
#1




How do you measure downforce?
How is downforce measured (the exact amount of of produced at speed..)?

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#2




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 Terry Phillips 
#3




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 
#5




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.
Taz Terry Phillips 
#6




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.

#7




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. 
#8




Quote:
So in a wind tunnel, they will measure how much "heavier" the car gets on the front and rear axles? 
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#9




Quote:

#10




Quote:
200# of down force has the same effect on ride height, regardless of the conditions that created it. Last edited by glasser1; 05052010 at 06:11 PM. 
#11




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.
Taz Terry Phillips 
#12




Or simply put load sensors under the tires to get a very accurate measurement.

#13




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 
#14




Is that with or without your double diffuser and FDuct?

#15




My FDuct? Why the hell are people asking me about my FDuct? So personal...
GT 
#16




Quote:
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; 05062010 at 12:19 AM. 
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#17




Thanks for the explanations !

#18




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.

#19




Quote:
Mitch, if only the track were flat! <grin>
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Peter www.peterkrause.net Driver Performance Analyst, Consulting and Tuition in the Art and Science of Going Fast! 
#20




Strain Rosettes;
125UY CEA 120, 350 0.125 3.18 Threeelement 60° delta singleplane 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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