The idea behind multiple gears on a bicycle -- whether it's an older "10-speed" bike or a modern mountain bike with 24 gears -- is to let you change the distance that the bike moves forward with each pedal stroke.­For example, a normal bicycle has wheels that are 26 inches in diameter. The "lowest" gear ratio on the bike might be a front chain wheel with 22 teeth and a rear gear having 30 teeth. That means that the gear ratio is 0.73-to-1. For each pedal stroke, the rear wheel turns 0.73 times. In other words, for each pedal stroke, the bike moves forward about 60 inches (about 3.4 mph / 5.4 kph at a 60-rpm pedaling rate). The "highest" gear ratio on the bike might be a front chain wheel with 44 teeth and a rear gear having 11 teeth. That creates a 4-to-1 gear ratio. With 26-inch wheels, the bike moves forward 326 inches with each pedal stroke. At a 60-rpm pedaling rate, the speed of the bike is 18.5 mph (30 kph). By doubling the pedaling rate to 120 rpm, the bike has a maximum speed of 37 mph (60 kph). A range of 3.4 mph to 37 mph is fantastic, and it lets the rider climb the steepest hill very slowly or race almost as fast as a car! That is why a bike has gears.

The gears at the front are called the chain wheels. Most bikes have two or three chain wheels that look like this:

Attached to the rear wheel is the freewheel, which looks like this:

The freewheel has between five and nine gears on it, depending on the bike. A freewheel spins freely in one direction and locks in the other. That allows the rider to either pedal or not pedal -- when not pedaling, the bike coasts (another feature that tricycles and penny-farthing bicycles lack).

To change the gears, a bicycle has front and rear derailleurs. Here's a shot of the rear derailleur:

The rear derailleur has two small cogs on it that both spin freely. The purpose of the arm and lower cog of the derailleur is to tension the chain. The cog and arm are connected to a spring so that the cog pulls backward at all times. As you change gears, you will notice that the angle of the arm changes to take up or let out slack:

The top cog is very close to the freewheel. When you adjust the gears with the lever on the handlebar, this cog moves to a different position on the freewheel and drags the chain with it.

The chain naturally slips from one gear to the next as you turn the pedals.

Everything about a bicycle is simple. That's what makes it such a great machine to ride -- and also a great mechanical work of art! For more information on bicycles and related topics, check out the links on the next page.

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