as a utility bike, usurping the role previously filled by the roadster. By 1990, the roadster was almost dead; while annual U.K. bicycle sales reached an all-time record of 2.8 million, almost all of them were mountain and road/sport models. BMX bikes BMX bikes are specially designed bicycles that usually have 16 to 24-inch wheels (the norm being the 20-inch wheel), which originated in the state of California in the early 1970s when teenagers imitated their motocross heroes on their bicycles. Children were racing standard road bikes off-road, around purpose-built tracks in the Netherlands. The 1971 motorcycle racing documentary On Any Sunday is generally credited with inspiring the movement nationally in the US. In the opening scene, kids are shown riding their Schwinn Sting-Rays off-road. It was not until the middle of the decade the sport achieved critical mass, and manufacturers began creating bicycles designed specially for the sport. It has grown into an international sport with several different disciplines such as Freestyle, Racing, Street, and Flatland. Mountain bikes In 1981, the first mass-produced mountain bike appeared, intended for use off-pavement over a variety of surfaces. It was an immediate success, and examples flew off retailers' shelves during the 1980s, their popularity spurred by the novelty of all-terrain cycling and the increasing desire of urban dwellers to escape their surroundings via mountain biking and other extreme sports. These cycles featured sturdier frames, wider tires with large knobs for increased traction, a more upright seating position (to allow better visibility and shifting of body weight), and increasingly, various front and rear suspension designs. By 2000, mountain bike sales had far outstripped that of racing, sport/racer, and touring bicycles. 21st century The 21st century has seen a continued application of technology to bicycles (which started in the 20th century): in designing them, building them, and using them. Bicycle frames and components continue to get lighter and more aerodynamic without sacrificing strength largely through the use of computer aided design, finite element analysis, and computational fluid dynamics. Recent discoveries about bicycle stability have been facilitated by computer simulations. Once designed, new technology is applied to manufacturing such as hydroforming and automated carbon fiber layup. Finally, electronic gadgetry has expanded from just cyclocomputers to now include cycling power meters and electronic gear-shifting systems. Hybrid and commuter bicycles In recent years, bicycle designs have trended towards increased specialization, as the number of casual, recreational and