To test the efficiency of pedals, a direct rotational torque is measured rather than reactive torque. A balanced load of 43lbs is placed on the cleat of the pedal under test. The cleat is snapped into the pedal. The weight is placed on the cleat rather than diretly on the pedal to simulate real operating conditions. 43lbs is the approximate load to which a pedal would be subjected during the down-stroke of a rider maintining steady-state 250W ouput, with 175mm crank arms. Of course, the load seen by the pedal would be much higher when a rider is out of the saddle accelerating hard, equalling the weight of the rider or more. However, the steady state riding condition is typically chosen to determine lab loading conditions.
The weight is static (balanced by gravity), along with the pedal, and the shaft is turned at 95RPM. The torque required to turn the shaft is measured by an axial rotational torque transducer. The shaft torque is a direct measurement of the friction of the pedal bearings, plus the two ceramic bearings in the tester. Energy consumption of the pedal is calculated from the shaft torque multiplied by the known RPM. The ceramic bearing equipment losses are removed from the final results to provide the losses found in the pedals alone. At 43lbs load, the system ceramic bearing losses are 0.08 watts.
It must be noted- Friction Facts would prefer to use the same efficiency method as the Reactive Torque Pulley Tester, with load cell attached to the balance beam and measuring reacitve torque. As explained in the RPTP section, this method is slightly more accurate and does not have to account for the bearings losses in the system. For the Pedal Efficiency Test, the equipment was set up to measure losses using the reactive torque method. However, it was quickly determined that many of the pedal's shafts are not perfectly aligned with the threaded portion of the shaft, causing the balanced weight to oscillate, negatively affecting the load cell readings to the point that this method could not be used. The misalignment was measured at a local machine shop. The 'wobble' at the center of the cleat varies from 0.001" on the pedal with the straightest shaft to 0.017" on the pedal with the most misalignment While this small amount of misalignment would most likley not be noticed by a rider, and would most likley not affect riding perfomance, the wobble is enough in the lab to warrant using the direct torque measurement.
The accuracy of the Pedal EfficiencyTest Method using direct torque vs the RPTP using reactive torque is +/-0.005 watts vs. +/-0.001 watts respectively.