A Moment of Science

Changing Gears

In our high-tech world it’s easy to forget the simpler things. Take a bicycle, for instance. No engine, no computerized braking system, no fancy fuel injection: all you need is air in the tires and off you go.

Simple, that is, until you take a closer look at bicycles with multiple speeds. How do all those notched gears generate more power when you switch from first to fifth?

The answer has to do with a basic mechanical principle called gear ratio. The gears on a bicycle come in different sizes, with the larger gears attached to the pedals and the smaller gears attached to the back wheel. Let’s say the smallest gear attached to the back wheel has a circumference of two inches. Now let’s say the bike’s largest gear attached to the pedals has a circumference of ten inches, or five times the circumference of the smaller gear. Since the two gears are connected by the bicycle chain, when the larger gear revolves, it causes the smaller gear to revolve five times, creating a gear ratio of 5 to 1.

Switching gears either increases or decreases the gear ratio. So switching from first gear to fifth increases the difference in circumference between the front and rear gears, which in turn increase the number of times the back wheel revolves each time you pedal. The result is that in fifth gear you travel several times the distance with each pedal as you would in first. But there’s no free lunch–in order to generate speed on a high gear, you have to work twice as hard to crank the pedals around.

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