Bicycle Performance - Energy Efficiency

Energy Efficiency

A human being traveling on a bicycle at 10–15 mph (16–24 km/h), using only the power required to walk, is the most energy-efficient means of transport generally available. Air drag, which increases with the square of speed, requires increasingly higher power outputs relative to speed, power increasing with the cube of speed as power equals force times velocity. A bicycle in which the rider lies in a supine position is referred to as a recumbent bicycle or, if covered in an aerodynamic fairing to achieve very low air drag, as a streamliner.

On firm, flat ground, a 70 kg (150 lb) person requires about 30 watts to walk at 5 km/h (3.1 mph). That same person on a bicycle, on the same ground, with the same power output, can average 15 km/h (9.3 mph), so energy expenditure in terms of kcal/(kg·km) is roughly one-third as much. Generally used figures are

• 1.62 kJ/(km∙kg) or 0.28 kcal/(mi∙lb) for cycling,
• 3.78 kJ/(km∙kg) or 0.653 kcal/(mi∙lb) for walking/running,
• 16.96 kJ/(km∙kg) or 2.93 kcal/(mi∙lb) for swimming.

Amateur bicycle racers can typically produce 3 watts/kg for more than an hour (e.g., around 210 watts for a 70 kg rider), with top amateurs producing 5 W/kg and elite athletes achieving 6 W/kg for similar lengths of time. Elite track sprinters are able to attain an instantaneous maximum output of around 2,000 watts, or in excess of 25 W/kg; elite road cyclists may produce 1,600 to 1,700 watts as an instantaneous maximum in their burst to the finish line at the end of a five-hour long road race. Even at moderate speeds, most power is spent in overcoming the aerodynamic drag force, which increases with the square of speed. Thus, the power required to overcome drag increases with the cube of the speed.