自行車儲能省力機構的效能分析與最佳化設計參數的探討

Abstract

自行車問世二百多年來，騎乘時上死點附近與下死點附近因力臂過小而難以施力的問題仍然存在。從生物力學的角度，於自行車騎乘時大小腿夾角大時踏力較大，夾角小時踏力較小，因此本研究設計一新式自行車儲能省力機構，在踏力大的行程中儲存能量，在踏力小的行程中釋放能量，以達到省力的效果。本機構以彈簧為儲能元件，有彈簧k值、彈簧預壓縮量及滾輪相位角等三項可調整參數。本研究以最大肌力上限值做為可施力範圍之指標，並以省力效果及可施力範圍為設計目標，分別找出平路騎乘及10度坡騎乘時的最佳化設計參數。分析結果顯示，彈簧k值及預壓縮調整可以增加省力效果，但使可施力範圍減少。將滾輪相位角提前，省力效果及可施力範圍同時增加。以騎乘時速為5 km/h做最佳化設計目標，平路騎乘時維持起始可施力範圍又不省力過度之情況，相位角提前10°、k值8 N/mm，可得到最理想的結果；坡度騎乘時，可將相位角提前10°，體能狀況差者可選擇較小 值為5 N/mm，體能狀況較佳者可以提高 值或增加彈簧預壓縮量來額外提升省力效果。依上述最佳化設計可得具體效果，體能狀況佳者，可施力範圍達76%至90%，省力效果為48%至63%、膝蓋減輕負荷5%至17%。體能狀況差者，雖然可施力範圍相對較小，但省力效果可達65%至66%，膝蓋減輕負荷43%至51%。

After about two hundred years since the current form of bicycles was established, the problems of upper and lower dead zones of the crank driving stroke due to smaller force arms are yet to be solved. Biomechanics analyses have shown the force that can be asserted to the pedals decreases when the angle between the shank and the thigh decreases, as in the upper stroke, and the force increases when the angle between the shank and the thigh increases, as in the lower stroke. Therefore the purpose of this research was to design a novel bicycle driving mechanism that can store energy while in the lower stroke and release this energy while in the upper stroke, thereby achieving labor-saving effects. The bicycle driving mechanism uses a spring as the energy storage device. There are three parameters that can be adjusted for better overall performance: the spring constant, the spring pre-compression and the phase angle. Optimized parameter values are found for level and 10 degrees uphill cycling based on the labor-saving effect and the pedalable range, which is defined as the degree range of the stroke where cyclists can assert enough force on the pedals to overcome the resistance based on their maximum muscular strength. It is found that increasing either the spring constant or pre-compression can increase the labor saving effect. But doing so will also reduce the pedalable range. Advancing the phase angle would increase both the labor-saving effect and the pedalable range. At a speed of 5 km/h in level cycling, the best combination of design parameters would be a phase angle of 10° in advance and a spring constant of 8 N/mm. By increasing either the spring constant or pre-compression could increase the labor-saving effect in uphill cycling for people in better physical conditions. A spring constant of 5 N/mm is better suited for people in poorer physical conditions. For people in better physical conditions, the labor-saving effect is from 48% to 63% with a pedalable range of 76%-90%, while the load on the knees could be reduced by 5%-17%. For people in poor physical conditions, the labor-saving effect of up to 65%-66% and a reduction of knee loading of 43%-51% is achieved.