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  #1  
Old 05-05-2010, 12:43 PM
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How do you measure downforce?

How is downforce measured (the exact amount of of produced at speed..)?
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Old 05-05-2010, 01:37 PM
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Andrew- It is usually measured just in pounds on the front and rear axles. Pretty much need a wind tunnel to get accurate results, although CFD can give you good approximations if it was grounded using real data. CFD models without real data around which to base them can give some squirrelly results.

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Old 05-05-2010, 01:44 PM
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You can measure it by using the same linear potentiometers that are used on any data acq. It can actually be extrapolated very easily.

jay
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Old 05-05-2010, 01:52 PM
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what is the Formula?
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Old 05-05-2010, 02:14 PM
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Jay- You are correct. If you want to put orange wires on the car and measure suspension compression, it is not that difficult. You could actually do the same thing with a laser measurement of ride height while at speed. Either set of numbers is easy to convert to pounds, since you know the spring parameters.

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Old 05-05-2010, 02:35 PM
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Quote:
Originally Posted by miketuason View Post
what is the Formula?
The difficulty in calculating downforce (or anything aerodynamic) is that you don't have one single formula, but have to use special numerical techniques with special sets of formulas to calculate your result.
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Old 05-05-2010, 03:18 PM
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Originally Posted by miketuason View Post
what is the Formula?
You find the linear position of the car sitting on a dead flat surface (ride hieght null point)
You find out the spring rates front and rear (WSM)
You determine the spring rate to wheel rate via suspension geometry (geometry and trig)

Then you take the car up to speed and measure the change in ride height versus speed on a dead flat road surface.

Then you back calculate via wheel rate out how much downforce (or upforce) existed

Alternately you can add weights to the front and rear of the car until the ride height matches that at speed.
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Old 05-05-2010, 04:29 PM
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Originally Posted by tazandjan View Post
Andrew- It is usually measured just in pounds on the front and rear axles. Pretty much need a wind tunnel to get accurate results, although CFD can give you good approximations if it was grounded using real data. CFD models without real data around which to base them can give some squirrelly results.

Taz
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Terry,

So in a wind tunnel, they will measure how much "heavier" the car gets on the front and rear axles?
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Old 05-05-2010, 04:35 PM
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Originally Posted by Mitch Alsup View Post
You find the linear position of the car sitting on a dead flat surface (ride hieght null point)
You find out the spring rates front and rear (WSM)
You determine the spring rate to wheel rate via suspension geometry (geometry and trig)

Then you take the car up to speed and measure the change in ride height versus speed on a dead flat road surface.

Then you back calculate via wheel rate out how much downforce (or upforce) existed

Alternately you can add weights to the front and rear of the car until the ride height matches that at speed.
you also need to know ambient air conditions - air density, wind speed
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Old 05-05-2010, 05:10 PM
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Originally Posted by mousecatcher View Post
you also need to know ambient air conditions - air density, wind speed
Although these factors affect downforce, they don't affect the relationship between downforce and subsequent ride height.
200# of down force has the same effect on ride height, regardless of the conditions that created it.

Last edited by glasser1; 05-05-2010 at 05:11 PM.
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Old 05-05-2010, 06:45 PM
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Andrew- In a wind tunnel, any of the methods mentioned above becomes very simple. A moving road wind tunnel like Ferrari uses makes it very simple to measure ride height directly or to measure spring compression directly without having to use orange wires. Just hook up sensors, measure directly, and record the data through a wire (or wireless) connection between the sensor and your data recording system. Either, or both for accuracy, gives you almost a direct reading of downforce if your system is calibrated for that particular suspension system.

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Old 05-05-2010, 07:49 PM
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Originally Posted by tazandjan View Post
Andrew- In a wind tunnel, any of the methods mentioned above becomes very simple.
Or simply put load sensors under the tires to get a very accurate measurement.
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Old 05-05-2010, 11:05 PM
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First, I stand on the scale and take a reading without any wind.

Then, I turn on one of those big ass shop fans and see what that scale reads. Depending on how I hold my hands and comb my hair, I'm up to 14lbs of down force.

GT
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Old 05-05-2010, 11:09 PM
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Originally Posted by GTHill View Post
First, I stand on the scale and take a reading without any wind.

Then, I turn on one of those big ass shop fans and see what that scale reads. Depending on how I hold my hands and comb my hair, I'm up to 14lbs of down force.

GT
Is that with or without your double diffuser and F-Duct?
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Old 05-05-2010, 11:11 PM
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Originally Posted by 1_can_dream View Post
Is that with or without your double diffuser and F-Duct?
My F-Duct? Why the hell are people asking me about my F-Duct? So personal...


GT


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Old 05-05-2010, 11:16 PM
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Quote:
Originally Posted by Mitch Alsup View Post
You find the linear position of the car sitting on a dead flat surface (ride hieght null point)
You find out the spring rates front and rear (WSM)
You determine the spring rate to wheel rate via suspension geometry (geometry and trig)

Then you take the car up to speed and measure the change in ride height versus speed on a dead flat road surface.

Then you back calculate via wheel rate out how much downforce (or upforce) existed

Alternately you can add weights to the front and rear of the car until the ride height matches that at speed.
Yup, in theory. If you've ever tried to do this in practice, you find out very quickly that there's no such thing as a dead level road. Even on the flattest road you can find, the car is still weaving and bobbing around on the springs, so, what you end up with it a whole pile of data points cross referenced to timing marks, and then you have to run statistical formulae to then try to build a model for speed v. average spring measurements. It ain't as easy as it sounds.

Another way to measure down force very precisely is to take your MG out on a straight road and drive it as fast as it will go. Pretty soon you'll feel the front end get so light that it's clear the car wants to fly. At the precise speed at which the front wheels leave the ground, then you've achieved exactly the (negative) downforce equivalent to the front axle weight of the car.

Last edited by CliffBeer; 05-05-2010 at 11:19 PM.
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Old 05-07-2010, 10:35 AM
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Thanks for the explanations !
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  #18  
Old 05-08-2010, 02:17 AM
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Originally Posted by glasser1 View Post
Although these factors affect downforce, they don't affect the relationship between downforce and subsequent ride height.
200# of down force has the same effect on ride height, regardless of the conditions that created it.
Of course, but you have to know the conditions for the number to have any real meaning. You could generate 1500# of downforce but your wing stalls at 50MPH after which maybe you even generate lift.
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  #19  
Old 05-08-2010, 07:55 AM
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Originally Posted by glasser1 View Post
Or simply put load sensors under the tires to get a very accurate measurement.
Load cells under the spring perches, shock pots and laser ride height front and rear along with a true air speed sensor (pitot tube) can supply the information you need.

Mitch, if only the track were flat! <grin>
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  #20  
Old 05-08-2010, 09:58 AM
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Question

Strain Rosettes;
125UY CEA 120, 350 0.125 3.18
Three-element 60 delta single-plane rosette.
Matrix size: 0.50L x 0.44W in. (12.7L x 11.2W mm)

Placed on the flexible CF wing standards, will give real world downforce mapping.
After they are calibrated in a wind tunnel..


Edwardo
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