training wheels
additional wheel mounted parallel to the rear wheel of a bicycle to assist learners until they have developed balance on a bicycle
en-US/ˈtɹeɪniŋ ˈwilz/
en-GB/ˈtɹeɪnɪŋ ˈwiːlz/
additional wheel mounted parallel to the rear wheel of a bicycle to assist learners until they have developed balance on a bicycle