The 'whippletree' had broken.
"The Continental Monthly, Vol. 4, No. 2, August, 1863" by Various
He sends me for firewood and when I gits it loaded, de wheel hits a stump and de team jerks and dat breaks de whippletree.
"Slave Narratives: a Folk History of Slavery in the United States From Interviews with Former Slaves" by Work Projects Administration
Manuel put his cowboy escort to work, but it was a difficult job, with one of the whippletrees splintered.
"When the Owl Cries" by Paul Bartlett
Twelve "Swingle trees" (whippletree, whiffletree, singletree) are listed in the inventory.
"The Cultural History of Marlborough, Virginia" by C. Malcolm Watkins