Prior to the advent of multiple sprockets and derailleurs, cyclists took to the roads and mountain trails on single-speed bikes. The front and rear sprocket combination on these bikes provided a particular gear ratio that you modified by changing one of the sprockets. Consumer demands brought about innovations that included two-speed bikes. Two-speed bikes remain somewhat of a rarity and not all types operate in the same way.
Invention
Initially, manufacturers and inventors stayed away from adding a second sprocket to provide another gear for single-speed bikes. Development centered around a system within the rear wheel hub and, as a result, internal components became a design standard. One of the early two-speed hubs provided a low gear when you pedaled forward and a higher gear when you pedaled backward. You might understand why this was short-lived by imagining pedaling backward to gain speed.
Pull-Shift
The first version of a two-speed bike that proved successful included a shift lever and a cable. Pull shifters work by altering the gearing within the rear hub when a pulling arm is engaged. The arm threads into the center of the rear hub and the shift cable attaches to the outer end of the arm. When you shift up or down, the arm pulls or pushes a cog out of one internal gear and into another. Engineering advances include lighter materials and more reliable components that don't depend on frequent lubrication.
Twin-Fixed and Auto-Shift
Riders who prefer sure-footed pedaling and selective gear changes might find other types of two-speed operation better suited for a recreational rider. The twin-fixed system consists of a primary cog and internal dual gears similar to pull shifters, but the components are inside the bottom bracket shell instead of the rear hub. To shift, you push one pedal crank inward and push the opposite crank inward to shift back. Auto shifters work in the same way as an automatic transmission. The unit is contained in the rear hub and automatically shifts up at a certain speed and back down as you slow.
Fixed-Free
As quick-release levers began replacing traditional axle nuts, another version of the two-speed system appeared that broke from the internal hub standard. The system has a larger fixed-gear sprocket on one side of the hub, a freewheel inside the hub and a smaller sprocket on the opposite side of the hub. To change from fixed to free, you loosen the quick-release levers and turn the wheel around. The smaller sprocket allows you to attain faster speeds than the fixed-gear ratio. The freewheel allows you to coast with your feet on the pedals.
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