In all the helmet discussions we hear about helmets being rated up to a certain speed, but I'm not sure what that really means. It seems reasonable to me that (1) the helmet might still be better than nothing even above that speed, and (2) the helmet is no guarantee even below that speed. If those assumptions are correct (are they?), then it seems that the speed rating is not just some magical point ensuring protection below and failure above, but rather a point on some kind of probability spectrum. I can envision some kind of curve showing level of protection (or perhaps probability of level of protection) plotted against speed, so is the rating simply the point on the speed axis where the protection falls below a certain threshold or the curve takes on a certain slope?