1) Carriage underpinning 2) Dead axle 
• (n.) A bar or beam of wood or iron, connecting the opposite wheels of a carriage, on the ends of which the wheels revolve. • (n.) A spindle or axle of a wheel.
Ax'le·tree` noun [ Confer Icelandic öxultr... .] 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. [ Obsolete]
[n] - a dead axle on a carriage or wagon that has terminal spindles on which the wheel revolve
fixed bar or beam of wood, etc., on the rounded ends of which the opposite wheels of a carriage revolve.
noun a dead axle on a carriage or wagon that has terminal spindles on which the wheels revolve
a bar, fixed crosswise under an animal-drawn vehicle, with a rounded spindle at each end upon which a wheel rotates.