Downforce
12-25-2005, 01:06 AM
#91
Nordschleife Master
I think the original question was if 944's generated downforce. The answer is no, they don't.
Go drive a 951 at 150mph and see how it sucks itself down to the road.
12-25-2005, 01:31 AM
#92
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Originally Posted by BigNNasty
To even more so approve that downforce on the rear makes lift on the front. Why do you think you can't make a sharp turn when going 50 mph? Mainly due to the fact that your car is lifting up in front, pulling the front wheels off the ground, therefor limiting turning substantionally (sp?).
Example...Tried to turn a bike when riding a wheelie? This is what I'm explaining.
Example...Tried to turn a bike when riding a wheelie? This is what I'm explaining.
.
12-25-2005, 03:54 AM
#93
Race Director
Originally Posted by Bill.e1
My point on ground effects is that it is an incorrect use of the term. The F-1 and champ cars make downforce by sealing a venturi under the car.
Originally Posted by Bill.e1
Chapman's original venturi tunnels looked like upside down wings. Since the downforce is from venturi effect, it is concentrated at the point of highest air velocity which was a narrow strip shortly behind the front wheels. While this design produced the most negative pressure, it varied a lot and was centered too far forward so losing suction would cause sudden understeer. The flatter bottom makes less "vacuum" but it acts over a larger area and, since that area is longer and centered further back, it doesnt make the car as squirrely. It is ironic that the flat bottom was partly driven by regulations trying to curb downforce.
I'm not sure I'd claim the "flat-bottom" cars make less downforce. The diffusers they use are extremely efficient and effective and the cars can produce in excess of 5 gs.
12-25-2005, 07:34 PM
#94
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Rich,
If I'm driving, how can I see it "hunker down?" If your car is still recognizable as a 951 and you can roll a softball under it, it doesn't produce downforce. Front engine cars in particular are hard to kill all lift- the engine compartment traps air. Mid and rear engine cars are much easier to make the bottom smooth enough to keep airflow attached.
members.rennlist.org/951_racerx/Porsche959vs911TurboS.pdf
http://www.tavlecentret.dk/Michael/Porsche/Racecars.htm
Porsche 959 reference about 1/4 way down the page. If the 944 was already making downforce, why would they make such a big deal about the 959's lack of lift?
http://www.tfd.chalmers.se/~lelo/rva...porsche911.pdf
I know the MIRA guys are English but they actually own a wind tunnel. I don't think they'd fudge the numbers to make Porsche look bad. If they could get the 944 glued down, wouldn't they do the same for the 911?
http://www.968.net/pressroom/star_news.htm
Article states 968 has near zero lift, not downforce.
http://www.automobilemag.com/reviews...t3/index1.html
A GT-3 test that talks about "no lift" not downforce.
http://www.mulsannescorner.com/aerod...sche91100.html
Tested a 2000 911, 600 pounds lift.
http://www.factoryfive.com/table/ffr...amicstest.html
Three road cars that produce downforce at 150 mph including one Porsche although I don't think the Carrera GT has many aerodynamic similarities with the 951.
http://www.ddavid.com/formula1/lotus79.htm
Lotus 79 design info citing Bernoulli's principle as the source of "ground effect."
George, it was hypocritical to complain about using "ground effect" and then do the same with the word "seal." You are correct that racers no longer "seal" to the ground. Other than that, I don't see anywhere we disagree. Race cars have, for the most part, moved beyond simple bernoulli chambers- See:
http://www.mulsannescorner.com/vortexlift.html
This website and the MIRA article are pretty good places to learn more about the topic.
Once again- 944, 951, 968 and 999 of 1000 other street cars don't create downforce. They are way too high off the ground. We are really arguing semantics- reducing lift is a very good thing and Porsche does a good job on their cars.
On my first post, I had written a longer intro but it got too long and I cut it down. Rereading, it does sound like I am offering to share the profound knowledge a degree bestows. Sorry about that. In fact, our prof was a WWII vet- he worked on adapting the Merlin to US fighters. In his opinion, nothing important had been discovered since. Not exactly state of the art.
Have fun and happy holidays,
Bill
PS- I don't mean to be rude but I am done with this thread. The links provide far more facts than I can.
If I'm driving, how can I see it "hunker down?" If your car is still recognizable as a 951 and you can roll a softball under it, it doesn't produce downforce. Front engine cars in particular are hard to kill all lift- the engine compartment traps air. Mid and rear engine cars are much easier to make the bottom smooth enough to keep airflow attached.
members.rennlist.org/951_racerx/Porsche959vs911TurboS.pdf
http://www.tavlecentret.dk/Michael/Porsche/Racecars.htm
Porsche 959 reference about 1/4 way down the page. If the 944 was already making downforce, why would they make such a big deal about the 959's lack of lift?
http://www.tfd.chalmers.se/~lelo/rva...porsche911.pdf
I know the MIRA guys are English but they actually own a wind tunnel. I don't think they'd fudge the numbers to make Porsche look bad. If they could get the 944 glued down, wouldn't they do the same for the 911?
http://www.968.net/pressroom/star_news.htm
Article states 968 has near zero lift, not downforce.
http://www.automobilemag.com/reviews...t3/index1.html
A GT-3 test that talks about "no lift" not downforce.
http://www.mulsannescorner.com/aerod...sche91100.html
Tested a 2000 911, 600 pounds lift.
http://www.factoryfive.com/table/ffr...amicstest.html
Three road cars that produce downforce at 150 mph including one Porsche although I don't think the Carrera GT has many aerodynamic similarities with the 951.
http://www.ddavid.com/formula1/lotus79.htm
Lotus 79 design info citing Bernoulli's principle as the source of "ground effect."
George, it was hypocritical to complain about using "ground effect" and then do the same with the word "seal." You are correct that racers no longer "seal" to the ground. Other than that, I don't see anywhere we disagree. Race cars have, for the most part, moved beyond simple bernoulli chambers- See:
http://www.mulsannescorner.com/vortexlift.html
This website and the MIRA article are pretty good places to learn more about the topic.
Once again- 944, 951, 968 and 999 of 1000 other street cars don't create downforce. They are way too high off the ground. We are really arguing semantics- reducing lift is a very good thing and Porsche does a good job on their cars.
On my first post, I had written a longer intro but it got too long and I cut it down. Rereading, it does sound like I am offering to share the profound knowledge a degree bestows. Sorry about that. In fact, our prof was a WWII vet- he worked on adapting the Merlin to US fighters. In his opinion, nothing important had been discovered since. Not exactly state of the art.
Have fun and happy holidays,
Bill
PS- I don't mean to be rude but I am done with this thread. The links provide far more facts than I can.
12-26-2005, 03:39 AM
#95
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I've noticed some disturbing things in this thread.
First we have failed to adhere to the Socratric method.
For us to have this discussion in a productive matter, we must first define our terms.
first define "lift".
Next define "Downforce".
Are they mutually exclusive or conditionally inclusive?
Once we have determined a commonly accepted defintition of each term, we can then attempt to apply those terms within the scope of our application.
To do so, we must fiorst define our application.
Let's do that, and define the accepted conditions in which we will apply the vehicle, and then, only then can we make real head way without being sucked down by semantics and guesses.
First we have failed to adhere to the Socratric method.
For us to have this discussion in a productive matter, we must first define our terms.
first define "lift".
Next define "Downforce".
Are they mutually exclusive or conditionally inclusive?
Once we have determined a commonly accepted defintition of each term, we can then attempt to apply those terms within the scope of our application.
To do so, we must fiorst define our application.
Let's do that, and define the accepted conditions in which we will apply the vehicle, and then, only then can we make real head way without being sucked down by semantics and guesses.
12-26-2005, 04:04 AM
#96
Nordschleife Master
Bill, before you finish with this thread, read what I have to say. I'm not an egineer, but I do hold a commercial pilot's licence, and have a couple hundred dollars worth of textbooks on aerodynamics and such.
I've driven my 951 with and without a splitter at 160+mph, and, I have driven my cavalier at 115+mph. Let me tell you, FROM EXPERIENCE: The cavalier, like an early 911, gets floaty... it HAS lift, at least on the rear end. The 951 is the EXACT OPPOSITE, you can FEEL it suck itself down to the road.. you don't need to "see" it. FWIW, i still have the 20 year old stock "soft" suspension... so maybe that makes it so noticeable. With the splitter on the front, I have BARELY 2 inches of ground clearance in the front. More in the back, but that actually helps the "venturi effect" (are you familiar with the term "strake"?)
The 959 was designed with zero lift and zero downforce so that it could get a higher TOP SPEED, in order to be more competitive with the F40. If you look at most 959's now, you'll find an additional "gurney flap" added below the rear wing, in order to actually get some rear downforce!
You cannot possibly compare a 911 to a 951. They are two completely different shaped cars. The 951 is technically a fastback design, or was intended to be... which is a million times more aerodynamically sound than the 911 which is shaped like the cross section of an airplane wing. Additionally, the rear end of the 911 is VERY low to the ground, which the front is a lot higher. The 951 is the exact opposite, which again is condusive to better aero.
The 968 is not the same car as a 951. It has a higher Cd than the 951 did. It has a totally different rear end. Porsche went "backwards" with aero in order to give the car an updated look. They were not concerned with high speed aero because the 968 was going in a different direction than the 944 turbo was.
These cars make about 300lbs of downforce at 150mph. Is it totally unreasonable to say that my 951 makes maybe 50 or 150 lbs of downforce at 150 mph??? Listen, sitting in your car, you SHOULD be able to feel a 50 to 150lbs person put all his weight on the hatch of your car. (regardless of whether you are parked or moving...)
Wasn't it you who told me not to get hung up on terminology? 
Coming from an aviation background, the term "ground effect" means something quite different than it does in racing. But the commonality is this: "Ground effect" are aerodynamic effects that make use of the airplanes or cars proximity to the ground. Whether you are talking about venturi's, diffusers, sliding skirts, etc... doesn't really matter to me. The only term I don't like is when people can a spoiler a wing, or when they call a wing a spoiler.. those two should not be interchangeable terms in aerodynamics!
Bill, I appreciate your input.. it's really valued to me because you are clearly well educated. However, that does not mean that you aren't overlooking some details, which could change the outcome of your opinion on this subject!
If I'm driving, how can I see it "hunker down?" If your car is still recognizable as a 951 and you can roll a softball under it, it doesn't produce downforce. Front engine cars in particular are hard to kill all lift- the engine compartment traps air. Mid and rear engine cars are much easier to make the bottom smooth enough to keep airflow attached.
Porsche 959 reference about 1/4 way down the page. If the 944 was already making downforce, why would they make such a big deal about the 959's lack of lift?
http://www.automobilemag.com/review...gt3/index1.html
A GT-3 test that talks about "no lift" not downforce.
http://www.mulsannescorner.com/aero...rsche91100.html
Tested a 2000 911, 600 pounds lift.
A GT-3 test that talks about "no lift" not downforce.
http://www.mulsannescorner.com/aero...rsche91100.html
Tested a 2000 911, 600 pounds lift.
http://www.factoryfive.com/table/ff...namicstest.html
Three road cars that produce downforce at 150 mph including one Porsche although I don't think the Carrera GT has many aerodynamic similarities with the 951.
Three road cars that produce downforce at 150 mph including one Porsche although I don't think the Carrera GT has many aerodynamic similarities with the 951.
Originally Posted by from your link
2004 Porsche GT ---------------- 343 lbs. DOWNFORCE @ 150 mph
1999 Ferrari 360 Modena ------ 294 lbs. DOWNFORCE @ 150 mph
2005 Factory Five GTM --------- 333 lbs. DOWNFORCE @ 150 mph
1999 Ferrari 360 Modena ------ 294 lbs. DOWNFORCE @ 150 mph
2005 Factory Five GTM --------- 333 lbs. DOWNFORCE @ 150 mph
http://www.ddavid.com/formula1/lotus79.htm
Lotus 79 design info citing Bernoulli's principle as the source of "ground effect."
George, it was hypocritical to complain about using "ground effect" and then do the same with the word "seal." You are correct that racers no longer "seal" to the ground. Other than that, I don't see anywhere we disagree. Race cars have, for the most part, moved beyond simple bernoulli chambers- See:
http://www.mulsannescorner.com/vortexlift.html
Lotus 79 design info citing Bernoulli's principle as the source of "ground effect."
George, it was hypocritical to complain about using "ground effect" and then do the same with the word "seal." You are correct that racers no longer "seal" to the ground. Other than that, I don't see anywhere we disagree. Race cars have, for the most part, moved beyond simple bernoulli chambers- See:
http://www.mulsannescorner.com/vortexlift.html
Coming from an aviation background, the term "ground effect" means something quite different than it does in racing. But the commonality is this: "Ground effect" are aerodynamic effects that make use of the airplanes or cars proximity to the ground. Whether you are talking about venturi's, diffusers, sliding skirts, etc... doesn't really matter to me. The only term I don't like is when people can a spoiler a wing, or when they call a wing a spoiler.. those two should not be interchangeable terms in aerodynamics!
Bill, I appreciate your input.. it's really valued to me because you are clearly well educated. However, that does not mean that you aren't overlooking some details, which could change the outcome of your opinion on this subject!
12-26-2005, 04:07 AM
#97
Nordschleife Master
I've noticed some disturbing things in this thread.
First we have failed to adhere to the Socratric method.
For us to have this discussion in a productive matter, we must first define our terms.
first define "lift".
Next define "Downforce".
Are they mutually exclusive or conditionally inclusive?
Once we have determined a commonly accepted defintition of each term, we can then attempt to apply those terms within the scope of our application.
To do so, we must fiorst define our application.
Let's do that, and define the accepted conditions in which we will apply the vehicle, and then, only then can we make real head way without being sucked down by semantics and guesses.
First we have failed to adhere to the Socratric method.
For us to have this discussion in a productive matter, we must first define our terms.
first define "lift".
Next define "Downforce".
Are they mutually exclusive or conditionally inclusive?
Once we have determined a commonly accepted defintition of each term, we can then attempt to apply those terms within the scope of our application.
To do so, we must fiorst define our application.
Let's do that, and define the accepted conditions in which we will apply the vehicle, and then, only then can we make real head way without being sucked down by semantics and guesses.
Lift is the opposite.
12-26-2005, 10:58 AM
#98
Nordschleife Master
Ack!
Someone needs to find the wind-tunnel numbers for these cars.
I'll bet a used headlight that the 944/951/968 has lift at speed. Without ground effects or tunnels under the car it is inevitable. Less than a Cavalier? I gosh derned hope so!
Someone needs to find the wind-tunnel numbers for these cars.
I'll bet a used headlight that the 944/951/968 has lift at speed. Without ground effects or tunnels under the car it is inevitable. Less than a Cavalier? I gosh derned hope so!
12-26-2005, 05:00 PM
#99
Race Director
Originally Posted by Rich Sandor
If you look at most 959's now, you'll find an additional "gurney flap" added below the rear wing, in order to actually get some rear downforce!
Originally Posted by Rich Sandor
These cars make about 300lbs of downforce at 150mph.
Originally Posted by Rich Sandor
Coming from an aviation background, the term "ground effect" means something quite different than it does in racing. But the commonality is this: "Ground effect" are aerodynamic effects that make use of the airplanes or cars proximity to the ground. Whether you are talking about venturi's, diffusers, sliding skirts, etc... doesn't really matter to me.
I'll try to find the article I read about all of this. Once again I think it was in Racecar Engineering (one of the few magazines I buy these days). It was very informative.
Originally Posted by Rich Sandor
Bill, I appreciate your input.. it's really valued to me because you are clearly well educated. However, that does not mean that you aren't overlooking some details, which could change the outcome of your opinion on this subject!
Never should anyone thing a good technical discussion is a chest thumping argument. They aren't for me at least. I like to share info and get info from others. More than once I've learned an important tidbit that changed my understanding of a topic.
12-26-2005, 05:01 PM
#100
Race Director
Originally Posted by GlenL
Ack!
Someone needs to find the wind-tunnel numbers for these cars.
I'll bet a used headlight that the 944/951/968 has lift at speed. Without ground effects or tunnels under the car it is inevitable. Less than a Cavalier? I gosh derned hope so!
Someone needs to find the wind-tunnel numbers for these cars.
I'll bet a used headlight that the 944/951/968 has lift at speed. Without ground effects or tunnels under the car it is inevitable. Less than a Cavalier? I gosh derned hope so!
12-26-2005, 05:11 PM
#101
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Thanks Geo and Rich for this one. 
I don't think anyone who has stuck through all 7 pages thinks this is an argument anyway. I do think this has been a great opportunity to lean some things in regard to aerodynamics, lift and down force.
BTW has anyone here considered looking to fluid dynamics for some answers? With a scale model of a 951 you could maybe prove some thing regarding air flow and how it effects the body of the car. I saw a similar experiment on Mythbusters on a truck and weather or not the tailgate down or up was better. Turns out it was better up, go figure.
I don't think anyone who has stuck through all 7 pages thinks this is an argument anyway. I do think this has been a great opportunity to lean some things in regard to aerodynamics, lift and down force. BTW has anyone here considered looking to fluid dynamics for some answers? With a scale model of a 951 you could maybe prove some thing regarding air flow and how it effects the body of the car. I saw a similar experiment on Mythbusters on a truck and weather or not the tailgate down or up was better. Turns out it was better up, go figure.
12-26-2005, 05:23 PM
#102
Race Director
Originally Posted by my84-944
Thanks Geo and Rich for this one. I don't think anyone who has stuck through all 7 pages thinks this is an argument anyway. I do think this has been a great opportunity to lean some things in regard to aerodynamics, lift and down force.
BTW has anyone here considered looking to fluid dynamics for some answers? With a scale model of a 951 you could maybe prove some thing regarding air flow and how it effects the body of the car. I saw a similar experiment on Mythbusters on a truck and weather or not the tailgate down or up was better. Turns out it was better up, go figure.
I don't think anyone who has stuck through all 7 pages thinks this is an argument anyway. I do think this has been a great opportunity to lean some things in regard to aerodynamics, lift and down force. BTW has anyone here considered looking to fluid dynamics for some answers? With a scale model of a 951 you could maybe prove some thing regarding air flow and how it effects the body of the car. I saw a similar experiment on Mythbusters on a truck and weather or not the tailgate down or up was better. Turns out it was better up, go figure.
The bottom line is that aerodynamics is anything but intuitive.
12-26-2005, 05:29 PM
#103
Nordschleife Master
Originally Posted by Geo
Originally Posted by Rich Sandor
If you look at most 959's now, you'll find an additional "gurney flap" added below the rear wing, in order to actually get some rear downforce!
http://www.theautoappraiser.com/88%2...20959%20RR.jpg
http://img41.exs.cx/img41/4769/Porsche959spoiler.jpg
12-26-2005, 05:48 PM
#104
Race Director
Originally Posted by Rich Sandor
Ha! I knew someone would try to correct me on that one! I didn't say on the bottom side of the rear wing, I said below it! Yes, gurney flaps lengthen the chord of the wing... but, it doesn't need to be on a proper "wing" to work! Geo, take a look at these photos and tell me what you'd call the add-on strip, if not a gurney flap! (I couldn't think of any other term!)
A Gurney Flap, or wickerbill is a very special aerodynamic device used with a wing. IIRC it also delays flow separation and that is what makes it act like a wing with a longer chord.
Damn, I HAVE to find my copy of Tun to Win. I was out of town for Christmas and I'm in the office today so I didn't get a chance over the weekend to look for it. I have a party tonight so I won't be able to find it tonight either.
12-26-2005, 06:02 PM
#105
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Originally Posted by Geo
The bottom line is that aerodynamics is anything but intuitive.
Then they should try playing in a flowbench with modeling clay....they'd get some revelations... Air almost never does what you think it does...