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The aerodynamic resistance that opposes the car's forward motion. More downforce typically creates more drag, so setup decisions involve finding the right trade-off between cornering grip and straight-line speed.
Katz's Race Car Aerodynamics: Designing for Speed explains the fundamental relationship: "the total resistance, including aerodynamic drag, increases very rapidly and shows a parabolic curve fit (D = C * V-squared), which indicates that aerodynamic loads increase by the square of speed." He notes that "at higher speeds the curve-fit is far better — aerodynamic forces are very important at racing speeds." McBeath adds that the optimum wing location involves trade-offs between "the rear wing's potency, profile(s) and plan-form shape" and the resulting drag penalty. For HPDE cars, drag primarily comes from form drag (the shape of the car) and any aftermarket aero devices — reducing drag improves straight-line speed but may sacrifice the downforce needed for corner grip.