A bicycle transmission is like a lever, it is used to change the ratio of the force required to perform work, this changed ratio is called gear ratio.
When the crank complete rotation 1 turn, 52 teeth large gear disk will drive 13 teeth chain disk rotation 4 turns, gear ratio is 4:1. 28 teeth large gear disk can drive 28 teeth chain disk rotation 1 turn, gear ratio is 1:1. 52/13 teeth gear ratio is large, help to accelerate, and 28/28 teeth gear ratio is small, although the speed is slower, but help to climb in a stable state of travel The ride produces an average of 0.125 horsepower and the most effective use of energy when the pedaling rhythm is 55-58 revolutions per minute. The purpose of the derailleur is to help the rider maintain an effective cadence, and the key to using the derailleur effectively is the ability to anticipate, that is, to shift early before the actual need to shift, so that a smooth cadence can be maintained.
When switching from high to low speed for hill climbing, the ability to anticipate is most important, because although modern transmissions shift quickly and surely, shifting half a beat faster before climbing can reduce the pressure on the pedals. However, you should never change from the smallest disc to the smallest chain disc or from the largest disc to the largest chain disc when shifting, because such shifting will make the chain switching angle too big, which will not only reduce the efficiency but also increase the wear and tear.
When the chain is on the small toothed disc and the large chain disc with the crank turning 1 turn, the distance the bike moves is shorter; when the chain is on the large toothed disc and the small chain disc, the distance the bike moves is longer.