Calculating power based on force and rpm


New Member
Jun 6, 2005
Hi everybody,

[SIZE= medium]Bike is hooked to a trainer on a certain cog.[/SIZE]

Is it possible, considering the accurate weight hanged on the horizontal pedal arm that makes it starting turning down , and certain RPM on the same cog, calculate the power output ?

[SIZE= medium] (say, from a static condition, a 10kg weight is required to get the arm start moving downward.[/SIZE]

[SIZE= medium]Weight is removed and rider pedals 1 minute at 90 RPM on that same cog )[/SIZE]

[SIZE= medium]Thanks ![/SIZE]

[SIZE= medium]J[/SIZE]

I don't think so, but it's hard to know exactly without knowing what type of trainer you have. I'll assume yours is like mine and has a viscous fluid resistance unit. The force stopping the wheel from turning when it is not moving is caused by static friction, but once it starts moving it is impeded by friction caused by the movement of a viscous fluid. These work in different ways and apply differing magnitudes of force. Since the wheel is always turning when you are pedaling, you are only interested in 2nd type of friction. To measure that friction, you need to measure the terminal angular velocity of the crank when a known force is applied to it. Terminal means the final steady state velocity. When a force is applied to the crank, the crank will accelerate until a a steady state velocity is reached. In theory it never reaches a steady state, but in practice it does. Measuring the terminal velocity is important because the resistance of a viscous fluid increases with velocity; much like air resistance increases with velocity. So what you are measuring is the velocity at which the resistance force equals the known force. I don't think it is practical to make this measurement by hanging a weight on the pedal because the force is dependent on the angle of the crank.

It's possible the power curve for your trainer has already been measured for you as it has for my trainer.

More information on friction is here:
[SIZE= medium]Hi, thanks.[/SIZE]

[SIZE= medium]As it is not possible for me to measure forces on a rotating crank, I referred to a particular static position of crank-arm at 9 o'clock.[/SIZE]
[SIZE= medium]My trainer is magnetic (Tacx Swing)[/SIZE]
[SIZE= medium]It takes a cerian value (kg) of force to move the crank arm from 9 o'clock toward 8 o'clock..7..6..[/SIZE]
[SIZE= medium]So my question was whether or not it is possible based on this force (which does tell something on the wheel's resistance) And based on the cadence of pedaling against this resistance, to determine the power output[/SIZE]

[SIZE= medium]Thanks again :)[/SIZE]
[SIZE= medium]J[/SIZE]
IF you knew the force required to turn the pedals at a constant angular velocity, then yes you could calculate the power. This is how commercial units work.

As stated previously, the scenario you offered does not measure this, however.

If the goal is to calibrate your trainer's speed to power relationship, you would have better luck renting a powertap wheel. Though some trainers resistance may change as the unit heats up.

If you desire a Macgyver solution, perhaps you can drive the rear wheel with a highly efficient electric motor, using a roller, while it is mounted on the trainer. The voltage multiplied by current going into the motor (minus losses) would be the power required to drive the wheel.