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plural whippletrees
a wooden crossbar for a plough or carriage, pivoted in the middle, from which traces are fastened to a draught animal. quotations examples
The single whippletree should have a chain attached to bring it out even with the other two.
1889, George E. Blakelee, Blakelee's Industrial Cyclopedia, page 364
Like the whippletree, the wheelbarrow only became known in the Middle Ages.
1964, Albert C. Leighton, Early Medieval Transport, page 128
Note the two-horse whippletree on the beam at the lower left. (A whippletree, of which this is the simplest possible, balances the load on the animals.)
2016, Steven Vogel, Why the Wheel Is Round: Muscles, Technology, and How We Make Things Move