自行车简史
g enough to make a fine chain and sprocket18) small and light enough for a human being to power, the next design was a return to the original configuration of two same-size wheels; only now, instead of just one wheel circumference for every pedal turn, you could, through the gear ratios, have a speed the same as the huge high-wheel. The bicycles still had the hard rubber tires, and in the absence
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