Helmets pinging off, no matter how well fitted, is a fact of motorcycling life. The biomechanics are simple enough. You need to put it on your head and it follows you must be able to take it off.
So, it is fixed by a strip of strong webbing that usually sits somewhere under your chin which contains your throat. The throat is intrinsically soft which means with the wrong force applied in the wrong way, no matter how well your helmet fits it can be forced off. Luckily, it tends to be forced off after it has absorbed much of the accident’s force.
Insurers defending head injury claims get very focused about helmets coming off if there is a head injury.
There are experts who will offer opinions on whether your helmet was strapped up properly. If your helmet is unbuckled and that worsened your injuries then it is established law that a 10% discount on those head or facial injuries to account for your carelessness will be applied.
If no head injury, then no discount as foolhardiness alone is not enough. The foolhardiness must be causative of the harm suffered.
While riding with your helmet unbuckled is foolish, it only becomes relevant in a head injury. That said, when it is explored the experts tend to be vague as to the likelihood of the helmet being strapped; forensic tests can sometimes reveal stretching in the fibres of the chin strap and significant deformation of the lining.
I have more often than not dismissed the unstrapped helmet argument with evidence of the angry wheal left on the throat as tough nylon webbing forces its way over skin.
Failure to strap
Because failure to strap a helmet is an argument on contributory negligence the insurers must prove it was more likely than not that the helmet strap was undone, a tough burden to get over, as all the experts say much the same thing.
Helmets can and do come off, even if strapped. In my experience, which luckily coincides with what the experts say, the key points arc fit (pretty obvious) and shape of the face; a smaller featured, say a female face, without an adolescent dose of testosterone to bulk the jaw up is more likely to allow a strap to slide over it. Long hair is a known factor in helmet slippage, the hair providing a low friction surface for the helmet to slide over. A bit of pudge on the chin creates yet further soft points for physics to exploit as the laws of nature and evolution conspire to remove your helmet when you really want it to stay on.
A basic test when you are in the shop buying your lid is to see if you can pull the normally tightened strap close to your chin. If it gets within touching distance it’s not the helmet for you.
Finally, even in a modest bump, the slighter built you are the worse the strains put through your neck will be. Proportionately a nine-stone woman with a 13-inch neck and a 1500g helmet is going to have a lot more pain in her neck than a 15-stone man with a 17-inch neck in the same helmet even in a modest roundabout bump.
Bike Magazine April 2021