Experimental optimization of pivot point height for swing-arm type rear suspensions in off-road bicycles.

Abstract

Towards the ultimate goal of designing dual suspension off-road bicycles which decouple the suspension motion from the pedaling action, this study focused on determining experimentally the optimum pivot point height for a swing-arm type rear suspension such that the suspension motion was minimized. Specific objectives were (1) to determine the effect of interaction between the front and rear suspensions on the optimal pivot point height, (2) to investigate the sensitivity of the optimal height to the pedaling mechanics of the rider in both the seated and standing postures, (3) to determine the dependence of the optimal height on the rider posture. Eleven experienced subjects rode a custom-built adjustable dual suspension off-road bicycle, [Needle, S., and Hull, M. L., 1997, "An Off-Road Bicycle With Adjustable Suspension Kinematics," Journal of Mechanical Design 119, pp. 370-375], on an inclined treadmill. The treadmill was set to a constant 6 percent grade at a constant velocity of 24.8 km/hr. With the bicycle in a fixed gear combination of 38 x 14, the corresponding cadence was 84 rpm. For each subject, the pivot point height was varied randomly while the motions across both the front and rear suspension elements were measured. Subjects rode in both the seated and standing postures and with the front suspension active and inactive. It was found that the power loss from the rear suspension at the optimal pivot point height was not significantly dependent on the interaction between the front and rear suspensions. In the seated posture, the optimal pivot point height was 9.8 cm on average and had a range of 8.0-12.3 cm. The average optimal pivot point height for the seated posture corresponded to an average power loss for the rear suspension that was within 10 percent of the minimum power loss for each subject for 8 of the 11 subjects. In the standing posture, the average height was 5.9 cm and ranged from 5.1-7.2 cm. The average heightfor the standing posture was within 10 percent of the minimum power loss for each subject for 9 of the 11 subjects. While the optimum height was relatively insensitive to pedaling mechanics in both the seated and standing postures, the choice of the optimal pivot point height in production bicycles necessitates some compromise in performance given the disparity in the averages between the seated and standing postures.