Energy required to make the bicycle "go"

Let's analyze the forces that act on a bicycle at all times:

  • Thrust is provided by the rider's muscles.
  • Air resistance is the largest retarding force. The cyclist, bicycle, and trailer have to plow through the air. The recumbent design reduces the amount of air resistance experienced by the rider. Headwinds increase and tailwinds decrease the effect of air resistance.
  • Friction comes from two main sources:
    • Rolling resistance of the tires on the bicycle and trailer(reduced by using low rolling resistance tires at high pressure).
    • Friction in the moving parts of the drivetrain (axles, chain, sprockets, chainrings, pedals, bottom bracket, rear derailleur, idler wheels).
  • Gravity is a constant force that does not greatly affect the rider unless the bicycle is going uphill or downhill.

Energy analysis

Energy can neither be created nor destroyed, only transformed.
-Law of conservation of energy

The energy that propels a bicycle forward comes from one source: the food the cyclist eats. Food is a form of chemical potential energy that is broken down by the digestive system. Large molecules in the food are broken down and recombined to form new molecules that can be used as the body's fuel. This fuel then powers the body's muscles which makes the bicycle move forward.

Energy of motion is called kinetic energy. So, in a way, the chemical potential energy is tranformed into kinetic energy. Kinetic energy is measured with the following formula:

Ek=1/2 mv2

where Ek is the kinetic energy (in Joules), m is the mass (in kilograms), and v is the velocity (in meters per second).

The faster a rider travels, the greater the kinetic energy. Since the velocity term is squared the kinetic energy varies exponentially as the velocity. So if a rider rides twice as fast, he has four times as much kinetic energy as he did before. If he rides four times as fast, the kinetic energy increases by a factor of sixteen.

A cyclist cannot ride uphill as fast as on level ground. This is because some of the energy is being transformed into potential energy. Potential energy is the energy required to overcome gravity (which is pulling the rider back down the hill). Potential energy is measured with the following formula:


where Ep is the potential energy (in Joules), m is the mass (in kilograms), g is the local acceleration due to gravity (near 9.80 m/s2 in North America). So the higher the hill, the greater the increase in potential energy. This energy is not "lost." When decending this potential energy gets transformed back into kinetic energy which makes the rider go faster with less effort than when riding on flat ground.

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