Non veni pacem mittere, sed gladium.
I was on my Kawi 500 Triple, driving home from work one day when I was sideswiped by a clumsy driver. I went over the bars at about 35 mph and skidded a ways before I could roll with the fall. The bike was doing cartwheels in the air behind me.I kept the shattered remains of that old Bell Star 120 for years to remind me to always wear a skid lid.Replaced it with an expensive Simpson full-face helmet.As my Old Man used to say, "If you have a $35.00 head, buy a $35.00 helmet." =BCE56=
That looks like a smooth cut with a bandsaw. Notice the pristine shell edges and parallel marks on the inner protective foam. The scratch marks on this side may even be where the vice jaws clamped it while the cut was made. Why? I dunno. But that's my forensic analysis.
I think it was hanging on a helmet hook next to the seat and was ground down while doing switch back turns.
I'd say the helmet must have been trapped under the bike otherwise there wouldn't have been enough downward force to grind it like that. Whether there was a head in the helmet is an open question. Bayouwulf has a point, I've seen lots of helmets strapped to the seat instead of worn. My Dad hit a dog at 70+ and slid forever. His visor ground off and his helmet was scarred up badly, but not like that. It was scarred all over as he rolled several times.