Throughout the last century, bicycles have gone through a steady evolution, gradually improving over time. From humble beginnings of the single gear, direct drive bicycles of yesteryear to the sophisticated derailleur-type road bikes of today, several gear types have come to the forefront and bicycle technology.
Fixed Gear
According to Sheldon Brown of Harris Cyclery, fixed-gear bicycles are the oldest and simplest form of bicycle. The fixed-gear bicycle has a direct connection by the drive chain from the pedals that drive a chain ring gear to a single, fixed gear on the back wheel hub. This bicycle can be pedaled either forward or backward and is impossible to coast on, since the rear wheel drives the pedals as much as the pedals drive the rear wheel. Cyclists do not require brakes for this bike; slowing down pedaling stops the bike. Track bikes used in velodromes for racing typically have fixed gears.
Single Speed
Similar in form to the fixed-gear bicycle, single-speed bicycles have only one gear operated by the pedals and another gear connected to the hub of the rear wheel. Unlike fixed-gear bikes, however, single-speed bicycles use a freewheel device to allow the rear gear to engage while pedaling, but that allows the rear wheel to turn forward freely while the gear is stationary. This allows the cyclist to stop pedaling and coast on the bicycle. To stop a single-speed bicycle, separate cable brakes or a special coaster brake installed in the rear hub of the bike are required.
Multispeed Gears
Multispeed bicycles typically use two sets of gears. The pedals of the bicycle operate the front chain ring, which usually has three stacked, concentric gears, while the rear gear set attaches to the rear wheel hub. The rear gear set may have from three to nine stacked gear sprockets. The cyclist uses levers connected to cables that operate two devices, called derailleurs, to move the chain up or down on the gear sets. The front derailleur is a cage assembly that wraps around the chain, pushing or pulling the chain up or down the gears of the front chain ring. The rear derailleur is a spring-loaded armature that moves inward or outward to align the chain onto any given sprocket and maintains constant tension on the chain in any gear. The rear gear set rotates freely from the wheel, allowing coasting.
Internal Hub Gears
Some older bikes have a special gearing system internal to the hub of the bicycle's rear wheel. The cyclist operates a shifter lever on the handlebars to move a cable leading to a chain and pin assembly, which inserts into the rear wheel hub. This assembly operates an internal transmission within the hub to change the gear ratio between the single gear on the hub and the hub itself. Older bikes with internal hub gears typically had three separate gears to choose from. While largely replaced by derailleur-type bikes in the 1970s, today some cruiser and comfort-style bikes continue to use traditional internal hub gears with up to seven speeds.
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