Originally Posted by
rpenmanparker
Let's take that further. So 28 mph headwind. Supposing you have impeccable balance and don't pedal at all. The wind will accelerate you backwards (decelerate you) until you reach a speed where the combination of the frictional forces and differences in drag coefficient front and back balance out. That won't be exactly -28mph, but if we assume a frictionless bike and same drag, close enough. So the wind would be exerting some force (proportional to the square of the wind velocity) over distance and time. Energy is being expended by the wind and power is applied as well not by the rider but by the wind. Quite significant I would think. Now in order to just "speed up" from -28 mph to 0 mph, would you not have to apply the same power in the forward direction.
Power will be applied to the frictionless bike to accelerate it. Once the bike reaches a steady state speed, no more power is applied to the bike from the wind. To oppose this (assuming the bike is still stationary and you intent it stay stationary), you need force to counter the wind drag, but you don't need power because the bike is not moving relative to the ground (where the force is being applied).
A good rule of thumb is, if you can do the job with an inanimate object, you are not applying power. A block of wood or your brakes cannot apply power to anything. If you can do the same job with a block or your brakes as you can with your legs, then you are not applying power to the bike. From the bike's point of view, it doesn't know if it's your legs or a block of wood or your brakes that is holding it in place.
Your powermeter will read zero under two conditions:
- the force on your drivetrain is zero
- the drivetrain is not turning
This is because
power is
force times
velocity.
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