Well, the design may be simple, and winding on may be simple as well, but drawing such a thing? It is not. Which I used to think would be the reason why we do have images of yarnwinders of that design that are depicted somehow... alternatively.
This ape winding wool from a 14th century manuscript is a typical example. Or this ape winding wool from a different 14th century manuscript. (Hm. Do we sense a pattern here?)
Medieval images are not always true to perspective, and they are not always mechanically correct when they are depicting tools that are a bit more complicated, like this wool wheel:
[caption id="" align="alignnone" width="500"]
You can clearly see that this would not work as depicted. The wheel has the requisite one-sided axle, and there is only one post holding it up, but it is drawn on the wrong side - facing the viewer. You could theoretically spin with that setup by shoving the wheel and letting go of it again, but this is not really compatible with the usual style of spinning with this tool, where the fine control of the wheel's (even) speed lets you draw your thread out evenly.
There's a similar error in the image in the Luttrell Psalter - again, you clearly have one post only, and it is on the wrong side.
[caption id="attachment_3679" align="alignnone" width="682"]
So there are obvious difficulties in getting the details right, and it looks like the difficulties are similar, and it seems to be a pattern. In the case of the wheel, though, it really makes no sense at all to set it up this way.
Which is why I used to think that the alternatively depicted yarnwinders were also some kind of artistic thing - something not gotten quite right.
However... I'm going through the wood things again, and I am thinking of adding a yarnwinder based on medieval sources to my assortment of goods, and I've been looking at images again. There is a pattern - but the pattern is not what I expected, and it has led me to take a closer look, and start to doubt my previous assumptions - that it's just a matter of how things are drawn, and that you cannot wind yarn on a yarnwinder of this style if the crossbars are parallel to each other.
So. I'm very excited right now, and very much in research mode, and will look at more pictures, and try out things. And tell you more about the results tomorrow.