Cycling gears are circular discs with teeth around the circumference that are used to generate torque (rotating) motions on bicycles. The use of several gears on a bicycle provides a variety of gear ratios for peak efficiency or comfort for the cyclist.
The gear ratio is the proportional relationship between the speed of rotation and the used gear. Particular gear ratios are used in consideration of the type of terrain that the cyclist is traversing and of the particular ability and style of the cyclist. For instance, it may be more efficient to use a higher gear when biking down a hill, but it may be better suited to switch to a medium gear when cycling on a flat path and to switch to a lower gear when cycling up another steep hill.
In such cases, a lower gear will require the rider to pedal at a faster pace but with less force (forward motion). Conversely, a higher gear provides for a higher speed at a given pace, but necessitates a greater force be exerted by the cyclist. Different cyclists have different preferences for pace and pedaling force in various situations.
The switching of gears increases or decreases a particular pace by changing the amount of resistance on those gears. Such gearing can affect how much or how little the cyclist's leg muscles, heart muscle, and lungs are exercised. In addition, the more gearing available on a bicycle, the easier it is to find the best pace (what is often called cadence). The number of speeds a bicycle possesses can be calculated by multiplying the number of chain rings (type of gear located near pedals) by the number of the cogs (type of gear located near rear hub). For instance, a 27-speed bicycle with three chain rings in the front and nine cogs in the rear has 27 possible gear ratios. A 3-speed bike with one chain ring and three cogs has only three possible gear ratios. Some of the earliest bicycles, which are now considered single-speed or fixed-gear bicycles, had one chain ring in front and one cog centered around the rear hub, which multiplies into only one possible gear ratio.
Power is transmitted primarily from the cyclist's legs to the rear wheel through the pedals, crankset (metallic piece that connects a pedal to the frame), chain, and rear hub. A cyclist can produce only so much power and, thus, cycle at only so much speed. Gearing allows the cyclist to optimize this restriction in power generation. That optimization occurs through the shifting mechanism mounted on or near the handlebars. It gives the cyclist the ability to choose which gears the chain uses. During the shifting of the gears, the chain moves up and down on a cassette (series of cogs) and between the chain rings, thus, allowing the cassette and chain rings to work in unison with the pedals.
The type of shifters varies depending on the style of bicycle, such as off-road (mountain), hybrid (cross between mountain and racing), and on-road (racing). However, in almost all cases, cables are connected from the shift levers to the derailleur gears. Most bicycles use derailleur gears to change the gears. That is, when triggered by levers, derailleurs mechanically guide (derail) the chain—by pushing it closer to or further away from the bike's frame—so that it moves the chain from one gear to another while minimizing any excess slack in the chain.
Specifically, a cable from the right shift lever runs to the rear derailleur that controls the rear cassette around the hub of the rear wheel. The rear derailleur moves the chain across the cogs for the smaller, more precise, changes in gear ratios needed for most shifting. Another cable runs from the left shifter to the front derailleur that controls the front chain rings (large circular rings) near the pedals. The front derailleur moves the chain across the chain rings to alter the large changes in shifting. For instance, one shift in the front derailleur can quickly and efficiently change from a slow uphill climb to a fast downhill sprint.
All bicycles work better and last longer when properly maintained. Cyclists should regularly maintain their bicycle by cleaning and lubricating exposed moving parts. Especially important are the cycling gears, which will last longer when the chain and gear mechanisms are cleaned and lubricated on a regular basis. Gears should also be regularly checked to assure that they are working properly and that cables and other related mechanisms are moving freely. Many simple bicycle repairs can be done by the cyclists, though most gear repairs should be performed by professional cycle mechanics.
see also Cycling.