Does this rely on a optimum speed?
Yes, speed is one of the factors used to calculate a drag coefficient. IIRC that drawing's from a NACA paper, so I suspect the relevant speeds are rather higher than those achieved by most bikes.
@Brommers started
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Yes, speed is one of the factors used to calculate a drag coefficient. IIRC that drawing's from a NACA paper, so I suspect the relevant speeds are rather higher than those achieved by most bikes.