Bike porn

With a strain gauge.
Power is the force x velocity produced by the rider. One of the first places in the drivetrain to experience this is the crank. So it makes sense to measure it there.

yes but the force has to take into account the actual cog in back, otherwise it's meaningless, especially if were talking velocity of the bike.

i honestly don't see the meaning in measuring realtime power before it hits the road.

a force of 5 newtons exerted by the rider at the crank when in the small cog, means a very different thing than when in the big cog. unless the velocity slows down exactly linearly as the mecanical advantage increaces???