Torque!
Torque is what makes the wheels on the bike go round. Great research has been but into the sport in order to figure out how to increase the torque applied by the rider to the rear wheel, wile decreasing the torque required to make the wheel turn. Torque is produced the rider using a device called a crank which looks like this:
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The torque in the series of parts that drive the bicycle forward (called the drive train) is dependant on the size of the chain ring (the large gears mounted on the crank) being used, and the size of the rear cog being used. When the chain is on the smaller chain ring, the force through the chain must be greater because the chain ring is closer to the axis of rotation and must apply a larger force to equal the torque produced by the pedals.
Likewise, if a larger cog is used in back on the wheel, an larger torque is exerted on the wheel, and so produces a larger forward force at the tire. These higer-torque gears are good for climbing, where large forward forces are need at low speeds, but for flats and downhills, different gearing is needed, because the lower gearing does not provide much forward rotation (the rear wheel will not rotate as much for a single stroke of the pedal).
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Detail of the drive train on a bicycle. The smaller the gear on the crank combined with a large gear on the wheel will produce large amounts of torque and large forward forces.
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A couple of racer-boys torquein' it up on a climb
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