Single-speed bikes are referred to as fixed-gear bikes in many circles. The bikes have a pedal sprocket and rear sprocket, and the circumference of each is relative to a particular gear ratio. BMX and other single-speed riders often change the gear ratio for a particular type of riding. Some use complex tables that calculate precise gear ratios. Others have a favorite setup that may have proved effective in the past.
The ratio of a particular single-speed bike is determined by the diameter and the number of teeth, or holes, in the front and rear sprockets. A typical front and rear, single-speed setup would consist of a 42-hole front sprocket and a 22-hole rear hub sprocket. This combination provides a low- to mid-range gearing ratio for the typical range of performance that suits most riders. The majority of production single-speed bikes have a gear ratio in this range.
A chain ring is the front sprocket that generates the power synonymous with single-speed bikes. The sprocket is noticeably larger than the rear sprocket. The circumference of the chain ring and the relative length of the pedal crank arms affect the speed or power that can be attained. In most instances, top-end speed is improved by going with a smaller chain ring and longer crank arms. One way to increase low-end torque and gain power without changing the stock chain ring is to install shorter crank arms. This is easy to understand by comparing the length of BMX pedal cranks with those on fixed-gear road bikes.
Over any given distance, the rear hub sprocket makes the same number of rotations as the rear wheel. The number of rotations the chain ring makes over the same distance is substantially less. If this were not the case, cycling might not be a popular recreational activity. The gear ratio here is given in a set of numbers, such as 4.5.1, which translates to four and a half rotations of the rear sprocket for one rotation of the pedals and chain ring. Again, changing the diameter of the rear sprocket to increase or reduce the number of teeth affects the gear ratio of single-speed bikes. A sprocket with fewer teeth adds top-end. A sprocket with more teeth would improve low-end power.
Quick-change axles make conversion of the rear wheel sprocket easier and faster. The chain tension needs to be relaxed with any rear hub work. The absence of typical axle nuts makes conversion faster. Flipping the quick-release lever open releases the wheel from the frame arms of the bike. Once the wheel is off the bike, removing the bolts that hold the sprocket to the hub takes minutes. Bolting the replacement sprocket to the hub, reinstalling the wheel into the bike frame and adjusting the chain tension completes the work. Pressing the quick-release lever back into place secures the wheel in the frame arms.