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Wheel chocks (or chocks) are wedges of sturdy material placed closely against a vehicle's wheels to prevent accidental movement. Chocks are placed for safety in addition to setting the brakes. The bottom surface is sometimes coated in rubber to enhance grip with the ground. For ease of removal, a rope may be tied to the chock or a set of two chocks. One edge of the wedge has a concave profile to contour to the wheel and increase the force necessary to overrun the chock. Most commonly, chocks are seen on aircraft and train cars. Automobiles usually have parking brakes on the rear wheels. If the rear axle is jacked off the ground with only the parking brake set, the vehicle may roll on the front wheels and fall. Chocking the front wheels prevents this mishap. Motorcycle and bicycle chocks are bifurcated and fit around the wheel, supporting the bike and preventing its movement. The mining industry uses wheel chocks to protect lubrication trucks and heavy maintenance vehicles from slipping on off-road terrain when placed in Park. The huge haul trucks, which can weigh up to 450 tonnes, require a much larger wheel chock that itself will weigh almost 40 kilograms. These circumstances will benefit from urethane wheel chocks that are lightweight enough to be maneuvered, yet can withstand the responsibility of holding a truck if a brake should fail. The Mine Safety and Health Administration (MSHA) has established standards that wheel chocks are used when a vehicle is parked on a grade, and OSHA has guidelines that require wheel chocks during loading or unloading of a heavy truck. ==How to use wheel chocks== To use, wheel chocks must be selected to match the size of the tires on the vehicle and used in pairs, as the size of the tire is designed to be proportional to the vehicle's weight and size. For a maximum slope of approximately 15.6 degrees, use a chock 1/4 height of the wheel as per SAE J348. The maximum slope angle for any given chock is . For example, for a chock 1/6 the height of the wheel, the maximum slope angle would be 10 degrees. Some manufacturers recommend wheel chocks that are slightly bigger than 1/4 the height of the wheel. Three main factors affect the performance of wheel chocks: # The angle of the road surface, called slope or grade. Parking on an angle greater than 10 degrees increases the risk of the car rolling off or over the chock. # The size (height) of the wheel chock compared to the wheel height. Extra large wheel chocks are suitable for e.g. parked air craft where wind forces may come into effect. # The surface slip or friction of the road surface, as the wheel chock may slide downhill due to wet grass or ice. Increasing the size of the wheel chock will not necessary allow the parking on inclines greater than 30% gradient (16.6 degrees). Wheel chocks have to be used on a firm surface; some rubber wheel chocks have steel grates and ice cleat accessories. The vehicle is placed in park (on vehicles with automatic gearboxes) and the parking or emergency brake is applied. The wheel chocks are then snugly positioned against the center of the tires in the direction of the grade, on both the left and right side of the vehicle. On even surfaces, chocks are applied to the rear-facing as well as the front-facing side of each tire. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Wheel chock」の詳細全文を読む スポンサード リンク
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