cross-country bicycle is a bicycle suitable for children's physical exercise. Referred to as B.M.X. car. Its appearance is similar to that of a moped, and its structure is solid (see the picture [off-road bicycle with auxiliary wheels]). The frame and front fork are exposed welded structures. Wheel diameters are available in 510mm (20 inches) and 405mm (16 inches). The rim is a hook edge type, generally a steel structure, and also made of high-strength plastic injection molding. The rims are equipped with 510×54mm or 510×45mm (20×2.125 inches or 20×1.75 inches) and 405×45mm (16×1.75 inches) rough tread tires. The handlebars are adjustable and upturned. Both the frame top tube and the handlebar cross tube are fitted with protective foam covers. The brakes generally use pincer brakes, and there are also foot brakes. The flywheel and sprocket are single-stage transmissions, the speed ratio is generally 2-2.5, and the stroke is 2.6-4m. Some cars are also equipped with auxiliary wheels on both sides of the rear wheels, which play an auxiliary role for children who are just learning to ride bicycles, to prevent them from falling when riding unbalanced.
