Quoted for truth! After I saw my lady get her second concussion (she claimed the first was just a freak accident) with the padding she was using in her 14gg stainless helm, I took it away and re-padded it. Like so many helms I see these days, it was closed-cell only, and a pretty hard closed cell at that.However the type of foam can have alot to do with how that helm absorbs all that energy from a blow. For instance if all you use is open cell foam you are going to probably end up bottoming out if you take a good solid blow but you will also kind that it takes alot of the kinetic energy out of the blow. On the other side if you use only close cell foam you will be hard pressed to bottom the helm out but at the same time you will reduce very little of the concussive force that comes along with that shot.
While I love period solutions to period problems, this is one place I still use modern technology. I pad helms with one layer of Ensolite (tm) camping pad as the closed cell, or similar. Looking for a gooshy foam here, usually blue or black. The green military sleeping pads are way too hard, and concussive force just zings right through them. Then I put in a very thin layer of open cell foam (1/4 inch is plenty) for comfort and force dissipation. Finally, I sew a layer of linen over the whole thing to hide the modern foam and hold the padding together. As a bonus, this makes a nice sweatband and is removable after fighting to dry out.
Last, but far from least, is fit. I can't tell you how often I've seen padding that presses against the forehead and leaves air gaps over the temples. This is an issue of helm fit, but can also be corrected by tailoring padding.
Frankly, I think padding issues and chin straps that allow helm movemement are responsible for far more concussions than weight of steel.
