Only thing I know is I've had 2 offs in the last month and hit my head hard both times (especially the last one) whilst wearing a helmet. I suffered no rotational injuries and I am glad I was wearing a helmet. I believe my injuries would have been much worse had I not been wearing a helmet. My rather simplistic take on it is that if I had to headbut a brick wall (or a tarmac road) and was given the choice of doing so with or without a helmet, I would go with the helmet. I'm totally happy for others to make their own choice though. Edit. I also know I should look at taking my cycling proficiency test again!