Downforce
12-17-2005, 08:43 AM
#16
Geaux Tigers!
Rennlist Member
Rennlist Member
Originally Posted by bleucamaro
Option B: I saw this once in a Lincoln Mark VIII comercial.
Take your car to a long straight away, like 2/3 the way down the backstretch of your local highschool track, and setup a high jump bar so that it just hits the roof. Get a running start at it and go under the bar as fast as you can. If there's no loud 'CLANG' then the body does creat downforce. To quantify it, dangle a string from the bar and apply chalk to the roof panel just behind the windshield. When you go back, see how far from the bar the chalk on the string is. But to make sure, try the 'CLANG' method first.
Take your car to a long straight away, like 2/3 the way down the backstretch of your local highschool track, and setup a high jump bar so that it just hits the roof. Get a running start at it and go under the bar as fast as you can. If there's no loud 'CLANG' then the body does creat downforce. To quantify it, dangle a string from the bar and apply chalk to the roof panel just behind the windshield. When you go back, see how far from the bar the chalk on the string is. But to make sure, try the 'CLANG' method first.
12-17-2005, 09:33 AM
#19
Drifting
Just worm your way into you local windtunnel and print out the data for everyone to enjoy. You may have to do it at 3am on a Sunday and run like hell when you're done, but it'll end the debate. 
12-17-2005, 11:49 PM
#21
Drifting
Join Date: Apr 2005
Location: Elk Gove, CA
Posts: 3,400
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Predator
Come on guys, damn. Of course the 944 makes downforce. If you don't believe it, take your shoe off and let it roll over your foot.
12-18-2005, 12:53 AM
#22
Addict
Rennlist Member
Rennlist Member
I was thinking about making a measuring device that would show flex and movement of components. you could do a live measurement of average ride height at stop and at speed. The calculate how much weight has to be duplicated to get that same height?
I was wanting to do this to figure supension and tub flex in cornering. Could be used to measure any movement of any component tho.
I was wanting to do this to figure supension and tub flex in cornering. Could be used to measure any movement of any component tho.
12-18-2005, 06:31 AM
#23
Burning Brakes
Join Date: Oct 2001
Location: Abbotsford, B.C.
Posts: 818
Likes: 0
Received 0 Likes
on
0 Posts
I would think that you'd have to build some sort of rig to mount on your car, using some sort of sensors that measures the vehicle height at stop and at speed.
12-18-2005, 10:21 AM
#26
Originally Posted by Predator
Last time I talked with Sir Issac, gravity was a force. And it sure as hell doesn't go up. Hence.... down......force. Getit, getit? It was a JOKE! Jeez..............
But, once you reach teh core of the earth.....oh, nevermind.
12-18-2005, 10:24 AM
#27
Originally Posted by MPD47
Reduces lift? Sure, downforce? I'm skeptical.
It would have to. Bernoulli's Principle. Just think of it as negative lift; especially with both the splitter and spoiler on your car.
12-18-2005, 10:54 AM
#28
Good article at www.en.wikipedia.org/wiki/downforce. According to the article, lift and downforce are the same thing, only using the principle in reverse. Lift creating an upward force, downforce creating a downward force. The only difference being the direction of force being applied. Therefore, in the case of the 951 rear valence, would not its effective reduction of lift also be considered an increase in downforce?
I always thought the 951 rear valence was designed to eliminate the vortex created by the air traveling above and below the car meeting at the rear. This vortex elimination allows the air traveling under the car to move faster and creates more low pressure both along the underside of the car and on the underside of the hood.
The shape of a car's bumper, hood, fenders, and windshield (and in some cases the rear spoiler) cause a deflection of air upwards (known as up-wash) resulting in an equal downward force (downforce).
I would think all cars, with few exceptions and some more efficiently than others, create downforce.
I always thought the 951 rear valence was designed to eliminate the vortex created by the air traveling above and below the car meeting at the rear. This vortex elimination allows the air traveling under the car to move faster and creates more low pressure both along the underside of the car and on the underside of the hood.
The shape of a car's bumper, hood, fenders, and windshield (and in some cases the rear spoiler) cause a deflection of air upwards (known as up-wash) resulting in an equal downward force (downforce).
I would think all cars, with few exceptions and some more efficiently than others, create downforce.
12-18-2005, 12:43 PM
#29
Race Director
Originally Posted by shiners780
Good article at www.en.wikipedia.org/wiki/downforce. According to the article, lift and downforce are the same thing, only using the principle in reverse. Lift creating an upward force, downforce creating a downward force. The only difference being the direction of force being applied. Therefore, in the case of the 951 rear valence, would not its effective reduction of lift also be considered an increase in downforce?
I always thought the 951 rear valence was designed to eliminate the vortex created by the air traveling above and below the car meeting at the rear. This vortex elimination allows the air traveling under the car to move faster and creates more low pressure both along the underside of the car and on the underside of the hood.
The shape of a car's bumper, hood, fenders, and windshield (and in some cases the rear spoiler) cause a deflection of air upwards (known as up-wash) resulting in an equal downward force (downforce).
I would think all cars, with few exceptions and some more efficiently than others, create downforce.
I always thought the 951 rear valence was designed to eliminate the vortex created by the air traveling above and below the car meeting at the rear. This vortex elimination allows the air traveling under the car to move faster and creates more low pressure both along the underside of the car and on the underside of the hood.
The shape of a car's bumper, hood, fenders, and windshield (and in some cases the rear spoiler) cause a deflection of air upwards (known as up-wash) resulting in an equal downward force (downforce).
I would think all cars, with few exceptions and some more efficiently than others, create downforce.
It's all a fair bit more complicated than that.
While lift and downforce are opposite sides of the same coin, is reducing lift the same as creating downforce? Perhaps it's semantics, but I don't think so, although they do get interchanged a lot.
Airdams, splitters, and spoilers all work to reduce the amount of air underneath the car. The reduction in air under the car will lower the pressure producing some amount of downforce. Speeding up the air under the car will also reduce the pressure and produce some amount of downforce. Based upon what Scott and others have posted here, that is perhaps what is happening with the 951 lower rear valance.
BTW, most of what the "tuners" call splitters are anything but. A true splitter is a flat panel that extends forward from the leading edge of the airdam. It separates the air flow and prevents spill underneath the car. They produce downforce by reducing the pressure underneath the car. Racecar Engineering had a great couple of articles about the effects of airdams and splitters last year using CFD (computational fluid dynamics_ analysis.
Spoilers and such tend to try to keep airflow attached and as such do more to reduce lift. Again, semantics, but for my mind one is about creating and effect and the other is about reducing an effect.
BTW, most car bodies produce lift. In profile they are shaped somewhat like and airfoil. They have a flat bottom, but a rounded top side, so air must travel faster over the top, reducing pressure, and thus causing lift. Some things you can do will reduce the lift. Others will create downforce. All semantics though I guess.
12-18-2005, 12:58 PM
#30
BTW, most car bodies produce lift. In profile they are shaped somewhat like and airfoil. They have a flat bottom, but a rounded top side, so air must travel faster over the top, reducing pressure, and thus causing lift. Some things you can do will reduce the lift. Others will create downforce. All semantics though I guess.
In air (or comparably in any fluid), lift is created as an airstream passes by an airfoil and is deflected downward. The force created by this deflection of the air creates an equal and opposite force upward on an airfoil according to Newton's third law of motion. The deflection of airflow downward during the creation of lift is known as downwash.
Common misconceptions
Many readers new to this topic may be looking for the explanation that is commonly put forward in many mainstream books, and even in scientific exhibitions which touch on flight and aerodynamic principles. Known as the "equal transit-time" explanation, it states that the parcels of air which are divided by an airfoil must rejoin again, and due to the greater curvature (and hence longer path) of the upper surface of an aerofoil, the air going over the top must go faster in order to "catch up" with the air flowing around the bottom, therefor due to its faster speed, pressure of the air above the airfoil is lower, etc. Despite the fact that this "explanation" is probably the most common of all, it must be made clear that it is utterly false. It has recently been dubbed the "Equal transit-time fallacy." There is no requirement that divided parcels of air must rejoin again, and in fact they do not do so. In addition, such an explanation would predict that an aircraft could not fly inverted, which is demonstrably not the case. The explanation also fails to account for aerofoils which are fully symmetrical yet still develop significant lift.
It is unclear why this explanation has gained such currency, except by repetition by textbook authors, and perhaps the fact that the explanation is easiest to grasp intuitively without mathematics. However, since it is wrong, the assumed intuition which serves it is also wrong, and the wise reader would do well to discount this approach. Note that any text book claiming to be a serious work on the topic will never promote the Equal Transit-time fallacy.
It is interesting to note that Albert Einstein, in attempting to design a practical aircraft based on this principle, came up with an aerofoil section that featured a large hump on its upper surface, on the basis that an even longer path must aid lift if the principle is true. Its performance was terrible.
Taken from www.en.wikipedia.org/wiki/downforce.