Down Force

[yamato]

Just read the howstuffworks site about Enzo Ferrari http://auto.howstuffworks.com/enzo3.htm. I was surprised to read "At 135 mph, almost a half-ton of downforce is pressing down on the Enzo." That seems to be a lot of force...

Is there any similar measurement or study done on our 3ers?

Thanks.

[Miles]

I'd suspect the measurement of the 3-Series would be in "lift" pounds not downforce.

[Nick325xiT 5spd]

BMWs generate lift.

[Desertnate]

I don't know if there are any "normal"cars that produce any downforce. I am sure cars like the Enzo, the Carrera GT, McClaren F1 and a few others might produce downforce, but their design is much different from a standard car.

[yamato]

Thanks guys. You are right. You just reminded me the shape of a normal car is like a plane's wing... We shouldn't compare with those high end sports car.

[my3rdbimmer]

actually it is a function of suction i believe.. i read something about it in popular science.. there where a couple of teams that actually developed fans to suck their car to the ground.. but they where too efficient and relaly teste the human limits of speed and g force. they where banned.

[Desertnate]

actually it is a function of suction i believe.. i read something about it in popular science.. there where a couple of teams that actually developed fans to suck their car to the ground.. but they where too efficient and relaly teste the human limits of speed and g force. they where banned.

It's all a matter of the aerodynamics of the car. You can get downforce from the top from wings and front air dams. In the case of the Enzo they were able to control the flow of air under the car creating a low pressure zone which does "suck" the car downwards.

A few years ago, Road & Track held a contest for the best handling car in the world. One of the more unique vehicles they tested was a cart that had some sort of vacuume on back to literaly suck it down on the road.

[my3rdbimmer]

that was it.. that was the car i was talking about..

[Miles]

actually it is a function of suction i believe.. i read something about it in popular science.. there where a couple of teams that actually developed fans to suck their car to the ground.. but they where too efficient and relaly teste the human limits of speed and g force. they where banned.


The (in)famous "Sucker car" of Jim Hall.
http://www.mulsannescorner.com/chaparral2j.html

[The Roadstergal]

http://www.bmwpugetsound.com/vbb/showthread.php?s=&threadid=26826

[The Roadstergal]

This isn't an E46-specific topic. :confused:

[yamato]

This must be old news. I just discovered G35 has zero lift from their web page.

[Fast Bob]

That settles it....this weekend I`m gonna install an inflatible skirt and mount a couple of leaf-blower motors under my car so it can really suck the road....downforce, here I come! :-)

Happy Thanksgiving !
Bob

[jsc]

This must be old news. I just discovered G35 has zero lift from their web page.
I'll believe it when I see some wind tunnel measurements for the G35, until then I'd assume the statement is more due to "marketecture" than "architecture". Could be they are quoting the aerodynamic lift at v=0?

[Fast Bob]

I'll believe it when I see some wind tunnel measurements for the G35, until then I'd assume the statement is more due to "marketecture" than "architecture". Could be they are quoting the aerodynamic lift at v=0?
Naaahh....they just neglected to mention that it generates zero lift *while parked*... :-)

Regards,
Bob

[Mr Paddle.Shift]

Woah..old thread.

I am actually working on mearsuring pressure points on my bumper. Static for now. Pitot tubes to be used soon, I hope. Read about it here (http://home.comcast.net/~vsengineering/track_reports/WS_Nov_2005.htm).

EDIT: since we are on this topic, take a look at coeff. of drag as well. IIRC, BMW has one of the worst Cd out there. G35s and MB have better Cd.

[jsc]

since we are on this topic, take a look at coeff. of drag as well. IIRC, BMW has one of the worst Cd out there. G35s and MB have better Cd.
The thing is though, Cd is just a derived coefficient that is normalized from the drag and cross-sectional area at a given speed, so it easy to stretch the truth a bit (increase the area by including the area between the bottom of the car and the road, for example, will reduce Cd for a given drag). Also, a lower car will have a smaller cross-sectional area, but as many "draggy" components underneath, so it's Cd will be higher, even though it has the same or less drag at a given speed. Wide tyres (BMWs tend to have wider ones than Mercedes or Infinitis for equivalent models) also cause a significant amount of drag.

The bottom line is that drag, not Cd is a better measure of a car's aerodynamic efficiency, if the aim is to determine which car will have the highest top speed or lowest fuel consumption. If gearing is set such that top speed can be reached at peak power, then for cars with equivalent power, the fastest is likely to have the lowest drag. I don't think you will find BMWs any more "draggy" than equivalent Mercedes or Infinitis.

[Mr Paddle.Shift]

You do bring up a good point. I understand the tricks that car manufacturers can play to lower Cd.

Fact is it's difficult to increase the area without affecting drag. If you recall, drag (D), mathematically is defined by the sum of two integrals over area: shear stress and normal stress. You might just get away by increasing dA without affecting D, thereby lowering Cd. But how much tolerance do you have with dA?

I need to review my books but IIRC, it may not be the draggy things that cause the Cd to increase but the shape of the vehicle. There is an example based on two basic vehicle geometric styling: symmetric ellipsoid and semi-ellipsoid. While both have the same frontal area, same length, smooth underbelly, yet their Cd and CL behaves differently with decreasing ground clearance (or lowering the car as we say). I haven't really looked into the underbelly of the different makes. But I am sure C-class, G35s and E46s have very different underbelly design.

I agree Cd is not the best. But it is a widely used dimensionless alternative. It allows engineers, designers and experimentalists to approximate the values without having to deal wttih shear stress and pressure distribution over, what seem these days to be, complex geometrical shapes.

The drag you mentioned for gauging fuel consumption is CdA (Cd x frontal area). Which you could still argue is a function of D given a V, but does not have the units of force.

The thing is though, Cd is just a derived coefficient that is normalized from the drag and cross-sectional area at a given speed, so it easy to stretch the truth a bit (increase the area by including the area between the bottom of the car and the road, for example, will reduce Cd for a given drag). Also, a lower car will have a smaller cross-sectional area, but as many "draggy" components underneath, so it's Cd will be higher, even though it has the same or less drag at a given speed. Wide tyres (BMWs tend to have wider ones than Mercedes or Infinitis for equivalent models) also cause a significant amount of drag.

The bottom line is that drag, not Cd is a better measure of a car's aerodynamic efficiency, if the aim is to determine which car will have the highest top speed or lowest fuel consumption. If gearing is set such that top speed can be reached at peak power, then for cars with equivalent power, the fastest is likely to have the lowest drag. I don't think you will find BMWs any more "draggy" than equivalent Mercedes or Infinitis.

[jsc]

I need to review my books but IIRC, it may not be the draggy things that cause the Cd to increase but the shape of the vehicle. There is an example based on two basic vehicle geometric styling: symmetric ellipsoid and semi-ellipsoid. While both have the same frontal area, same length, smooth underbelly, yet their Cd and CL behaves differently with decreasing ground clearance (or lowering the car as we say). I haven't really looked into the underbelly of the different makes. But I am sure C-class, G35s and E46s have very different underbelly design.

Yes, shape is very important in determining the drag, the other problem with vehicles is that ground effect has a major influence on the aerodynamics, usually distorting the lift beyond the values generated by the pure body shape itself. With road vehicles that have an overall aerodynamically efficient profile, the drag generated by the trailing vortices and the underbody drag are the two most difficult to control, the former being limited by the length and slope of the tail, the latter being limited by the need to package and cool underbody components.

In terms of handling trailing vortices, consistency of shedding is the best approach on a vehicle, to try and stabilise the airflow behind, smoothing the turbulence as far as possible. If an optimum teardrop or trailing wedge shape is used, then the tail would need to be about as long as the rest of the vehicle. Most vehicles act like the first half of a wing, with the centre of pressure consequently near the rear axle, leading to more lift at the rear than the front.

No main stream vehicle I know of has any optimum underbody aerodynamics (Ferraris and Lotus Elises aren't mainstream in my book). For optimum downforce, a smooth underbody that has low cross-sectional area near the front axle and a higher area at the rear axle is preferred, but leads to packaging problems with the exhaust, rear differential and rear suspension, too severe for "practical" cars. The worst underbody design is where large amounts of air enter and stagnate, this can be alleviated with a front air dam (often termed a spoiler) to some degree, the approach taken by most road car designers. The G35, C-Class, E46 all employ an airdam with little attention to smoothness and fairing of the underbody, making little or no use of ground effect.

I agree Cd is the best dimensionless value to use, but on vehicles it is a value much more open to "distortion" compared with an aircraft. Also, I would question whether Cd is being quoted with or without the use of a rolling road moving at the same velocity of the airstream around the vehicle, causing significant changes in airflow and consequent aerodynamic forces.

A simple way of evaluating the overall aerodynamic efficiency of a given vehicle is probably a coast-down test - reach a given speed of say 60 metres per second, put the vehicle in neutral, then measure the distance taken to reach, say 40 metres per second. The longer the distance, the more aerodynamic efficient the vehicle is. The speeds need to be kept high to make sure that rolling and driveline resistance is small compared with aerodynamic resistance. Using this method will eliminate test methods which have inconsistencies in the handling of ground effect.

Hopefully I'll have a better understanding myself of ground effect when I finish my project car that I'm engineering to have a completely smooth underbody. I have a clear ground clearance path between the tyres of 0.08m at the front axle and 0.12m at the rear axle, which should lead to significant downforce. It is always counter intuitive to try and force as much air as possible under the car to create more downforce, but this is the way the accelerated air will create lower pressure because of the Bernoulli effect through the venturi created between the bottom of the car and the ground.

I'd be interested to see any more of the pitot results you have from your track tests on your E46, especially if you have anything from just above the ground where you would place a ground effect plate (assuming a flat plate protruding out from and around the bottom of the two front corners of the air dam.

[FenPhen]

The worst underbody design is where large amounts of air enter and stagnate, this can be alleviated with a front air dam (often termed a spoiler) to some degree, the approach taken by most road car designers. The G35, C-Class, E46 all employ an airdam with little attention to smoothness and fairing of the underbody, making little or no use of ground effect.I've always wondered... what part of the front bumper exactly is the air dam? It's always described as a "deeper air dam."

And then, where is it directing air? The M-Tech II kit has two small splitters(?) under each fog lamp at curb height (I know this for a fact :( ) and then the middle opening.

[iateyourcheese]

You do bring up a good point. I understand the tricks that car manufacturers can play to lower Cd.

Fact is it's difficult to increase the area without affecting drag. If you recall, drag (D), mathematically is defined by the sum of two integrals over area: shear stress and normal stress. You might just get away by increasing dA without affecting D, thereby lowering Cd. But how much tolerance do you have with dA?

I need to review my books but IIRC, it may not be the draggy things that cause the Cd to increase but the shape of the vehicle. There is an example based on two basic vehicle geometric styling: symmetric ellipsoid and semi-ellipsoid. While both have the same frontal area, same length, smooth underbelly, yet their Cd and CL behaves differently with decreasing ground clearance (or lowering the car as we say). I haven't really looked into the underbelly of the different makes. But I am sure C-class, G35s and E46s have very different underbelly design.

I agree Cd is not the best. But it is a widely used dimensionless alternative. It allows engineers, designers and experimentalists to approximate the values without having to deal wttih shear stress and pressure distribution over, what seem these days to be, complex geometrical shapes.

The drag you mentioned for gauging fuel consumption is CdA (Cd x frontal area). Which you could still argue is a function of D given a V, but does not have the units of force.


Crikey, are you two Mechanical Engineers. Aerospace Engineers, etc.?

[jsc]

Crikey, are you two Mechanical Engineers. Aerospace Engineers, etc.?
Ex-Aerospace, now IT Industry type, nearly became a Vehicle Aerodynamicist with Ford, but kept the automotive interest as a hobby instead.

[jsc]

I've always wondered... what part of the front bumper exactly is the air dam? It's always described as a "deeper air dam."

And then, where is it directing air? The M-Tech II kit has two small splitters(?) under each fog lamp at curb height (I know this for a fact :( ) and then the middle opening.
The air dam is really the area below the bumper to the lowest point of the front facia. The air that hits the air dam is either sent through the engine compartment, cooling ducts or around the sides of the car - it creates a "dam" to the air and prevents it from flowing under the car or over the top as far as possible.

In general, the splitters on road cars have very little benefit, as they are too small to direct much air or create any noticeable downforce. The basic principle is to split the air going under the car and the air going around and over it. The problem with most road cars is that air going under the car is not beneficial to aerodynamics. A splitter really needs a smooth underbody to have much effect.

[exdos]

I've come across this thread after doing some searches on Google about aerodynamics and find the content just what I've been looking for.

I have a BMW Z3 M Coupe on which I have fitted a Strongstrut buttstrut, and since the buttstrut is just a piece of half inch bar lying transversely across the underside of the car, I've been thinking of ways of employing this part to have an aerodynamic function as well as its stabilising effect on the car's handling.

Should I make this part into an inverted wing with a chord length of about 5 inches or should I attach a plate to it which is attached to the underfloor of the car in front of the buttstrut so that all the air passing under the car must increase in velocity to pass under the plate, thereby causing the air to lessen in pressure giving downforce? The thought does occur to me though that the a plate set across the car in this way might actually create lift in the same way that a water skier is able to stand on water.

What do you guys think?

[Jspeed]

I've come across this thread after doing some searches on Google about aerodynamics and find the content just what I've been looking for.

I have a BMW Z3 M Coupe on which I have fitted a Strongstrut buttstrut, and since the buttstrut is just a piece of half inch bar lying transversely across the underside of the car, I've been thinking of ways of employing this part to have an aerodynamic function as well as its stabilising effect on the car's handling.

Should I make this part into an inverted wing with a chord length of about 5 inches or should I attach a plate to it which is attached to the underfloor of the car in front of the buttstrut so that all the air passing under the car must increase in velocity to pass under the plate, thereby causing the air to lessen in pressure giving downforce? The thought does occur to me though that the a plate set across the car in this way might actually create lift in the same way that a water skier is able to stand on water.

What do you guys think?
I can't completely visualize what that would look like (not familiar w/ the underside of the Z3). The underbody should be kept as clean as possible, allowing the air to pass thru smoothly.

[exdos]

This is what the underside of my M Coupe looks like with buttstrut in place as seen from the front:
http://www.z3mcoupe.com/photopost/data//500/53Ground_clearance_from_the_front.jpg


What I am thinking of doing is either: 1. making an inverted aerofoil to fit on the buttstrut, or 2. fixing a sloping aluminium panel from the underfloor of the car, say 15inches in front of the buttstrut, and which descends rearwards from the floor of the car and is attached to the underside of the butt strut, so that from the same view as in the photo, you would't see anything above the strut and so that all the air passing under the car would have to pass UNDER the panel. See thge diagram:
http://photos.fotango.com/p/eba00443822f00000009.jpg



As I see it, the air in front of my proposed panel has approximetely 4.5 inches of ground clearance whereas it has only 3.25 inches of ground clearance to pass under the buttstrut. This should mean that the air will increase in velocity to pass under the panel with a consequent drop in pressure which might produce additional downforce beneath the strut brace. My concern is that fitting such a panel might act as a kind of surf board which could encourage the car to "ride" on the air passing beneath it. OTOH, the panel may act like placing the back of a spoon into a stream of water where the bowl of the spoon is "sucked into the water stream. Which will be correct?

The buttstrut is already present and causes me the greatest risk of ground contact (but has never been a problem) therefore, fitting either an aerofoil or a panel to it doesn't add to the potential for ground contact. Since the butt strut is already a potential point of attachment for aerodynamic devices it seems such a waste not to use it, if at all possible.

Ideas and suggestions much appreciated.

[allaboutme]

I'm not sure how effective this aerofoil for you would be seeing as how the underbody of the car isn't flat to begin with. I imagine the effects of your efforts would be minimal at best.

I'm thinking a better way to achieve downforce with these cars are to slap on a good size inverted wing to the rear and a "real" splitter to the front.


As for the Enzo, it appears the downforce (as well as the driver) wasn't enough to keep it on the ground in the recent crash...

[Jspeed]

Now that I can picture it better, what you need to do to reduce lift is to ensure that the buttstrut isn't the only part sticking out. The air gets compressed traveling from the 4.5" clearance area to the buttstrut with 3.25" clearance and creates lift. You need to fill up the area in front of it so that there's no more than 3.25" clearance.

[exdos]

Now that I can picture it better, what you need to do to reduce lift is to ensure that the buttstrut isn't the only part sticking out. The air gets compressed traveling from the 4.5" clearance area to the buttstrut with 3.25" clearance and creates lift. You need to fill up the area in front of it so that there's no more than 3.25" clearance.

What do you mean? Put a false flat floorplate on the full underside of the var which has a 3.5inch ground clearance, or is my idea of attaching a sloping plate (as per my diagram above) correct?

Alternatively, how about me fitting a vertically suspended plate on the underside of the front bumper to act as an airdam which should reduce the volume of air getting under the car in the first place?