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Originally Posted by GretchenGotGrowl
Although it has been a while since I took physics, I think there are two flaws with this. First, there are opposing forces to the downforce that must be accounted for. Cars a generally flat on the bottom and rounded on the top, which creates lift (like a wing).
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Yes, I ignored that -but with good reason. When a car is said to be producing downforce, the number quoted is the net result of the lift and downforce being generated. In theory, the body shape of the Z06 might be producing say 100 lbs of lift, while the various aerodynamic devices tacked on produce a sum total of 300 lbs of downforce, for a net result of 200 lbs downforce.
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Additionally, the suspension (especially the coils) and tires provide greater opposing forces as they are compressed. That is why acceleration is near zero.
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Yes, this is expressed in the increased rolling resistance. And as far as the tires & suspension are concerned, it doesn't really matter if its extra mass or aerodynamic downforce thats pressing down on them. So in that respect, you were right before when you said that aero & extra weight were the same.
But at speeds where aerodynamics become important, rolling resistance plays a fairly small role. I'm not 100% certain, but I think rolling res is a constant (based on the normal force at the tire). Aerodynamic drag is proportional to speed squared. So as speed increases, drag will quickly come to dominate over rolling resistance.
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It is also why they use manometers to measure pressure differentials in the wind tunnels. These differentials are extrapolated across all the surface area experiencing lift (upward and downward) to get the total lift. Downforce is just negative lift. I could be wrong, but that's how I remember it.
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Thats right. Create a pressure map, integrate over the surface area and you get a net force either up or down, for lift or downforce respectively.