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Originally Posted by UltArc
It was clear, now it is not clear lol. Does anyone have resources I could review to expand my knowledge on this? I understand how a wing is going to hit the air and become an issue, but if it's creating less force than the underbody (which has less drag) wouldn't it noticeably increase the rolling resistance?
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I would recommend
Race Car Aerodynaimcs - Designing for speed by Joseph Katz.
If you add downforce you'll add Crr, but it's not going to be an issue at road speeds. At 93MPH, that GT-R will be making 44lbs of downforce, nothing to loose sleep about. At legal speeds, you might see 20lbs.
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Ain't nothing for free...and since all LIFT and DOWN forces derive their respective results from the forward air motion, anything that creates either one, manifests itself as an increase in total drag.
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Sure, my point is it's possible to take a standard car, add an undertray and diffuser, and improve both Cd and Cl. That makes it 'free' downforce. Yes it could have even less drag if designed purely for Cd. But:
To use the GT-R example, let's just assume that the rear wing is enough to create a lift neutral car, and the splitter/ underbody create the rest at the 10:1 ratio.
At 186MPH, 176lbs of downforce is costing 17.6lbs of drag. About the same as sticking your palm out the window. At road speed that 20lbs of downforce is costing 2lbs of drag, barely measurable. This is a great cheat that lets designers create a low drag street car, that can still demolish the odd race track.