Determining Power on a Mechanically-Braked Bicycle Ergometer

In the exercise science field, a mechanically-braked bicycle ergometer is often used as a mode of exercise. In situations in which the physiological response to exercise is measured, it is important to know exactly how much exercise is being done, ie. The power output. Power can be easily determined on a bicycle ergometer.

Power is defined as the rate of doing work. Work is defined as force x distance. Therefore, power is:

Power = (force x distance)/time.

If you know the amount of force or resistance and the distance through which the force is being applied and the time the force is being applied, you can compute power. Power can be quantified using two sets of units. One set of units is kgm.min-1 (kilogram meters per minute) and a second set of units is watts.

Watts = (kgm.min-1 ) / 6 Because bicycle ergometers do not actually travel over a distance, the distance through which the force is applied can be determined by the travel of the flywheel. For each pedal revolution the flywheel travels 6 meters. If the number of pedal revolutions per minute (rpm) are known, then the total distance the flywheel travels is rpm X 6. The amount of force being applied is determined by how tight the belt around the flywheel is made and is measured by the travel of the pendulum (A in diagram 1) up the scale (B in diagram 1). The scale is in units of kilograms so if the pendulum is at the 1 mark, the force (or resistance) is 1 kilogram.

So, on the bicycle ergometer power in units of kgm.min-1 is the product of the flywheel travel x the force or resistance.

Power (kgm.min-1 ) = F x rpm x 6

You simply solve the equation for whatever unknown you are looking for. For example if force is 2 kg and rpms is 100, then power is 2 x 100 x 6 = 1200 kgm.min-1 . The power in watts is 1200/6 or 200 watts. A shortcut to calculating watts is F x rpm. If you know any two of the unknowns in the equation you can solve for the third unknown. It is just a simple algebraic equation.