Accurate assessment of the brake torque on a rope-braked cycle ergometer

A Monark cycle ergometer is a device globally used in physiological studies to measure the work and energy levels of exercising humans. In this paper a rope-braked cycle ergometer is examined to determine accurate measures of work and power. The work done is generally assumed, by physiologists, to be the load (masses suspended in a basket to apply tension to the rope) multiplied by the distance through which a flywheel braked by the load is moved (Åstrand). In this case the flywheel of the ergometer is designed such that any point on the circumference moves through 6 m for one complete revolution of the pedals. This is a simplistic approach and fails to take into account rope-brake theory and mechanical analysis of the brake mechanism. The dynamic coefficient of friction between the steel flywheel and nylon rope was determined experimentally to be 0.175. The physical dimensions of the ergometer brake system were taken and the theoretical brake torque calculated for a series of loads. It was found that this calculated brake torque was 10.8% less than the assumed brake torque. This error means that the work and power measurements obtained from the ergometer are overestimated by 10.8% for steady speed tests.

The braking force was measured experimentally using specially designed load cells to determine the tensions in the brake rope. The experimental results confirmed that the brake force is the same as that predicted using rope-brake theory and that the values of power and work used by sports scientists and physiologists are overestimated by approximately 11%.