axletree

noun

1. A bar or beam of wood or iron, connecting the opposite wheels of a carriage, on the ends of which the wheels revolve.
2. A spindle or axle of a wheel.
3. A stimulus or driving force.

   “Soon after his arrival, that patent axletree of action, the not knowing what to do with himself, domesticated him during some weeks of the shooting season at Sillery House...”