The thing I've always found somewhat puzzling is: If the fastest pace that I can sustain on a flat road with no wind is 20mph, how is it that I can ride into a 20mph headwind and sustain any pace at all? Maybe a partial answer involves other sources of friction besides wind, e.g. tires. Riding at 20mph, you have a resistance A due to the wind and T due to tires and the maximum resistance that you can overcome is A+T. Riding into the 20mph headwind at, e.g. 4mph, you face a higher resistance AA since your relative airspeed is now 24mph, but you have lower tire resistance of TT because your tires are only going at 4mph instead of 20mph. So if AA+TT < A+T then you can still push harder into the wind! Is there something else I'm not thinking of, here?