I have a motorcycle helmet and a bicycle helmet. Each of these has a “chin” strap, which is perhaps a bit of a misnomer, as it doesn’t fasten around my chin: it lays across my throat.
OTOH, if I look at a US Army helmet, or an NFL football helmet, these have true-to-the-name chin straps: the strap/webbing is sewn to form a cup, or is affixed to an actual molded cup, that fastens across the wearer’s chin.
Why the difference?
The first place I would go is to whatever paperwork (manuals, etc.) came with those helmets. Let’s verify that it really is a throat strap (which sounds dangerous to me).
“When al else fails, read the instructions.”
I think most bike helmet straps go under the chin, rather than on the chin. You could say they rest under the jaw, maybe would be another way to describe it? Definitely in front of the throat. But also not cupping the chin, as you say.
AIUI, a so-called “helmet harness” that fits over the chin is designed to keep the helmet securely in place during strenuous activity involving potential vigorous head movement (and in the case of military helmets, possibly having devices attached to the helmet that affect its balance).
Soldiers and football players need to be able to run, dodge, dive at the ground, roll and come up, etc., while retaining their helmets securely in the proper position on their heads.
Motorcyclists and bicyclists, on the other hand, basically just need a sturdy wearable shock absorber strapped to their heads firmly enough to protect them during a single sudden impact. They don’t change their head positions very much in the ordinary course of riding.
A motorcyclist thrown through the air in an accident would probably undergo very similar actions.
I’d WAG it’s about keeping the helmet straight when knocked.
A motorcycle helmet isn’t going to get knocked unless you’re in an actual accident, in which case we only care that the helmet is protecting the wearer; a little “yaw” rotation or whatever isn’t so important.
Soldiers and football players though might get bumped enough that even a tiny bit of rotation would be annoying. So a chin strap keeps it more secure.
Here’s a couple of examples:
Maybe I overstated the case when I said that my helmet’s chin strap lays “across my throat,” but it’s pretty consistent with those pictures, i.e. it’s definitely a good distance away from my chin.
The responses suggesting that football and military helmets use a chin cup to help assure consistent helmet orientation are intriguing…except that hockey helmets tend to use a strap like a motorcycle/bicycle helmet, and players often wear it very loose, and they tend to get bounced around a lot during routine gameplay. Not sure what to make of that.
Motorcycle helmets have chin straps for keeping it on your head not for chin protection. Only a full faced helmet will protect your chin and jaw. Half faced, and skid lid styles do not offer the same protection.
Agree completely (mine is FF), but not really relevant to my question, which was about the position of the strap rather than its mere existence.
I knew a guy who was in the army when it transitioned from the M1 tin pot to the PASGT. He said he preferred the latter because the M1 sat higher on his head and with all the webbing inside the liner and such would lag a bit with rapid head movements plus made the head’s CG higher so even tilting it to one side felt weird to him. The PASGT is lighter and sits lower so he preferred it.
This was right after the transition though, before they started hanging all that stuff off of it. I haven’t kept in touch so I don’t know his opinion after that happened.
I’d disagree with that statement as bicycle helmets now have MIPS to help with the ‘yaw’ rotation upon impact
My guess is that a chin cup style strap is less comfortable, and inhibits talking – as in, that’s even less comfortable, not as in, you can’t do it. A bike or motorcycle helmet is held effectively enough in place with their style of strap, and people are more likely to wear them.
With a football or military helmet, you are much more likely to have forces that act to strip the helmet from your head, possibly right before an impact. And you have to have the helmet in place properly for successive impacts in a short time. And wearing them is not optional, so you don’t have to worry about it not being worn.
A bike helmet is made for one impact. Then it must be replaced.
That’s not what I meant by ‘yaw’; by yaw I meant turning your head to look to your left and right, that axis.
Anyway, it was just a WAG, it’s probably wrong.
No, the point is that the soldier or football player may be doing such actions without significant head impact, and consequently need their helmet to stay in position as they keep moving after coming up.
A motorcyclist or bicyclist who gets knocked off their bike and hits their head on the road, on the other hand, is not going to be immediately jumping up to resume their ride without having time to readjust the helmet. In their case, the helmet just has to be strapped on the skull securely enough to successfully absorb the shock of a single impact, as eschrodinger noted.
I wonder how the British Lip Strap figures into this thread:
Hat moves with lip movements, maybe helps lip-readers? 
