heinrich

Interesting :-) Manual, brochure and story all say .34 with flaps closed and .352 with them open. I don't remember where I saw .32. .34 is the number du jour then.

Btw to all you naysayers, .34 is nothing at which to smirk.

heinrich

Hammer, total downforce listed where? by whom? Total downforce is total. Not nett.

So, if you tell me that our S4's aerofoils create a total downforce of 10,000lbs at 200mph, but at the same time total lift is 10,000lbs, life is good. The nett downforce for a good turbo 911 was neg a few pounds, ie nett lift.

I'm perfectly willing to accept that other manufacturers may quote nett downforce of 6,000lbs, but again, I have a very very hard time imagining a suspension that can send a car around a racetrack chicane at 60mph with its suspension load at say 2,000lbs, only to quadruple that (when adding 6,000lbs nett downforce) ending up with an end result of 8,000lbs total pressure downward on the tyres, suspension and frame.

Suspensions these days must be incredibly versatile and robust if you accept that .... so I'm not buying it. What happens to a suspension when a sherman tank rolls over it?

Nicole

/it was pretty darn good for 1987 - especially for a car with such width and wide tires.

Hammerhead

Heinrich,

Please look at the following link:
http://www.mulsannescorner.com/data.htm

And then look through the rest of the website.
http://www.mulsannescorner.com/

Most of the data is collected from the various manufacturers, engineers who worked on those vehicles, or racecar engineering magazines. I was just trying to alleviate confusion. You can believe what you want to believe. Many people still think synthetic motor oil will destroy their engine. I can tell you right now, that those cars were NOT engineered to produce x-lbs. of downforce to counteract x-lbs. of lift. No one in their right mind would design a car that creates thousands of pounds of lift. Even a VW Beetle only produces 742 lbs. of lift at 124mph. No one is stating that regular road cars are producing thousands of pounds of downforce, anyway. The Carrera GT only produces 640 lbs. at 205mph. You're correct in that normal road cars could not handle thousands of pounds of downforce, however racecars (F1, LeMans Prototypes, old IMSA GTP) are designed and manufactured to withstand those loads. If you look at the link listed above, go to the bottom of the page and work your way up, you'll see the progressive improvement in aerodynamics from the late 60's to today. The old GT40's used to produce lift at speed.

If you're really interested in finding out the truth, you should ask any current of former engineer from Grand Am, ALMS, F1, or IMSA or purchase a book on racecar aerodynamics.

Hammerhead

drnick and Greg Gray, please check your PM's.

Doug Hillary

Hi,

just a thought - Mark Webber and Daimler Benz may have some ideas on this, I seem to recall a few unplanned flips not so long ago!

Perhaps it was just that he was reaching for his coffee....................

Regards

Gnarly 928

Interesting discussion on a topic I've been studying. 928s are not purpose built racecars, by any means. No sports cars really are. So forget comparing one to an F-1 car. Use one AS a road race car, and improving it's downforce by any and all means also means you can improve your laptimes, significantly. Take the 996 Cup cars as an example, they are much faster lappers than their 996 street car cousins. Partly due to weight reduction, partly due to aerodynamics. The underside of a Cup car is a "very neat thing" shaped to help downforce *as much as practical* with a full undertray. They also have thier wings and splitters, all of which were wind-tunnel optimized to work as efficiently as possible, yet still allow the car to work on real life roads. Us 928 racers have done similar aero-improvments, and seen good increases in traction, due to help from the air around the car. A few proper modifications will give a significant improvment in lap times. Don Hanson

Realist D.

OK Hammerhead, I give you points for being the only person arguing here with some actual facts. "I once saw a program on ..." does not qualify as a fact ;-) I checked the website you recommended and did note that as I expected the GTP cars from the late 80's and early 90's created pretty much the most downforce in history (with the exception of the '78 Lotus which carried Mario to a World Championship he could not have earned otherwise), and as you point out it is primarily rules and secondarily the limitation of tire technolgy that limits what is possible in this area.

Was it you who claimed that a modern F1 car creates as much downforce as these earlier GTP cars? I didn't see any data on modern F1 cars and I very much doubt that to be true. The aero rules in F1 are so closely controlled and change so frequently that I very much doubt these cars are creating 6,000 lbs. of net downforce. Also, because F1 cars are obliged to use 13" diameter wheels with very high tire sidewalls it would very difficult to control weights on these tires that varied by 6,000 every few seconds. As an engineer that would be a daunting problem to control.

Anyway, kudos for having some facts! To all the others who like the urban legend type stories about Ferrari Enzos and Porsche Carrera GTs drivng upside down, the facts just simply don't support those stories. By the way, I once heard about a guy who bought a 1 month old Corvette for $50 ...

MBMB

0-100 times might provide some indication of drag; 100-150 times probably a more precise indication (since drag is more significant at higher speeds). There might even be a formula out there to calculate relative drag, all else being equal, from different delta-v's.

But lower drag might not be an "improvement." The addition that lowers drag (improving 0-100 times) might increase lift (to the detriment of cornering). Better lap times might be a better measurement of overall improvement.

It seems to my untutored mind that the downforce created by any aerodynamic addition could be measured by comparing the compression of the springs at speed with and without the addition. Measure the additional compression at each wheel, multiply by the spring rate, add them up, and you should have the downforce (a sillier word the more I type it).

Mark.

heinrich

"No one in their right mind would design a car that creates thousands of pounds of lift"

Yet we've all seen racecars that weigh thousands of pounds, take off and flip end-over. Which means that their total lift would have had to exceed their total mass *plus* downforce at that speed. If the numbers quoted were nett, then as I said before, the suspension of the car would have to bear not only the 6,000lbs of downforce, but also the weight of the car in addition, say 2,000lbs, making it a TOTAL of 8,000lbs, and additionally, whatever downforce was expended to combat lift, ending in more than 8,000lbs total. Driving a 2,000lb car at 200mph with no aerodynamic aids would certainly cause it to lift and flip. This means that at that speed, lift has exceeded 2,000lbs.

From http://www.tvmotorsports.com/lolaart.htm:
"A spring rated at 1,000 pounds per inch moves one inch when the first 1,000 lb. bears on it and another inch with every additional 1,000 lb. Let’s say a car has a 1,000 lb. per inch spring at each corner and generates 500 lb. of downforce at 60 mph. 500 lb. supported by 4,000 lb./in only deflects an eighth of an inch, but, at 120 mph, you’ve got 2,000 lb. of downforce which moves the springs 1/2 in. "

As I've said before, the numbers listed aren't nett downforce but total downforce. There is no reason to submit a car to 6,000lbs of downforce if there isn't something to be gained by that downforce. Particularly because that downforce comes at a high cost of additional drag.

Hammerhead is no the only one talking with the backing of facts, his facts are derived from one website. And that website doesn't qualify whether it is giving total or nett numbers. It seems logical to me that if technical specifications list downforce as 6,000lbs, then it is total downforce.

heinrich

Here's more on "net downforce". It doesn't appear that a net downforce of 6,000lbs is desirable. Only enough to keep the car on the road.

http://rclsgi.eng.ohio-state.edu/cou...o/6Report.html

http://autozine.kyul.net/technical_s.../tech_aero.htm

http://www.tricklefan.com/guide/draft.html

http://edog.mne.psu.edu/pdfs/AIAA-2002-5608.pdf

http://www.autospeed.com/cms/article.html?&A=0157

http://www.luxfamily.com/jimlux/robot/windball.htm

heinrich

MBMB wrote: "But lower drag might not be an "improvement." The addition that lowers drag (improving 0-100 times) might increase lift"

Absolutely. And the converse is true, as you add downforce you **slow your car down**, so you want to add enough downforce to stick the car to the road. If 6,000lbs is required, then that will be your target. If one pound is needed, then you'll shoot for one pound of ***total downforce***

bcdavis

When you see a racecar go over a bump. get the nose up a bit, and the air flips the car upside down, that is a pretty obvious example of the wind exerting at least 2000 pounds of force, or enough to lift the car. And airplanes! They generate enough lift with their wings, to lift an entire airplane. Which is sometimes very heavy. So it seems pretty reasonable to assume that the force of the wind at 100mph or more, is pretty significant. Both in terms of lift, and downforce...

6000 pounds? Maybe not on a car like ours.
But 1000? Maybe. I have heard people say that other automotive wings produce around 600 pounds of downforce.

It's all a tradeoff. I'd rather have downforce as a helpful addition to my handling stability, and lose a few MPH from my top speed, due to drag from wide tires, spoilers, wings, etc...

2V4V

For those who just must have a link...


http://8w.forix.com/fancar.html


I'm sure that most people who have read this thread already know that I was speaking of facts as well. Those who are too young to remember (myself included) might have done about a minute of web-based research and figured out what I was talking about.

A nice summary of the "sucker" or "fan" cars that I referenced more than once can be found on the above captioned site - and thousands more if you search it - just in case you think I made up a website for this post.
(Didn't I do a great job on Photoshopping in that rear skirt?)

I know for another fact that every serious auto racing engineer, and many enthusiasts, that I have ever met knows about "fan" cars. I didn't feel the need to elaborate, because I prefer to think that most people know this stuff or are, at least, thoughtful enough to do some research before telling me that I'm blowin' smoke - maybe I am, but I seem to have a lot of trained aero-engineers who agree with my wacky, non-factual, "$50 for a Corvette" stories.

If you disagree with the aerodynamicists, fine, enumerate the why. But just because I won't post a link to archived footage of the freakin' moon landing everytime the subject is posted, does not for a moment suggest that it did not happen. It does mean that if you are not familiar with the event, maybe you should do some research. I would have been happy to give more data had you indicated an interest in learning of what I was referencing instead of some faulty-logic blather that I'm not providing "facts".


Greg

Realist D.

Just a bit of attitude there huh gbyron? Sorry if I offended you. I am fully aware of the "fan" cars. I was around to see them. I saw Jim Hall drive his Chaparal at Mosport. I didn't see the Brabham F1 fan car because it was outlawed before they came to Mosport - it only ran once. There was some other interesting aero experimentation back then that was also outlawed - for example the "dual chassis" cars. These were F1 cars that had all of the aero effects attached directly to the wheels (so to speak) so that the suspension could still work effectively even at high speed - one of the flaws of high aero downforce is that it compresses the suspension until it is effectively closed, causing the car to function like a gokart with no suspension at all. All of this stuff was outlawed because there were no tires available (and there still aren't) that could handle the loads created by the downforce.

All of this is irrelevant to the arguement I was making, which is, there are no passenger cars that make "net" downforce. Calling a late 80's GTP car a passenger car doesn't count. The simple fact is that closed passenger cars create tons of lift, just like an airplane wing. We attach inverted wings to these cars to create downforce to counteract the lift. We also create ground effects under the car to counteract this lift. The bottom line is that most cars are left with a small amount of net lift after being designed carefully to create a ton of downforce. There are a few exceptions that create net downforce of a few hundred pounds, which is not enough to drive the thing upside down.

Also, I hope you caught the significant difference between downforce created by a wing and downforce created using the ground plane (ground effects) - there is a significant difference in the stability of the downforce created and in the amount of drag created between these two.