Improvement of Cycling Efficiency for Drivetrains with Elasticity

Abstract

Test and modeling results are reported on a bicycle crankset with limited elasticity. Like record-breaking running shoes, the crank set has spring action which mitigates the effect of the dead zone during the pedal stroke. Fiber composite leaf springs are inserted inside the hollow carbon crank arms. The crank arms are not directly attached to the crank axle. Instead, sleeve bearings allow the crank arms to rotate by up to about five degrees relative to the crank axle. The rotation is counteracted by the springs and is proportional to applied torque at the pedals. The novel crank set and a conventional crank set with forged aluminum crank arms were both tested on a stationary bike. The ratio of effective speed to input power is used as a measure of cycling efficiency. Depending on the difference in torque during the downstroke and in the dead zone, this ratio is typically a few percent higher for the novel crankset than for a conventional crankset. Multiple tests show efficiency improvements in the range of 1 to 4% at power levels of 200 W and cadence of 71 rpm with average of around 2%. Details of a test with 2.3 % improvement are presented. This would translate, for example, into a one minute advantage in a 45 minute time trial. In an attempt to understand the test results computer modeling of bicycle speed and crank arm angular velocity vs. time was performed for non-elastic and and elastic crank arms. It is difficult to explain the test results with computer modeling unless it is assumed that conventional crank sets introduce energy losses in the drivetrain from twisting of the crank arms and flexing of the bicycle frame under load at the pedals and that these energy losses are reduced for the crank set with built-in elasticity.