The formula is Power (In Watts ) = Torque (in nM) x Angular speed(In RPM) /9.55 . This is same formula above, we just converted it to be used with rpm, 2 PI x RPS = (2 PI /60) x RPM = RPM/9.55.
Plug the values in you get,
8x300/9.55 =251.3 ~ 250 W.
I fully agree with this (the formula shows the dependencies of NM and W). We have:
- 300 RPM = 5 rotations per second
- 8 NM = given value
- 6.2832 = 2 * pi
- 0.95 = efficency of the drivetrain
The key point is: Why do we have 300 RPM?
It's because we have the rider's power and the motor's power together to achieve 300 RPM. And what will happen if we reduce the power of the motor but the rider still contributes with the same power? We will have e.g. 240 RPM. But as mentioned, the rider makes stil it's 265W. The formula is saying now: 4 * 8 * 6.2832 / 0.95 = approx 265W riders input. So, the formula claims the rides has reduced input -but this is not true, as we said, he maintains his power. We only reduced the contribution of the motor.
The reduction of the RPM can also occur, if the demand rises. E.g. if the wind increases or a climb is there. Then motor and rider contribution still with the same power values, but the speed (=RPM) decreases.
The RPM are not a given value, its variable which can change independently from the other values in the equation.
You see the dilemma I'm in?