Bicycles are machines that mostly attract the interest of the kids. It is a machine which lets you travel faster to destinations using the minimum of energy, compared to the energy that you would spend by walking or running to reach the place that you would want to go to. The mechanics that drive the bicycle are all open and exposed, and none of the machineries used are covered with sheet metal. Many of the kids, having curiosity in mechanical tendencies, cannot help but take their bike apart and put it back again.
You might have seen bicycles ridden in the movies or in a circus, where the front and the rear wheels of the bicycle have a funny "penny-farthing" shape. The bicycle used usually had a huge front wheel and a tiny wheel at the back. This type of bicycle appeared in the year 1870, but by the end of the century the bicycle was replaced by what we see normally these days. Just as we find in a kid's tri-cycle, the front wheel of the penny-farthing bicycle is directly connected. This would mean that as you pedal once, the wheel goes through one revolution. If the front wheel of the tri-cycle is taken as 16 inches, the circumference of the wheel comes to 50 inches. Therefore, with each revolution of the pedal, the front wheel travels through 50 inches. Pedaling at the rate of 60 rpm, the tri-cycle would be moving 50 inches per second, which comes to about 2.8 mph. If the pedaling goes up to 120 rpm, which is very unlikely, the tri-cycle would be moving at 5 mph.
The concept of gears in bicycles came from the fact that, it provides the rider with the facility to move faster without pedaling rapidly. If you take an example of a normal bike, the wheels are 26 inches in diameter. With the front chain wheel having 22 teeth and the rear having 30, a bike would have the lowest gear ratio. This would give a gear ratio of 0.73:1, meaning that with each pedal revolution, the front wheel would turn 0.73 times. This would make the bike move forward 60 inches with each pedal stroke. This comes to about 3.4 mph with 60 rpm of pedal revolution.
In the same way, the highest gear ratio in a bike is achieved with the front wheel chain having 44 teeth, and the rear wheel having 11 teeth, which provides a gear ratio of 4:1. Therefore, with 26 inches diameter of the front wheel, the bicycle moves forward by 326 inches with each pedal revolution. If the pedaling is done at 60 rpm, the bike would move forward at a speed of 18.5 mph. If the pedal speed can be doubled to the maximum of 120 rpm, the maximum speed of 37 mph could be attained. The speed range from 3.4 mph to 37 mph is an attractive feature, and that is the reason why bicycles have gears.
The gears at the front of the bicycle are known as chain wheels. This, what is called freewheel, is attached to the rear wheel. Depending upon what kind of a bicycle you have, the freewheel has 5 to 9 gears on it. There is only one direction that the freewheel can spin, and it is locked in the other. This way you can either pedal or you cannot.
There are two derailleurs at the front and back of a bicycle. These are used for changing the gears. The two small cogs at the rear derailleur spin freely. The tension of the chain is maintained by the arm and the lower cog of the derailleur. A spring holds back the cog and the arm. By shifting the gear, you are simply putting the chain on a different ring. Changing the left shifter, the ring at the pedal changes. There are three rings, where the smallest one is 1, the middle one is 2, and the third one is 3. By shifting down, you are changing to the smaller ring. To change the rings on the rear wheel, you use the right shifter, where the biggest ring is 1, and the largest is 6. There are markers in both the right and the left shift, and you can sift any of the shifters looking at the numbers.