Gil_M posted up a computer image of his skull post crash either here or on UKFatbikes which showed a clear dent/fracture of the brow portion of his skull. He wasn't wearing a helmet at the time of his crash and now thinks himself VERY lucky. There may only be anecdotal evidence of the value of helmets but things like this remind me never to ride without one !
Yes, I saw the images - scary.
I was riding alongside my lad when he came off, and so I had a slowmo view of it all. Speed was about 16mph. If he hadn't been wearing the helmet, he would have been in serious trouble.
The image doesn't really show up the damage particularly well. The crack runs right through the polystyrene, and into the polycarb. The polystyrene in bottom right of the image is about 10-20% thinner than the non-impact side. It basically took all the force out of the impact.
Wearing a helmet is a no-brainer IMO (no pun intended).
2012 CannondaleSupersix Ultegra (54cm)/2014 Saracen Kili Flyer X (M)