How to use Bicycle Gears - Gears on a Bike
Single-speed, or fixed-gear, bicycles feature a single chain ring in front that is centered on the pedals and only one cog in the rear, centered on the rear hub. Old three-speed bicycles had one front chain ring and an internal gear mechanism enclosed in a drum inside the rear hub. Bicycles having anywhere from 5 to 27 speeds dominated the bicycle market in western Europe, the United States, and other developed countries by the late 1990s. These bicycles offer a choice of gear ratios, or speeds, that result from the interaction of the bicycle chain with one to three front chain rings and a cluster of five to nine cogs of various sizes that are mounted on the freewheel or cassette mechanism of the rear wheel. The number of speeds a bicycle allows can be calculated by multiplying the number of cogs by the number of chain rings. A 16-speed bicycle, for example, has eight cogs and two chain rings (known in bicycling parlance as a double crank), and a 27-speed bicycle has nine cogs and three chain rings (known as a triple crank).
On most multispeed bicycles, gears are changed by means of derailleurs that, when activated by levers mounted on or near the handlebars, mechanically derail the chain by pushing it toward or away from the bicycle’s frame so it moves from one sprocket to another. The rear derailleur moves the chain across the cogs for the finer differences in gear ratios needed for most shifting. A spring-loaded arm under the cogs takes up or lets out slack in the chain to keep the tension consistent regardless of the gear ratio chosen. The front derailleur moves the chain across the chain rings to alter the whole range of available shifting (to change, say, from a fast downhill cruise to a slow uphill climb).
Although the derailleur was invented in 1899, it didn’t become popular until roads were paved because of its tendency to be fouled by mud and grit. Even with the ten-speed’s rise in popularity in the 1960s, derailleurs were a source of frustration to many novices because their operation required the cyclist to develop some skill in sensing the friction point at which the gears would change and the point at which the derailleur was securely in gear. Those frustrations were eliminated in the 1980s with the invention of reliable indexed, or click, shifting, which latches the shift lever with a secure click to let the rider know that a shift from one gear to the next is complete. Because indexed shifting is faster to use, particularly when downshifting at the base of hills in preparation for a climb, such systems are popular with racing and touring cyclists as well as with novices.
Some manufacturers have experimented with gears that shift automatically. These products have had some commercial success, particularly on recreational bicycles used by novice riders.