GlenL

"Valid" is pretty harsh. "Authoritative" might be better.

I've found good stuff on Wikipedia. Some is dubious or just wrong.

ehall

From an academic stand point it is not a 'valid" source because the material within is open to any individual input, regardless of qualification.
A "valid" source is qualified using multiple verifiable sources, from qualified institutions or testing groups. wikipedia is a good place to get abasic idea of a concept, but not a source for academic information. Asthis has been a really good thread, we might want to find authoritative sources.


Sorry Daniel. I only read your post via email, and didn't see the smiley. My bad. No harm no foul.

I'll add this for some interest, and to try to adsd to the actual topic: http://e30m3performance.com/myths/splitter/splitter.htm

shiners780

Understood, Ehal...however...

I did make mention of the fact that the informations source was unknown on wikipedia. Regardless of who wrote it, it was very interesting and made reasonable assumptions. A good read, if nothing else. There are many good ideas out there from people who are not verifiable "authoritative sources". On the flip side, information given by qualified institutions or testing groups is sometimes bullsh!t as well.

I understand your concern, but posed the information as a source of discussion, not as absolute fact. Sorry if it came across that way...

Geo

That site is generally quite good.

If, however, one would like a VERY authoritiative source on splitters, I would be happy to send scans of the Racecar Engineering article. I won't post it because that would violate the rule about copyrighted material. It's quite a good article, good tech, but understandable without being a nerd. Lots of CFD analysis by the CFD company owned by BarF1 (or is that Barf 1? ). Or perhaps it's owned by Honduh now. I don't know of the CFD company was part of the F1 team deal.

Rich Sandor

George, which issue is that article from?

Geo

November 2004.

It's a good article. Technically good, but not overly so that you need to be an engineer to understand it and benefit from it. If you don't have it I can e-mail the scans (they are BIG).

azmi951

An intersting item to note. The Infinity G35 coupe has an optional aerodynamic package and it is claimed to create zero total lift on the body. I dont know how true this is but Infinty claims it was devloped in the wind tunnel.

Also a good aerodynamics book is by Joseph Katz "Race Car Aerodynamics"



There are also ways to determine lift and drag forces using rotational potentiometers. They can measure suspension displacement at each wheel to later calculate aerodynamic forces. It would not be a hard thing to do if given good data collection hardware (read big $$$$).

xsboost90

yeah i always wondered why cars werent flat on top and round on the bottom...like an upside down vw bug. Of course that would make wheel placement interesting.

Geo

Besides the obvious packaging difficulties, it's not the most effective way to make downforce in a car (reference my post about Colin Chapman). What you find is many of the very high end cars have some light ground effects to produce some downforce. It's not terribly likely any of the established manufacturers would build cars with serious ground effects simply because true ground effects cars are not easy to drive.

So, why don't most cars have smooth underbellies with small ground effects tunnels or rear diffusers? Cost. Cost to manufacture. Cost to develop. Cost to assemble. Doing proper aero design is not cheap. Thus, you only find it on supercars.

ehall

My comments were just directed at the source. I see it quoted all the time as an authority, and it'sa bit of a peeve of mine. My comments weren't meant to deride you persoanlly. I apologize if they came off that way.
You see bone headed wikipedia posts in OT way too often.
Anyway, this is a really good thread. 'Legitimate Rennlist'.
Have we decided now that we are mostly battling a semantics issue vis a vis downforce as opposed to reduction of lift? I'm still a bit foggy on where we stand.

Bill.e1

Hello again and happy holiday.

I am fairly new to this website and 944 ownership. I've asked a few questions and posted a few answers. I have an engineering degree in IC engines but work on medical devices.

Cars don't act as wings- the are too long for their width so spillage from the sides quickly fills in any local low pressure areas under tha car. Look at airplanes that fly at car-type speeds. They all have long, skinny wings. Don't get hung up on what a device is called either. A spoiler is called a spoiler because that is the name that stuck, not because it "spoils" anything. (Although the original 911 duck tail is a true spoiler- it breaks the airflow down the long, continuos rear surface. The whale tale is not a spoiler- it is too big and actually takes over as the car's trailing edge)

True Ground Effect occurs when an airplane is near the ground when the proximity and relative velocity between wing and ground "tumbles" the air under the wing, creating a "pillow" of air that adds lift. Absent some sort of Chicken Little event, ground effect cannot create ground force.

What racers call "ground effect" is actually venturi effect. A venturi requires a long, fully enclosed, expanding channel to reduce pressure inside the channel. If you lose seal or if the channel gets bigger too fast, you lose the effect. Venturi cars need to be low to the ground, preferably sealed with draggin skirts or spillage kills you. If the nose drops, air separates from the channels and suction is lost. If the nose lifts, flow increases and suction is lost. A bit more air under the nose and you get Mark Webber's Mercedes.

From around 1970 to 1990, you can see how computer power has effected racing. Porsche 917's grew longer and longer tails as comparative testing showed this helped. 935's are slab-sided and flat-topped so computers of the day could analyse flow. Notice how the 935's aero fences are next to rising surfaces? They are there to keep the hiqh pressure on top of the car and force the car down. Falling upper surfaces are not fenced since preserving low pressure would create lift and drag.

Today's racers are all curves again because today's computers are fast enough to handle the complexity.

For a road car, lift costs horsepower so everyone works to reduce it. Cars have smoother bottoms, under-nose radiator intakes, and high, sharp trailing edges. The 944's "spoiler" gives the car a cleaner, higher, more rearward trailing edge. This reduces drag and lift. If you want to see what it would take to develop useable downforce at highway speed, look at an outlaw sprint car or SCCA open autocrosser. Huge wings with barndoors for endplates. Better find a few hundred more horsepower too unless you're OK with a 75mph top speed.

Hope this helps and happy holidays,
Bill

Geo

Bill, I respect your engineering degree and contribution. However, understand that there are some highly specialized areas of engineering where everything is not always as it seems. Also never underestimate the amateur with a passion on a subject because they often can and do keep up with some of the technical matters.

I'm sure the Champ Car guys and Forumla Atlantic guys will be surprised to hear they are not making serious downforce. Sliding skirt ground effects cars have long been outlawed everywhere because they are crazy to drive. The faster you go, the more downforce and the faster you can go, right up until the car loses grip and then the car loses virtually ALL grip. Still, after sliding skirts were banned, cars were built with ground effect tunnels. They made a considerable amount of downforce too. F1 was the first major series to ban the tunnels, but Champ Cars and Atlantic cars still have tunnels today. Over the years the tunnels have been legislated to ever smaller and narrower but they are still there and they still make considerable downforce, even without the sliding skirts. The vast majority of the downforce on these cars is from the tunnels and the wings are more for trim than anything else.

This is far more true with the "flat bottom" cars than the tunnel cars. In fact, this is why Colin Chapman's team switched to the tunnels - they were not only more efficient, but they were far less pitch sensitive. Calling the flat bottom cars "flat bottom" is something of a misnomer since they are only flat between the trailing edge of the front tire and the leading edge of the rear tire. The F1 cars also have a stepped flat bottom. In the back is a diffuser of course that begins at the leading edge of the rear tire and serves to speed air flow under the car, thus creating the downforce.

Yes, it does so by delaying flow separation (Smith, Tune to Win) as already mentioned.

Nicht so. Again, this is where specialized knowledge overcomes general knowledge. In a Racecar Engineering article on Touring Cars in the last year or so they discussed the effect of wings in the development process. The addition of a simple, but efficient small wing dropped multiple seconds consistently from their lap times. Well designed, efficient wings have more impact at highway speeds than most people think.

Welcome to Rennlist in general and this discussion in particular. Always great to have good technical discussions.

BigNNasty

Hmm...

I'm not in for all the technical mumbo jumbo, but...

Has anyone ever noticed that an Enzo's underside and a Murcielago's are like, completely flat? This would make the car rise right? Considering the cars would then be shaped like an airplane wing?

In the R/C world, if your car can move at a clip of 100 mph or so, your fast! I have no clue what I'm proving, but let me tell you about this one experience.

Guy's car went Like 103 mph or so, using a C5 corvette body, with a rear wing. At around 103mph, his car lifted up, and flew. Why you may ask? The bottom of the car is indeed flat, but air got up under the body. This is called lift, correct me if I'm wrong. His car most likely produces downforce at around 40-50 mph where air is not traveling fast enough under the car, to actually lift it, but traveling fast enough over his rear wing to swquat the *** end. I have no clue if that's on track or what, but this is what I'm picturing.

Basically, the wings do create downforce in R/C. The sole reason the guy with the C5 corvette car lifted up at 103mph was due to him not having downforce on the front of the car to keep it planted. Sure, he probably had GOBS AND GOBS of downforce on that rear end, but if he had none on the front to compensate, it's going to lift up if you don't compensate for the other. At the speed he was traveling, it was basically like someone standing on the back of his car, lifting the front end up, making the car try to wheelie. If he would have had downforce on the front, his car would have been less prone to lift.

For anyone that still does not believe me, you can make a car go 800+ mph in Gran Turismo 3. Know why? You put ZERO downforce on the front of the car, and MAX on the rear. Know what the car is doing at 800mph? A wheelie with sparks flying out the back. The car does 800mph because the air is lifting the front up, there for the car doesn't have to use power to osuh that.

Look at a dragster, it has a wing in the front of it, right infront of the front wheels. These are ONLY used to keep the front end planted. Indy cars have the exact same thing.

In the end, if your car has a wing/spoiler on the back, it's making downforce. If it doesn't have anything to equal the downforce being applied on the rear, on the front, then your car now experiences lift in the front because of the downforce in the rear.

The two forces depend on each other. When you have downforce, you have lift, period.

::Edit::

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.

my84-944

While I am not an engineer, I repeat NOT an engineer; this seems to kind of explain a few things here. While looking at high scientific data and “specialized” theory and fact, I find sometimes starting at the basic “jump off point” can help me to better understand more complicated principle. When looking at the fundamentals of a baseball and how it is thrown to get effect on spin and direction seems to be un-related to this subject, please just humor me. Here is an explanation of a “Knuckle Ball Pitch.
The ball drops and soars unpredictably because it doesn’t spin. The lack of rapid spin turns the seams of the baseball into tiny airfoils—surfaces that create lift and drag when they fly through the air. As the air passes over the seams, tiny swirls are created, causing pockets of low pressure around the surface of the ball. As air rushes in to fill the pockets, the ball is pushed in different directions. If the ball rotates too much, the seams will present a more consistent surface to the wind, and the ball will likewise follow a smoother path.
Ok from this I see:
1. The ball not spinning is moving through the air or space the same as a non-spinning car. Both are matter and air is displaced by both.
2. The seams or ridges in the ball act to disrupt the flow of air over the surface of the ball causing it to “dance” good in a pitch, bad in a car. But they act the opposite to a wing, and the same as a spoiler, just used in opposite ends and for the opposite function.
3. When spinning the seams effects on the ball are no longer as dramatic on overall travel.
If this is the case it would explain why on many Prototype and specifically built cars, Indy, Champ, F-1, Supercars, and the like, it is necessary that the driver carry a window of speed into a turn.

Looking again in simple terms:
A .Too little speed than the car just turns, this is slow and the Ground effect or spoilers have zero effect in travel. They are only added weight and with out the proper “wind” do nothing; I suspect this is the case in 99% of street cars with wings and spoilers in use today. (Speaking of course of Civics, Mitsu, and the like).
B. If you were to increase the speed and the line between the “ground effect” and the total loss of “stick” blur. Too fast and the cars slides by breaking the effect of your ground effects, spoilers etc..,
C. Go too slow but not *slow enough*(see A) and the loss of “effect” and the car will cause lift, or loose the benefit of the equipment.
What we are or should be pondering is not if the 951, 911, 944 or any other car creates down force, but weather the effect of positive lift over negative lift is an important effect on the car in terms of handling.

So does the 951 or 911 or any car for that matter create negative lift or positive lift?
Yes and no. I think a lot will depend on the speed traveling and the equipment on the car. Rear down force will I think be directly proportional to the effect of air travel over the “top side” of the body. While negative lift would be caused by under chassis effects. The same for the front. Air over the top moving to meet back up in the rear making lift in a wing, will also push against the front slope of the nose and push down (down force) while a splitter or front diffuser can cause the air pattern to change, thus reducing the ability of the air (on the bottom) to produce lift, causing negative lift.

I could be way off, but like I said I am no engineer, but Merry Christmas anyway.
Chris

Bill.e1

Geo,

I think the original question was if 944's generated downforce. The answer is no, they don't. When Porsche introduced the 959, they called it their first road car with neutral lift.

Don't get too hung up on the words. 100 pounds less lift helps handling just as much as 100 pounds of downforce plus it will probably reduce drag.

Don't look at a Ferrari and think it's design has anything to do with road driving. An Enzo is designed to separate some wealthy guy from his money and make the rest of us like Fiats. Any useful driving characterstics are purely coincedence ;-)

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. When Webbers car pitched up and caught air, that was ground effect ;-)

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.

The improvements and article you cited for small wings is consistent with what I've written. Losing 100 pounds of lift helps just as much as adding downforce. Aerodynamic trimming is not all about downforce. Touring car aerodynamics have more to do with what the regulations allow then what works best. I can't imagine the team would tell the writer how the car got slower as they "improved." it.

Anyway. please don't take this as arguing. I am writing my understanding of the topic. I think it is right mostly because it fits what I've seen/read/learned. Hopefully someone finds the discussion informative or, at least, entertaining.

Happy holidays,
Bill