BMX bikes have many different applications from racing to street riding and dirt jumping; as such, sprocket size is dependent largely on desired application. Limitations in gear ratios, brake mounting positions and rear hub shell designs also play a large part in front sprocket sizing choices. Typically the best sprocket size is determined by offsetting the intended purpose of the bicycle against a functional gear ratio dependent on rear cog attachment sizing and brake mount placement.
Common gear combinations
A gear ratio chart like the one at bmxultra.com will come in handy when trying to decide which size sprocket to use. Until recently, the most common BMX sprocket size was 44 teeth. When paired with the most common 16-tooth rear cog on a 50 cm (20-inch) wheel, this causes a BMX bike to travel roughly 1.35 m (54 inches) each time the pedals complete one full revolution. A 44/16 gearing ratio is approximately 2.75:1. Modern common BMX gear combinations resulting in a ratio that will deliver a similar distance travelled with every pedal revolution are 42/15, 39/14, 36/13, 25/9 and 22/8. Unfortunately, due to hub and brake mount limitations, not every BMX bike can accept every size option.
Sprocket size hub limitations
The rear hub shells of BMX bikes typically have one of two thread sizes machined in the drive side -- the side the chain attaches to -- of the hub. These threaded hub shells accept two different sizes of driver cogs. Some hubs -- called flip-flop hubs -- have both sets of threads on either side of the hub shell to allow for a greater diversity of possible gear combinations. Another style -- called cassette hubs -- builds the cog directly into a driver that attaches to the hub and allows for very small rear cogs. Larger threaded hub shells typically offer rear cog sizes of 16 or 15 teeth. Smaller threaded hub shells come in 14 and 13 teeth. Cassette hub drivers typically come in nine- or eight-tooth versions. Acceptable sprocket sizes are dependent on the style of hub, as pairing a 36-tooth sprocket with a 16-tooth rear cog would result in a bike that pedals very fast but moves forward very slowly.
Brake mount limitations
Many BMX bikes use a U-style rear brake mounted to the top of the frame chain stay. With this style of brake mount, a very small sprocket and rear cog will lower the chain so much it contacts the top of the brake and inhibits pedalling. Typically the smallest gear combination a bike of this style will allow is a 36/13. This size usually allows the chain to pass above the brake by only a few millimetres, whereas a 25/9 gearing would lower the chain almost an inch, causing the chain to snag the brake arm.
Big sprocket advantages
According to Odyssey BMX, the larger a sprocket size is, the less force it exerts on a chain. Therefore, a BMX bicycle with a larger sprocket will experience less broken chains or rear cog teeth than a BMX bike with smaller gearing. Large sprocket teeth also contact the chain surface less frequently and usually wear longer than small sprockets.
Small sprocket advantages
Small sprockets bend less easily than large sprockets as contact with an object results in more leverage force on a sprocket, having more contact surface. Small sprockets are also easier to keep from contacting inert objects while tricks are being performed and generally experience less damage than larger sprockets when performing stunts. Lastly, small sprockets are lighter than large sprockets, and weight is a major concern with most cyclists and other operators of human-powered vehicles, as a lighter ride result in less effort from the rider.