For optimal performance, you must reduce your frontal area as much as possible. Your tires rolling resistance is also very important to bike efficiency, which is why you see racers running the narrowest and thinnest high-pressure tires available.
Depending on the exact configuration of the bike and the tires you are using, the power consumed by air resistance and the tires rolling resistance is typically about equal at 9-12 MPH. But because the power to overcome rolling resistance is linear with speed, and the power consumed by air resistance is a cubic function with speed…by the time you get to 20+ MPH typically over 70% of the overall power you deliver to the pedals will be consumed by air drag!
For example, let's say that the tires rolling resistance power/wattage equals the aero drag power at 12 MPH, which is pretty typical. So at 12 MPH, we have one unit of power going to overcome rolling resistance (RR), and one unit of power going to overcome aero drag at let's say a total of 40 watts at 12 MPH. That would be 20 watts for aero drag and 20 watts for rolling resistance at 12 MPH.
When we double our speed to 24MPH, we now need 40 watts of power for RR because it is linear with speed, or 2 X 20 watts. Since the aero power is proportional to the cube of the speed, when we double our speed the aero drag component becomes (2^3) x20 watts, or 8 X 20 watts= 160 watts.
When we add the aero and RR components together at 24MPH, we now have 40 watts + 160 watts= 200 watts @ 24 MPH.
We only doubled our speed from 12 to 24 MPH, but the power required went up by a factor of 5 (40 to 200 watts) because of the dominance of the aero drag at higher speeds.
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