Bicycle motocross or BMX is the sport of racing bicycles in motocross style on tracks which use an inline start and have obstacles, and also refers to the bicycle itself, which is designed for dirt and motocross cycling.
BMX began in the early 1970s when children began racing their bicycles on dirt tracks in southern California, inspired by the motocross stars of the time. The size and availability of the Schwinn Sting-Ray and other wheelie bikes made them the natural bike of choice for these races, since they were easily customized for better handling and performance. BMX racing was a phenomenon by the mid-1970s. Children were racing standard road bikes off-road, around purpose-built tracks in California. The 1972 motorcycle racing documentary On Any Sunday is generally credited with inspiring the movement nationally in the United States; its opening scene shows kids riding their Sting-Rays off-road. By the middle of that decade the sport achieved critical mass, and manufacturers began creating bicycles designed especially for the sport.
A BMX bike or BMX is the name of a popular bicycle used for both casual riding and sport, and designed mainly for dirt and motocross cycling. "BMX" is the usual abbreviation for bicycle motocross.
Though originally denoting a bicycle intended for BMX Racing, the term "BMX bike" is now used to encompass race bikes, as well as those used for the dirt, vert, park, street, flatland and BMX freestyle disciplines of BMX. BMX frames are made of various types of steel, and (largely in the racing category) aluminum. Cheaper, low end bikes are usually made of steel. High range bikes are mostly chromoly or high tensile steel, although the latter is noticeably heavier with respect to strength. High-performance BMX bikes use lightweight 4130 chromoly, or generation 3 chromoly.