Bicycle gearing is slightly confusing, as there are many different sizes of available gear combinations. However, with a little knowledge of gear ratios and intended purposes, deciding on the proper bicycle gearing is simple. Many available sources, such as the BMX gear chart on the BMX Ultra website, exist to aid in the selection of proper BMX gearing.
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Understanding Gear Ratios
A gear ratio is the amount of distance in inches a bicycle will travel given one full rotation of the pedals. Front gears are attached to the pedal/crank assembly, and rear gears are attached directly to the rear wheel hub. A larger front gear means a greater distance travelled, while a smaller rear gear also means a greater distance travelled.
When racing, riders are less concerned with starting power--as most BMX courses start downhill--and more concerned with maintaining speed. As such, BMX racers typically use large front chainwheels--42 to 46 teeth--and midsize--typically 16 tooth--rear cogs. A 46/16 combination results in a 56.06 gear ratio.
BMX Street/Park/Dirt Riding
Most street, park and dirt riders ride all three surfaces and desire smaller gears to simulate a gearing similar to BMX gearing. The small gears have less chance of bending or breaking when practicing tricks as well as less surface area of dangerous pointed chainwheels if and when a trick goes awry. Typically these riders use what are called "lo-lo" gears with a small 25 to 28 tooth chainwheel up front paired with a 9 tooth cog in the rear. A 26/9 combination results in a 56.33 gear ratio.
Flatland trick riders typically use a ratio much lower than other BMX riders because their style of riding involves balance tricks frequently performed on one wheel. Smaller gear ratios make pulling a bike up on the rear wheel far easier as a low gear transfers power that is able to upset the balance and stand the bicycle up to transition to other tricks. Typically, flatland riders use gearing between the racers and the street/park/dirt riders as there are more available options---34 to 39 tooth chainwheels, and 14 tooth rear cogs--between these two. A 34/14 combination results in a relatively low 47.36 gear ratio.
Applying Gear Charts
Gear charts are typically laid out as tables with one axis containing front gear tooth counts and the other axis containing rear gear tooth counts. Once the gear ratio desired is found, any combination of gears resulting in that ratio will work for the intended purpose, but most bicycles will only accept certain sizes of rear cogs. Therefore, converting a BMX race bike to lo-lo gearing will require changing the rear hub to accept an eight- or nine-tooth rear cog in place of the 16-tooth cog typically found on such a bike.
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