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== Description ==
Gears are used on the bicycle in two places: on the rear wheel and the bottom bracket. These two gears are connected by a chain, and depending on the size of the gears used, can either increase or decrease the effort needed to gain the desired speed and torque. On the rear hub there can be anywhere from one gear, usually on older bicycles, up to eight different sized gears on newer mountain bikes and racing bikes. There are usually one to three gears in the front as well. The ratio of the number of teeth from the front to the back is called the gear ratio, a very important concept.


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== How it works ==
[[image: yeoldbicycle.jpg|thumb|The large wheel was necessary to increase speed]]
The reasoning behind using gears comes down to numbers. When bicycles were first invented, the pedals were attached directly to the front wheel. Because of that, for every revolution of the pedals, the bicycle moved forwards a distance equal to the circumference of the wheel. In order to increase speed, either the person riding would have to pedal faster, or the wheel would have to be bigger. This led to some rather impressive bicycles, some of which are still seen in parades and movies.


To decrease the size of the wheels, gears were added. The gear ratio, mentioned above, is how they did it. For instance, if the front gear had 28 teeth while the rear gear had 14 teeth, it would result in a gear ratio of 2. When applied to the bicycle, it means that the rider would only have to use a wheel with half of the diameter previously needed to reach a certain speed. This breakthrough made the bicycle much easier to ride. It also removed the pedal from the wheel, meaning there was less side-to-side motion while pedaling.


While this addition was a major improvement, it would only get better. With the addition of the [[Bicycle_Derailleur|derailleur]], riders could shift the gear ratio while riding, to better suite their situation. For instance, if a person were going up a hill and needed more torque, they could shift to a lower gear ratio and have more of their work go directly to the wheels. On the flat open road, however, riders can shift to a higher gear ratio and pedal less for an equivalent speed.


Currently, there are a few main types of gear sets: enclosed hub gears, multiple rear gears with a single chain ring, and multiple rear gears with multiple chain rings. Each is used depending on the need of the rider. If the rider doesn't need very many gears, say, three to seven, an enclosed hub is the best choice. It differs from exposed gears because it is protected from the elements, doesn't need much maintenance, but doesn't have the variety the others do. The other two options are used if a greater number of gears are needed.


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== Evolution of the gears ==
'''1879''' - Henry J. Lawson patents a rear wheel, chain-driven safety bicycle, the “Bicyclette”

 
'''1885''' - Safety Bicycle invented, replaced the penny-farthing and included a chain drive and gears


'''1896''' - E.H. Hodgkinson patents a 3-speed Gradient gear, a pre-cursor of the modern derailleur.


'''1896''' - William Reilly patents a two-speed hub gear.


'''1910''' - The first, easy-to-use derailleur is invented by Paul de Vivie (Velocio) that shifted among four gears at the pedals


'''1998''' - Rohloff develops the Speedhub, 14 equally-spaced hub gears which are operated by a twist-grip with no overlapping ratios and a gear range as wide as a 27-speed derailleur system.


'''2002''' - Campagnolo offers a 30-speed derailleur drivetrain with the Record 3-x-10 drivetrain


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== 3D parts ==
The table belows lists the Bill of Materials for the Rear Hub & Gears:
{| border="1" align="center"
|+ '''Rear Hub & Gears Bill of Materials'''
! width="50"|Part # !! width="100"|Part Name !! width="50"|# Req'd !! width="120"|CAD File !! width="145"| Image
|-
! 1
| align="center"|Rear Axle
|align="center"|1 
|align="center"|<!--GICL Bot edit:-->[[axle_rear.zip Contents|Rear Axle]] 
|[[Image:axle_rear.jpg|center|thumb|100px]]
|-
! 2
| align="center"|Rear Axle Nut
|align="center"|1 
|align="center"|<!--GICL Bot edit:-->[[axle_nut_rear.zip Contents|Rear Axle Nut]] 
|[[Image:axle_nut_rear.jpg|center|thumb|100px]]
|-
! 3
| align="center"|Rear Bearing
|align="center"|2 
|align="center"|<!--GICL Bot edit:-->[[bearing_rear.zip Contents|Rear Bearing]] 
|[[Image:bearing_rear.jpg|center|thumb|100px]]
|-
! 4
| align="center"|Rear Hub
|align="center"|1 
|align="center"|<!--GICL Bot edit:-->[[hub_rear.zip Contents|Rear Hub]] 
|[[Image:hub_rear.jpg|center|thumb|100px]]
|-
! 5
| align="center"|Rear Sprockets
|align="center"|1 set of 5
|align="center"|<!--GICL Bot edit:-->[[sprockets_rear.zip Contents|Rear Sprockets]] 
|[[Image:sprockets_rear.jpg|center|thumb|100px]]
|-
! 6
| align="center"|Rear Tire 
|align="center"|1 
|align="center"|<!--GICL Bot edit:-->[[tire_rear.zip Contents|Rear Tire]] 
|[[Image:tire_rear.jpg|center|thumb|100px]]
|-
! 7
| align="center"|Rear Hub Assembly 
|align="center"|1 
|align="center"|<!--GICL Bot edit:-->[[hub_assem_rear.zip Contents|Rear Hub Assembly]] 
|[[Image:hub_assem_rear.jpg|center|thumb|100px]]
|}


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== References ==

[http://science.howstuffworks.com/bicycle4.htm Bicycle Gear Ratios]

[http://en.wikipedia.org/wiki/Bicycle_gearing Bicycle Gearing]

[http://en.wikipedia.org/wiki/Safety_bicycle Safety Bicycle]

[http://www.jimlangley.net/ride/bicyclehistorywh.html Bicycle History]