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Count your teeth.
Paul Doherty, Senior Scientist at the Exploratorium, explains the gear ratio on your bicycle. With this method you simply count the number of teeth in front chain ring and then the number of teeth in one of the rear chain cogs. In the example Paul created, the front chain ring had 54 teeth and the rear cog had 27. "That means every time I pedal around once on the front chain ring, the chain goes around 54 teeth. That means a 27-tooth (rear) cog goes around twice (rotating the rear wheel twice)." This would provide a ratio of 2-to-1. If the rear cog had 11 teeth, the ratio would be closer to 5-to-1 and so on.
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Rear Cog ....................................... Front Cog
This animation demonstrates the rotation of the front and rear cogs of a bicycle. Notice that for each turn of the larger front cog the rear cog turns twice. The animation also produces an interesting optical illusion where the "spokes" on the front cog appear to be moving backward.
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