Single-speed bikes have a single chainring sprocket and a single rear sprocket. In terms of ratios, the relative circumference of the sprockets determines the drive-gear ratio of a single-speed bicycle. The size of wheels used with a particular drive-gear setup also plays a role in the gear ratio. A BMX bike may have the same drive-gear ratio as a larger bike, but the smaller wheels used on these bikes equates to a different gear ratio.
Single-Speed Gearing
Most production single-speed bikes have a midrange gear ratio that suits a majority of riders. A typical single-speed setup consists of a 42-tooth front sprocket and a 22-tooth rear sprocket with 26-inch wheels. This combination allows the rider to get rolling with a reasonable amount of pedal power and comfortably maintain a midrange cruising speed. Considering the basics of transmission gearing, logic might suggest that a smaller 17-tooth rear sprocket translates to faster cruising speed. However, this changes the drive-gear ratio dramatically, making it harder to get the bike rolling.
Chainring and Cranks
The size of a chainring sprocket combines with the length of the pedal cranks to produce a particular amount of power at the front sprocket. One approach to adding low-end torque without changing the chainring and affecting the drive-gear ratio is to use shorter crank arms. This takes less time than changing sprockets for riding conditions with a lot of hills. BMX pedal cranks are typically shorter because of the riding style that requires more torque.
Wheels
A smaller wheel makes more revolutions over a given distance than a larger wheel, which is why road bikes and cruisers have 26-inch or 28-inch wheels. It also explains why BMX bikes with 16-inch or 18-inch wheels and shorter pedal cranks are not found in cycling races. Modifying the drive-gear ratio affects power and top-end speed. Changing the size of the wheels affects the single-speed gear ratio in terms of the number of rotations the pedals must make to produce a certain number of wheel rotations over a given distance.
Calculations
You can use a gear calculator table and still not completely understand the application of formulas that determine gear ratios. Calculators, like those available on the Harris Cyclery website, ask you to enter wheel sizes, sprocket sizes and the length of crank arms. Another required entry is the gain ratio of a particular gear at a certain speed. Considering the simplicity of single-speed bikes, the easiest way to determine the best gear ratio is to ride the bike and decide if you need more power or more speed. In most instances, shorter cranks increase power, and longer cranks improve the capability of going faster.
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