How to say - the size of the universal wheel

Universal wheel, adjust the feet manufacturers when we purchase castor, the size of the caster wheel diameter is generally represented by inches, so why castor with a 'to describe. Investigate its reason, have to talk about the development history of castor. About the development history of castor, although we know China early design and use of castor wheels, but the caster is from after the reform and development, batch export countries at a time when China is using imperial units such as the United States, the specifications are according to customers' requirements of castor, so of the specifications of the castor often USES inches inc said. Due to the international system of units use the decimal system is convenient, at present, the international system of units used around the world. From the point of the official, in addition to the United States, Burma, Liberia is not mainly adopts the international system of units, other countries in the international system of units as the main measurement system. That includes most of the former imperial countries, such as Britain, Canada, Australia and so on, they are in the 20th century to the transformation of international system of units. But in the air traffic control, still use English (international Such as altitude in feet) 。 About English unit length: 1959 years later, the American English in inches ( 英寸) And inch inch in scientific applications and commercial use of unity is 25. 4 mm, but American retained have slightly different measurement used in the 'measuring inches'. 1 inch ( 英寸) = 2. 54 cm ( cm） 1 foot ( 脚) = 12 inches = 30. 48 cm 1 code ( 院子里) = 91 = 3 feet. 44 cm