axle vs axletree

axle

noun
  • The pin or spindle on which a wheel revolves, or which revolves with a wheel. 

  • A transverse bar or shaft connecting the opposite wheels of a car or carriage; an axletree. 

axletree

noun
  • A spindle or axle of a wheel. 

  • A stimulus or driving force. 

  • A bar or beam of wood or iron, connecting the opposite wheels of a carriage, on the ends of which the wheels revolve. 

How often have the words axle and axletree occurred in a corpus of books? (source: Google Ngram Viewer )