I just read an interesting article by Marjorie Nice Boyer, "Medieval Pivoted Axles" in Technology and Culture, Vol. 1, No. 2, Spring 1960 (pp128-138, pub. Johns Hopkins University Press, JSTOR URL: <http://www.jstor.org/stable/3101055> requires access agreement through an academic, corporate, or library setting).
Ms. Boyer wished to counter the claim that pivoted axles were unknown in the medieval period. Alas, I find her argument to be mostly unconvincing. Her main premise seems to be "why would the people of the middle ages give up the use of a technology well known to the ancients, when to do so pretty well renders a wagon useless?"
That's what I've said! And indeed it doesn't seem to make sense, but...
We have extant ancient and roman, even late roman, examples (grave offerings) with pivot and underlock; and we have iconography of ancient and roman examples with pivot and underlock. We have no extant remains of a medieval wagon with pivot or underlock. We have no iconography that shows pivot or underlock. I am willing to concede that one of the iconographic examples she cites may show pivot, but again, none show underlock. Her analysis of medieval account books showing the purchase of wagon parts is (at least to me) unconvincing.
One cannot make the claim that in a picture that is very detailed in all other aspects, when it comes to the running gear and undercarriage lacks the detail showing the pivot as 'artistic convention.'
She also mentions the Oseberg wagon, but I have to regard this as inconclusive; and she only cites the sources that claim it was pivoted. The Oseberg remains could have a pivoting front axle, and the curved bed of the wagon would give it some underlock. But analysis over the years by undeniable experts in the field of archeology is not in agreement. The most recent analysis seems to indicate that was pegged in several places through the bolster and axletree locking them in place.
The thought that medieval people would have abandoned such a useful technology is hard to accept, but it remains that there is no reliable evidence to the contrary that is not colored by that premise.
I suspect that in truth they did have minimally pivoted axles, but did not use underlock. But this remains supposition, not supported fact.