need to do it under the entire car and not just at the back under the bumper. The point of a difusser is to create a vacuum effect under the car. This is done by having the underside of the car start low to the ground at the front and angle upward to the back. This allows only a small amount of air under the car and then forces it to expand as it moves toward the back. But in order for this to work you need to have the vertical pieces to keep new air from coming in along the sides. That is what those fins under the car are for. So, all those tuner cars with the difusser rear bumpers are just show and no function.

Hope that adds some light
It's late got to go to bed.


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## DVR (Apr 10, 2008)

Bugzuki said:


> This shape causes the wind on the top to spread out and apply less pressure per square inch then is applied on the under side.


No, not quite. What it does is force the air to actually compress and speed up as it travels over the car and this is what lowers the air pressure. It's because moving air applies less pressure than still air. The air traveling under the car is traveling less distance and therefore is going slower and has more pressure than the faster moving air above. If the underside of the car is draggy and the air "stagnates" as someone else here called it, it is slowed even further and the lift increases. That's why you can remove lift and decrease drag by smoothing out the underside of the car. This is however, *reducing lift*. It is not *adding downforce.

*As far as tear drops go your dead wrong. Drops of water fall as spheres, not tear drops.

From here.
*Q: What is the most aerodynamic shape? I've heard it is the shape of a falling raindrop but wonder why jet fighters have pointed noses. (Garrett, Surrey, Canada)*
*A:* You said the "shape of a falling raindrop", but I bet you meant a teardrop shape. Small raindrops fall like tiny spheres, not teardrops, and a sphere is a poor aerodynamic shape for objects moving at aircraft speeds. See figure.







In fact, a sphere disrupts air flow, and has about ten times the drag resistance of a teardrop-shaped airfoil.
The best aerodynamic shape for subsonic aircraft flight is a teardrop, because that shape interferes least with the surrounding air stream.


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## Bugzuki (Jan 15, 2008)

DVR said:


> No, not quite. What it does is force the air to actually compress and speed up as it travels over the car and this is what lowers the air pressure. It's because moving air applies less pressure than still air. The air traveling under the car is traveling less distance and therefore is going slower and has more pressure than the faster moving air above. If the underside of the car is draggy and the air "stagnates" as someone else here called it, it is slowed even further and the lift increases. That's why you can remove lift and decrease drag by smoothing out the underside of the car. This is however, *reducing lift*. It is not *adding downforce.*


Technically the air is always stopped it is the car/airplane that is moving and forcing the air out of the way. Everything else you said is correct. I was just going a little further to explain why the faster moving air causes lift. But, I guess I should have explained myself better. Thanks for the clarification. 

Then I as talking about adding a defusser not just putting a belly pan on. A defusser is a way of creating downforce by having less air pressure under the car.



DVR said:


> As far as tear drops go your dead wrong. Drops of water fall as spheres, not tear drops.
> 
> From here.
> *Q: What is the most aerodynamic shape? I've heard it is the shape of a falling raindrop but wonder why jet fighters have pointed noses. (Garrett, Surrey, Canada)*
> ...


An Airfoil is not the shape of a teardrop. As teardrops fall they become the shape of rain, which is not exactly round do to the drag on it. An air foil is more shaped like the wing you pictured above - with the same curve on both sides so that the air goes over both sides equally. But that is not really possible with a car since the ground is right there and the long length is not practical. It is usually better to have the back of the car end abruptly. This allows the air to break free before it starts turbulance.


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## Kreb (Apr 30, 2008)

> It is usually better to have the back of the car end abruptly. This allows the air to break free before it starts turbulance.


I'd take issue with this one statement. The ideal rear configuration would be a gradual point which would allow the air flow to merge with a minimum of turbulence. If you look at the flow pattern that was posted relative to the round shape, you have both turbulence and a low-pressure zone which "sucks" the car back.


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## Yaggo (Jul 9, 2008)

Bugzuki said:


> The better gas mileage turns out to be the money maker so that is why here in the USA almost every truck on the road is a conventional design.


Can this be translated as "The better gas mileage turns out to be the money maker so that is why here in the USA almost every person is driving a SUV."

(Sorry, had to


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## Kreb (Apr 30, 2008)

(Note: I am not an engineer, so any tech received from me is worth exactly what you paid for it)

My understanding is that drag increases as the square of speed. So it's worth noting that if you're mainly going to be tottering around town, aerodynamics is of significantly less import than if you expect to spend time at freeway speeds.


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## Bugzuki (Jan 15, 2008)

Kreb said:


> I'd take issue with this one statement. The ideal rear configuration would be a gradual point which would allow the air flow to merge with a minimum of turbulence. If you look at the flow pattern that was posted relative to the round shape, you have both turbulence and a low-pressure zone which "sucks" the car back.


You should have quoted the line I put before that one where I said that it is not practical to have a long pointed rear end of the car.



Yaggo said:


> Can this be translated as "The better gas mileage turns out to be the money maker so that is why here in the USA almost every person is driving a SUV."
> 
> (Sorry, had to


The term Heavy duty trucks refers to Semi-Trucks as in Kenworth (where I worked), Peterbuilt or Freightliner. If you have to have a large frontal area to haul a lot of stuff then you should break up the entry point.

To qualify some of my other theories on fast/slow moving air - they are just that - my theories. I have studied it some but am far from being an expert. So, I might be wrong, but it make sense to me.


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## DVR (Apr 10, 2008)

Bugzuki said:


> Technically the air is always stopped it is the car/airplane that is moving and forcing the air out of the way.


Yes that's right, the air is moved by the object passing through it and so is not always stopped.Moving air has less pressure than still or even marginally slower air. Its the diferential of the airspeed under and over an object that creates lift/downforce.



Bugzuki said:


> Then I as talking about adding a defusser not just putting a belly pan on. A defusser is a way of creating downforce by having less air pressure under the car.


A Diffuser is nothing but a belly pan with a fancy shape. It can be made to reduce turbulence(reducing lift) or beyond that, increase downforce.


Bugzuki said:


> An Airfoil is not the shape of a teardrop


 Actually it's a modified teardrop. but who's talking about airfoils anyway?


Bugzuki said:


> An air foil is more shaped like the wing you pictured above - with the same curve on both sides so that the air goes over both sides equally.


Just like a tear drop. But an airfoil is ANY wing shape whether it is symmetrical or not and that includes the wing above.
http://www.boeing.com/companyoffices/aboutus/wonder_of_flight/airfoil.html

What I posted was an extract from this article 
http://www.usatoday.com/tech/columnist/aprilholladay/2007-01-29-beer-plane-weight_x.htm
answering the Question.
*Q: What is the most aerodynamic shape?*
I suggest you read it.


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## Bugzuki (Jan 15, 2008)

I know that faster moving air creates less pressure then slow air, but I might be wrong in my theory about PSI - but no one seems to explain how fast air creates less pressure. So, I will stick with my theory until I understand it better.

And, I did not know that the teardrop shape was a common term for the airfoil shape. I thought it actually ment the shape of a teardrop - a ball with a tail due to the attachment to the skin. My mistake.

Now back to aerodynamics of Cars - and away from what is impossible to design in a car.
Here are some cars with low CD (coefficient of Drag) values:

VW 1-Liter-Car: CD value of 0.16 - closest to the teardrop shape
http://www.canadiandriver.com/articles/gw/vw1litre.htm

The Mercedes-Benz bionic car: a Cd value of 0.19
http://wwwsg.daimlerchrysler.com/SD...KIT/0,2970,0-1-68938-1-1-text-0-68935,00.html

The EV1: CD value of 0.19
http://ev1-club.power.net/bayintro.htm

Lexus LS430: CD value of 0.26
http://www.motortrend.com/cars/2006/lexus/ls430/specifications/index.html

Toyota Prius: CD value of 0.26
http://www.motortrend.com/oftheyear/car/112_04_coy_win/index.html

2006 Corvette: CD value (base package) 0.28 / (Aero Tuning Package) 0.34
http://www.motortrend.com/features/consumer/112_0502_2006_corvette_z06/index.html

2005 Mazda RX7: CD value of 0.29
http://www.motortrend.com/cars/1995/mazda/rx_7/specifications/index.html

Audi A2: CD value of 0.25
http://www.audiworld.com/news/99/a2_2/content.shtml


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## Yaggo (Jul 9, 2008)

It is also worth to note, that Cd is relative number. To compare real air drag of different vehicles, you need to multiple Cd with frontal area. (Sorry possibly wrong terms, English is not my native language.)


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## raceguy333 (Jun 18, 2008)

Yaggo said:


> It is also worth to note, that Cd is relative number. To compare real air drag of different vehicles, you need to multiple Cd with frontal area. (Sorry possibly wrong terms, English is not my native language.)


Absolutely right.

Just for reference here:


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## Yaggo (Jul 9, 2008)

Just couple of words about airfoils, although a bit unrelevant to cars. The common belief "a wing creates lift, because the upper air has to travel longer distance [over more curved top side of wing, causing lesser pressure due to increased speed]" is very over-simplified, if not even wrong explanation. Lift can be also generated by thin wing (sail, kite) or even by flat sheet, both having equal top & bottom surface area.

The real theory of airfoil (or hydrofoil) is very complex and has very less to do with common sense, as someone already mentioned in another post, but to summarize it, one can say "wing (or sail, blade, etc) creates lift, because it has ability to change the direction of surrounding air". This is regular subject to debate in sailing forums.

A ground effect is a bit different thing, but in electric (non-racing) cars, we don't want that (because again, it's increasing the total drag - nothing comes free, otherwise we just invented a perpetual-motion machine).


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## DVR (Apr 10, 2008)

Yaggo said:


> Just couple of words about airfoils, although a bit unrelevant to cars. The common belief "a wing creates lift, because the upper air has to travel longer distance [over more curved top side of wing, causing lesser pressure due to increased speed]" is very over-simplified, if not even wrong explanation. Lift can be also generated by thin wing (sail, kite) or even by flat sheet, both having equal top & bottom surface area.


Yes I agree with most of what your saying but you are wrong to say _"the common belief "a wing creates lift, because the upper air has to travel longer distance [over more curved top side of wing, causing lesser pressure due to increased speed]" is very over-simplified, if not even wrong explanation."_.

_This from Boeing (I think they know what they are talking about)

__As a wing moves through air, the air is split and passes above and below the wing. The wing’s upper surface is shaped so the air rushing over the top speeds up and stretches out. T*his decreases the air pressure above the wing*. The air flowing below the wing moves in a straighter line, so its speed and air pressure remain the same._
_Since high air pressure always moves toward low air pressure, the air below the wing pushes upward toward the air above the wing. The wing is in the middle, and the whole wing is “lifted.”_


I would also point out though that a sail acts as a highly "cambered" wing in those situations. The other way a sail is used is much like a parachute, it just grabs passing air and the sail is aerodynamically stalled. A flat sheet will not generate lift until it assumes the correct shape an it wont do that unless it is presented to the wind in the correct manner.
Just like a hang glider. When the wind flows over it in the correct way it assumes the desired shape. If the wind were to suddenly come at it from the rear, it will deflate and no longer create lift.

When inflated they are still an airfoil.
It doesn't matter whether it is a Clark Y, Symmetrical, cambered, laminar flow, some fancy NASA section, rigid or inflated, in the air or under water. It's an airfoil.

From wikipedia
An *airfoil* (in American English) or (*aerofoil* in British English) is the shape of a wing or blade (of a propeller, rotor or turbine) or sail as seen in cross-section.
An airfoil-shaped body moved through a fluid produces a force perpendicular to the motion called lift. Subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with asymmetric camber. Airfoils designed with water as the working fluid are also called hydrofoils.


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## Yaggo (Jul 9, 2008)

Sorry DVR, while I mostly agree with you, I still think that's somewhat simplified theory, or just an explanation. Also the Boeing link seems to be mostly "consumer stuff". From NASA:

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

Personally I don't really understand this stuff and that's why I pointed out aerodynamic is much more complicated subject than many of us tend to think. Anyway, I'm not going to argue more, let's build some electric cars


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