I’m seeing a lot of variance in the ratios. Some flails have longer handles, some have short. Some have chains as long or longer than the handle, some have almost non-existent chains. What are the advantages and disadvantages of various handle and chain lengths, and is there an “optimal” ratio?
The type of flail seen in your picture wasn’t used by the peasantry but nobility. Some crafty blacksmith probably saw drafted farmers take their threshing flail into battle as improvised weapon and thought about how to properly weaponize the concept. Also we are not talking about two flimsy sticks here. Flails used for threshing had to be sturdy as they needed to last the whole harvest season. They were made from hardwood and I can personally attest, that they are heavy indeed. The things one finds when clearing the grandparents barn …
As for the practicality of the weaponized flail: it was indeed of dubious use and it is questionable whether it saw widespread military adoption or whether it was the equivalent of “tacticool” gear of the middle ages. An iron club is indeed a more effective weapon and is commenly referred to as mace and if you take a look at a specific type of mace you can probably guess where the idea of the spiky ball came from.