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Old 6 Jun 2007, 09:06 (Ref:1930028)   #1
A. Mudge
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Ride Height vs Downforce

When going to a Hill climb in the season, a topic of conversation came up about Ride Heights and Downforce. Having an understanding of the ride height side of things, one thing I would like to know is that is there an equation which works out what level of downforce is going over the back wing if the car is doing a certain speed.

For Instance: if a car is racing at 100mph is there an equation which will show the weight of the downfoce which is being pushed on the back wing????

Any thoughts?
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Old 6 Jun 2007, 09:20 (Ref:1930044)   #2
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I dont think there would be an accurate equation, as this would depend on the density of the air flowing over the wind.

I would suggest that at circuits at sea level (where the air is most dense) it would have the most effect on ride height. At high altitude, it will have a lesser effect.

During a Hill climb, it would be possible for a car to ride lower at the bottom, and higher at the top with the same suspension calibration (depending on how long the climb to the top is). This effect would be minimal (probably only 1mm, and would depend on the amount of downforce used on the car).
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Old 6 Jun 2007, 09:40 (Ref:1930059)   #3
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Yup, i think that's also why races in high altitude locations need to run more wing than what they would if they were in Holland, for example.
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Old 6 Jun 2007, 10:09 (Ref:1930080)   #4
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Quote:
Originally Posted by 109E - Guess
For Instance: if a car is racing at 100mph is there an equation which will show the weight of the downfoce which is being pushed on the back wing????
You couldnt accurately tell without testing. You'd need wing speed, pressure, temperature, wing angle etc as it all affects the force generated.
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Old 6 Jun 2007, 18:24 (Ref:1930499)   #5
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But there must be some form of relationship between these variables and the product, which can then be expressed (if the value of the variables alters, then the result changes - but this will alter as according to the relationship between said variables and the product).

So, like, if we had the variable set [a,b,c,d,e,f,g], with each letter representing some downforce-affecting-thing, and with [x] as downforce-produced, then an equation could exist.

x = [a*b/e]*b-[g/c]/ga+f*[dae/fg]

Clearly, the above is an entirely arbitrary formulation. My point is there must be some kind of equation (well, more likely some kind of complex equation matrix - or something) that represents these things.

Life would be pretty difficult with regards development if there wasn't some kind of means like this to assist the process.

Still, there remains nothing quite like actual running to test the product of the theories.

Last edited by Dutton; 6 Jun 2007 at 18:31.
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Old 6 Jun 2007, 19:25 (Ref:1930568)   #6
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Although it seems simple it really isn't a straightforward question this. There *is* a formula for calculating what downforce is acting on your car at any given speed *but* you need to kinow the lift coefficient of the car in that partiuclar configuration in order to work it out. The lift coefficient is a "shape factor", which is kind of like a score for how much downforce an item or vehicle generates.

ok, for the more pedantically minded, it physically represents the proportion of the momentum of a mass of air of the same cross-sectional area as the vehicle which is converted into vertical force on the vehicle. For the even more pedantic, yes, you can get coefficients greater than 1 but that doesn't break Newton's laws, it just means that the area affected by the vehicle is greater than its geometric x-sectional area.

The effective lift coefficient of your car will vary with wing settings, ride heights and at least a dozen other things, in such a complicated way that there will be no analytical formula for it. You have to directly measure it, either by load cells in your wing mounts/suspension or ride height monitoring or in a wind tunnel.

Once you have a lift coefficient for the car in the posture you want to know about, you can calculate the force easily enough by using this formula:

Downforce = 0.5 * (air density )* (speed in m/sec)^2 * (car's frontal area in sqm) * (Lift coefficient)

Air density is roughly 1.1-1.3kg/m^3 over a realistic range of altitudes & temperatures and 100mph is 44.4m/sec.

Unfortunately since knowing the lift coefficient is the really critical but i'm not sure how helpful any of that actually is. Sorry.
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Old 6 Jun 2007, 20:40 (Ref:1930617)   #7
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And the bodywork/suspension/exhaust etc in front or around the wing will have a measureable effect on the 'efficiency' of the wing.
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Old 6 Jun 2007, 21:34 (Ref:1930662)   #8
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Here is an example:

Aerodynamic Test and Development of the Corvette
SAE Technical Papers

Title: Aerodynamic Test and Development of the Corvette C5 for Showroom Stock Racing
Document Number: 2002-01-3333
Author(s):
Larry Alan Kubes - GM Racing
James Ric Spaulding - GM Racing
Dwight Woodbridge - GM Racing
Abstract:
This paper documents a one shift (10 hour) wind tunnel test program conducted on a Corvette C5 prepared for Sports Car Club of America (S.C.C.A.) World Challenge racing. The testing was conducted at the Canadian National Research Center in Ottawa, Canada. Specific areas of test included front fascia and under tray, rear air discharge, rear wing configuration and angle, B-pillar configuration, and ride height. Standard wind tunnel test procedures were followed. In total twenty-six separate configurations were evaluated. Data for front and rear lift, total drag, and lift/drag (L/D) ratio are provided for each test configuration. The cumulative effects of the aerodynamic changes evaluated in his program, calculated at 192 KPH (120 MPH), increased front down force by 318 N (72 Lb.), and rear down force by 770 N (173 Lb.). Lift/drag ratio was improved from -0.597 to -1.016. These changes increased total drag by 381 N (86 Lb.). Further testing lowering ride height 2.5 cm rear and 5.0 cm front reduced drag by 326 N (73 Lb.) and resulted in a L/D ratio of -1.247. Although data are specific to the Corvette C5, the general principles studied may be applied to any production-based racecar. These modifications should be validated on the racetrack prior to competing to ensure the handling balance can be optimized for the driver and car combination.
File Size: 2642K
Product Status: In Stock
Included in: PT-118
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Old 6 Jun 2007, 22:02 (Ref:1930696)   #9
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ride height

A question that can not be answered with out knowing something about the vehicle
current issue of Race Tech has an article on this topic showing how to do simple tests with the car on a smooth flat road to arrive at some reasonable estimates of total downforce
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Old 11 Jun 2007, 20:24 (Ref:1934357)   #10
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"Race car Aerodynamics" By Prof. J.Katz (Pub.Bentley) deals with ride height in chapter 6. He shows that a front wing will achieve double the downforce of the wing alone if the ride height over chord ratio (h/c) is less than 0.3.

Elsewhere in his book, he says that "ffor ground clearance of less than h/c=0.5, the lift can easily increase by 50%"

But he also makes the point made above - airflow is dependent on so many other factors, in and around the car, that an absolute relationship is impossible.

I recommend his book to anyone with an interest in car aero.
John
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