Energy cost and mechanical efficiency of riding a four-wheeled, human-powered, recumbent vehicle

1439-6327

Key words Energy cost of cycling ; Human-powered vehicles ; Mechanical efficiency ; Best performance times

Springer Online Journal Archives 1860-2000

Medicine

Abstract Oxygen consumption at steady state (V˙O 2, l · min−1) and mechanical power (W˙, W) were measured in five subjects riding a human-powered vehicle (HPV, the Karbyk, a four-wheeled recumbent cycle) on a flat concrete road at constant sub-maximal speeds. The external mechanical work spent per unit of distance (W, J · m−1), as calculated from the ratio of W˙ to the speed (v, m · s−1), was found to increase with the square of v: W˙=8.12+(0.262 ·v 2) (r=0.986, n=31), where the first term represents the mechanical energy wasted, over a unit of distance, against frictional forces (rolling resistance, Rr), and the second term (k · v 2) is the work performed, per unit distance, to overcome the air drag. The rolling coefficient (Cr, obtained dividing Rr by m · g, where m is the overall mass and g is the acceleration of gravity) amounted to [mean (SD)] 0.0084 (0.0008), that is about 60% higher than that of a racing bicycle. The drag coefficient was calculated from the measured values of k, air density (ρ) and frontal area (A) [Cx=k · (0.5 · A · ρ)−1], and amounted to 1.067 (0.029), that is about 20% higher than that of a racing bicycle. The energy cost of riding the HPV (Ck, J · m−1) was measured from the ratio of metabolic power above rest (net V˙O 2, expressed in J · s−1) to the speed (v, m · s−1); the value of this parameter increased with the square of v, as described by: Ck=61.45 + (0.675 · v 2) (r=0.711, n=23). The net mechanical efficiency (η) was calculated from the ratio of W to Ck: over the investigated speed range this turned out to be 0.22 (0.021). Best performance times (BPTs) of a “typical”élite athlete riding the Karbyk were calculated over the distances of 1, 5 and 10 km: these were about 8% longer than the BPTs calculated, on the same subjects, when riding a conventional racing bicycle.

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