How do "ground effects" work???
11-12-2004, 05:24 PM
#1
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How do "ground effects" work???
OK, I understand the basic physics of lift, Bernoulli's effect, etc. With regards to a car, the typical car basically acts like a wing: Air flowing over the top of the car moves faster than air moving under the car. Thus, as speed increases a pressure gradient develops so there is less pressure pushing down on the top of the car and greater pressure pushing up from the bottom of the car. The car develops lift.
I believe I understand how accessories on the back of a car can reduce lift:
1. Wings. Putting an inverted wing on the back of the car will create a downward force.
2. Spoilers. These disrupt the flow of air over the top of the rear of the car and reduce the lift. (The same net effect as a wing, just achieved in a somewhat different manner.)
I do not understand how a front spoiler or "ground effects" work. I have read that these accessories reduce the flow of air under the car. Fine, but how does that reduce lift? It seems to me that reducing the flow of air under the car would increase the pressure gradient between the top of the car and the bottom of the car, thus increasing lift. Seems to me that you would want air flowing under the car as fast as possible.
Does the front spoiler and/or ground effects, by reducing flow of air under the car, create something of a vacuum effect under the car? That would, by definition, reduce the pressure under the car.
I am interested if someone can provide a good explanation.
Mark
I believe I understand how accessories on the back of a car can reduce lift:
1. Wings. Putting an inverted wing on the back of the car will create a downward force.
2. Spoilers. These disrupt the flow of air over the top of the rear of the car and reduce the lift. (The same net effect as a wing, just achieved in a somewhat different manner.)
I do not understand how a front spoiler or "ground effects" work. I have read that these accessories reduce the flow of air under the car. Fine, but how does that reduce lift? It seems to me that reducing the flow of air under the car would increase the pressure gradient between the top of the car and the bottom of the car, thus increasing lift. Seems to me that you would want air flowing under the car as fast as possible.
Does the front spoiler and/or ground effects, by reducing flow of air under the car, create something of a vacuum effect under the car? That would, by definition, reduce the pressure under the car.
I am interested if someone can provide a good explanation.
Mark
11-12-2004, 05:44 PM
#2
As Tom Smykowski would say (Office Space):
"You see... they're these effects that hang toward the ground..."
Theorhetically, adding more air to the top of the car decreases the pressure on the bottom of the car, therefore producing more downforce.
"You see... they're these effects that hang toward the ground..."
Theorhetically, adding more air to the top of the car decreases the pressure on the bottom of the car, therefore producing more downforce.
11-12-2004, 06:12 PM
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I believe the intention is more for high-speed parasite drag elimination. Drag increases as the square of the flow speed with so-called "parasite" drag (things that create turbulent flow behind them). Ground effects help to direct the airstream away from the typically less-than-aerodynamic undersides of vehicles and allow the flow to continue in a more-or-less laminar fashion whereas without them, the airstream would be made turbulent far sooner (creating parasite drag). The effect of ground effects would likely be negligable at anything other than fairly high speeds though (highway or above), but could prove quite significant at oval track speeds.
One more reason to laugh at ricers - they spend hundreds of dollars on expensive ground effects to put on cars that will never get over 100. Any benefits realized by adding them are certainly negated by the vehicle's lack of power (unless it's a highly modded engine / drivetrain). In most cases, I'd suspect dollars are better spent on engine / drivetrain / tire upgrades than on ground effects, unless you're intenting to run a high-speed track car.
One more reason to laugh at ricers - they spend hundreds of dollars on expensive ground effects to put on cars that will never get over 100. Any benefits realized by adding them are certainly negated by the vehicle's lack of power (unless it's a highly modded engine / drivetrain). In most cases, I'd suspect dollars are better spent on engine / drivetrain / tire upgrades than on ground effects, unless you're intenting to run a high-speed track car.
11-12-2004, 06:39 PM
#4
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OK, let's cut the BS here.....
Ground effects: Aftermarket slap-on crap is NOT ground effects. The plastic rice market (yes, even for German cars) calls this crap ground effects, but it's nothing of the sort. It's plastic, fiberglass, and CF tacked on stuff that may or may not work.
Real ground effects are caused by shaping the underside of the car to generate downforce. Ground effects were pioneered by Colin Chapman. On the Lotus 78 he shaped the underside of the sidepods like the underside of a wing. That was the start of ground effects. The following year was the real breakthrough when they discovered they didn't need to shape the underside like a wing, but rather it was much more effective to creat sloping tunnels. The air going under the car must not only accelerate, but it must expand to fill the volume of the tunnel creating a low pressure area under the car. This created downforce and this cemented the ground effects era.
Modern ground effects on F1 (and some other) cars use an angled flat bottom with a "diffuser" at the rear. The diffuser is upward sloping and outwardly expanding, much like ground effects tunnels, only shorter. They are nearly as effective as the ground effects tunnels used to be.
Splitter: The plastic rice market (even for German cars) call many spoilers and airdams splitters. The vast majority of what is called a splitter is not. A splitter is usually a flat piece of material (could be aluminum, CF, fiberglass, plastic, whatever) that is bolted to the underside of a airdam. It's quite effective in increasing the downforce by further limiting the amount of air that is allowed to enter under the car, thus creating an area of lower pressure. The farther forward the splitter extends, the greater the downforce. Sometimes a splitter will have an upturn after the airdam to aid in downforce (by making the same amount of air occupy a larger volume).
Airdams and front spoilers work by limiting the amount air that is allowed to enter under the car. This lowers the pressure under the car thus increasing downforce (reducing lift).
Rear spoilers work by delaying flow separation or minimizing flow separation and therefore decreasing lift (increasing downforce).
Ground effects: Aftermarket slap-on crap is NOT ground effects. The plastic rice market (yes, even for German cars) calls this crap ground effects, but it's nothing of the sort. It's plastic, fiberglass, and CF tacked on stuff that may or may not work.
Real ground effects are caused by shaping the underside of the car to generate downforce. Ground effects were pioneered by Colin Chapman. On the Lotus 78 he shaped the underside of the sidepods like the underside of a wing. That was the start of ground effects. The following year was the real breakthrough when they discovered they didn't need to shape the underside like a wing, but rather it was much more effective to creat sloping tunnels. The air going under the car must not only accelerate, but it must expand to fill the volume of the tunnel creating a low pressure area under the car. This created downforce and this cemented the ground effects era.
Modern ground effects on F1 (and some other) cars use an angled flat bottom with a "diffuser" at the rear. The diffuser is upward sloping and outwardly expanding, much like ground effects tunnels, only shorter. They are nearly as effective as the ground effects tunnels used to be.
Splitter: The plastic rice market (even for German cars) call many spoilers and airdams splitters. The vast majority of what is called a splitter is not. A splitter is usually a flat piece of material (could be aluminum, CF, fiberglass, plastic, whatever) that is bolted to the underside of a airdam. It's quite effective in increasing the downforce by further limiting the amount of air that is allowed to enter under the car, thus creating an area of lower pressure. The farther forward the splitter extends, the greater the downforce. Sometimes a splitter will have an upturn after the airdam to aid in downforce (by making the same amount of air occupy a larger volume).
Airdams and front spoilers work by limiting the amount air that is allowed to enter under the car. This lowers the pressure under the car thus increasing downforce (reducing lift).
Rear spoilers work by delaying flow separation or minimizing flow separation and therefore decreasing lift (increasing downforce).
11-12-2004, 06:50 PM
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I remember some 'Top Gear' test of the relevance of rear wings. If my memory is correct, the mph given of a rear wing starting to have an affect on a car/downforce/performance was 150mph!
Which = all our cars are rice
Which = all our cars are rice
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11-12-2004, 06:53 PM
#8
Race Director
Originally Posted by RMills944
would that be why the underside pan for the engine bay has louvers in it (much less why there's a pan at all) Geo?
11-12-2004, 06:54 PM
#9
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Originally Posted by flamingeye
I remember some 'Top Gear' test of the relevance of rear wings. If my memory is correct, the mph given of a rear wing starting to have an affect on a car/downforce/performance was 150mph!
Which = all our cars are rice
Which = all our cars are rice
11-12-2004, 07:04 PM
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11-12-2004, 07:11 PM
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If spoilers in the rear don't take an effect until 150mph, then why do the new Porsches have a spoiler that comes up in the 70-80 MPH?? Mostly the rear spoiler and how well it works is all depending on the aerodynamics of the car over all. If you have a nice smooth air flow over the car like most Porsches, the spoiler does have a good effect on the car, but take a Ford or Buick with a spoiler and it just doesn't make sense.
11-12-2004, 07:16 PM
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Get a HUGE pile of money, get yourself a large-scale rolling-road windtunnel, and a large group of highly-paid engineers... Aerodynamic changes to production cars are the highest, by far, cost/benefit ratio of anything one can do on these cars. Just my two cents...
11-12-2004, 07:55 PM
#15
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Ok, seeing as how I am I commercial pilot, and I've spent a good chunk of my spare time studying aerodynamics and fluid dynamics, I can give a little bit of credible insight into aerodynamics as it applies to cars.
Splitters: Used to seperate airflow between the top and bottom half of the car. Splitters are usually placed as low as possible to allow less airflow under the car, creating a low pressure zone or area. Since air always wants to flow from a high pressure area to a low pressure area, this would theoretically contribute to the overall downforce on the car. If the splitter does not actually decrease the amount of air flowing under the car from stock, it is not doing dick all.
Ground effects: As Geo pointed out, fibreglass riceboy kits are NOT real ground effects. REAL ground effects work like this: The sides of the underbody of a car and sealed off close to the ground, to prevent air from escapeing below the rocker panels. The REAR of the underbody is shaped like an inverted wing to expand and direct air upwards, creating an extreme low pressure zone. The entire underbody is as smooth as possible. this is real ground effects. The lower the car, the better this works. The faster the car, the more downforce is generated.
My good friend Ian M reminded me the other day of the difference between a rear wing and a spoiler.
A SPOILER, as is found on our 944s, 951s and yes, even the 968's, are NOT really wings, and they DO NOT generate any downforce. The cross section of a car is much like that of a wing, and that is why cars generally get floaty at high speeds. The rubber spoilers on our cars, and even the glassfibre bridge spoilers do not actually provide any lift. Just like the 911's RS ducktail and whaletail, they merely DESTROY lift generated by the car's body, and prevent downward flowing air at the rear of the car from generating a downwash causing lift on the rear end.
This can be shown and proven by studying the airflow over the rear hatch of the car and seeing what angle it hits the spoiler at. A real wing has what is called a critical angle of attack. This is where the wing is at such a great angle compared to the onciming air, that is does not creat any lift whatsoever. In it's stock position, the 968 bridge spoiler does not create any lift whatsoever.
A true WING, as is found on race cars, is SO high up on the car that it is able to meet with direct airflow, and thus, is able to produce proper downforce. Not to mention, race car type wings usually have a much more efficient crosssection for creating downforce. The 968 Turbo RS wing is a good example of this.. people complain that it is too ricey and guady, but the truth is, it isn't a wing if it doesn't meet clean oncoming air. If it's too low, it only halts body lift, and doesn't actually generate downforce!
Now, as far as speed having an effect on downforce, of course. Airspeed in directly proportional to Lift and Drag. As speed increases, so does downforce and drag!!! Only problem is, drag increases faster!!!! that's why you need so much HP to reach 200mph with a conventional car body!
Now as far as DRAG is concerned, it is divided into multiple categories:
clear as mud?
(I'm going nuts on a bottle of Bailey's so if there are any errors in my post - you know why!) HA
Splitters: Used to seperate airflow between the top and bottom half of the car. Splitters are usually placed as low as possible to allow less airflow under the car, creating a low pressure zone or area. Since air always wants to flow from a high pressure area to a low pressure area, this would theoretically contribute to the overall downforce on the car. If the splitter does not actually decrease the amount of air flowing under the car from stock, it is not doing dick all.
Ground effects: As Geo pointed out, fibreglass riceboy kits are NOT real ground effects. REAL ground effects work like this: The sides of the underbody of a car and sealed off close to the ground, to prevent air from escapeing below the rocker panels. The REAR of the underbody is shaped like an inverted wing to expand and direct air upwards, creating an extreme low pressure zone. The entire underbody is as smooth as possible. this is real ground effects. The lower the car, the better this works. The faster the car, the more downforce is generated.
My good friend Ian M reminded me the other day of the difference between a rear wing and a spoiler.
A SPOILER, as is found on our 944s, 951s and yes, even the 968's, are NOT really wings, and they DO NOT generate any downforce. The cross section of a car is much like that of a wing, and that is why cars generally get floaty at high speeds. The rubber spoilers on our cars, and even the glassfibre bridge spoilers do not actually provide any lift. Just like the 911's RS ducktail and whaletail, they merely DESTROY lift generated by the car's body, and prevent downward flowing air at the rear of the car from generating a downwash causing lift on the rear end.
This can be shown and proven by studying the airflow over the rear hatch of the car and seeing what angle it hits the spoiler at. A real wing has what is called a critical angle of attack. This is where the wing is at such a great angle compared to the onciming air, that is does not creat any lift whatsoever. In it's stock position, the 968 bridge spoiler does not create any lift whatsoever.
A true WING, as is found on race cars, is SO high up on the car that it is able to meet with direct airflow, and thus, is able to produce proper downforce. Not to mention, race car type wings usually have a much more efficient crosssection for creating downforce. The 968 Turbo RS wing is a good example of this.. people complain that it is too ricey and guady, but the truth is, it isn't a wing if it doesn't meet clean oncoming air. If it's too low, it only halts body lift, and doesn't actually generate downforce!
Now, as far as speed having an effect on downforce, of course. Airspeed in directly proportional to Lift and Drag. As speed increases, so does downforce and drag!!! Only problem is, drag increases faster!!!! that's why you need so much HP to reach 200mph with a conventional car body!
Now as far as DRAG is concerned, it is divided into multiple categories:
- Parasite Drag (all drag created by objects that do not contribute to lift)
- Form Drag = The drag created by shape or form of a body as it resists movement thru the air
- Skin Friction = The drag created by the tendancy of air moving over a wing to cling to it's surface.
- Interference Drag = When Resistance is caused by the effect of one part on another.
- Form Drag = The drag created by shape or form of a body as it resists movement thru the air
- Induced Drag (all drag created by lifting bodies) Induced drag is created by lift. The lower the aspect ratio, the higher the drag. Also, the faster it moves, the higher the drag. A long thin glider wing, would have less inducted drag than a short, stubby wing.
clear as mud?
(I'm going nuts on a bottle of Bailey's so if there are any errors in my post - you know why!) HA