Is helmet size really such a big issue?

[bert_is_evil]

Most people agree it's dangerous to wear a helmet that is too big, on the face of it this seems like good advice - you don't want your head rattling round in there right? But I got to thinking about it the other day..... say you have two riders (let's call them Bert and Ernie) who are wearing the same helmet except Bert's helmet fits perfectly and Ernie's helmet is a couple of sizes too big. Bert and Ernie are out for a ride and (shock horror) are not riding in staggered formation but instead precisely side by side. At the same moment Bert and Ernie T bone a cage that has driven through a stop sign and both go flying over the bonnet at precisely the same tragectory. Both land head first on the road the same distance from the cage. So what's the difference? They both receive the same benefits of the couple of cm of cusioning goodness from the helmet interior etc, the only difference being that the Ernie's helmet impacts the road a couple of mm's before his head hit's the inside of the helmet.
Is it really enough to make a difference? Or is the correct size issue more to do with other stuff such as strap fitting? Am I missing the point completely?

[yod]

it can come off

[Nagash]

And as Ernie's head slides down the road the back of the helmet rises and slips off Ernie's head leaving his face to fend for itself.

There's also other things.. I got a helmet as a gift and it was too big for me and when riding it would slip down blocking my view and i'd have to push it up again so I just use it for pillions now.. and it's better than my normal helmet! But yeah.. Visibility would be another thing.

[ManDownUnder]

Ernie's helmet is a LOT more likely to come off leaving a trail of "Ernie's High Protein Cranial Pate" to assist the investigators as they try to find the initial point of impact, and answer the question "where did his head go?"

[Sidewinder]

if you cant find one that fits dont wear one

[Sully60]

the only difference being that the Ernie's helmet impacts the road a couple of mm's before his head hit's the inside of the helmet.
Is it really enough to make a difference? Or is the correct size issue more to do with other stuff such as strap fitting? Am I missing the point completely?

The couple of mm that's needed for the impact momentum to increase so it's not a controlled deceleration anymore. Hmmm, no thanks!
Head injuries suck!

[Shaun]

To big a hemet, can also twist and break your neck very easy

[James Deuce]

Yes. The lining needs to be in contact with as much of your skull as possible to absorb the energy in an impact. If it isn't touching your head and the helmet will move relative to each other and the velocity difference between head and helmet will add to the force that your skull will be absorbing. This differential may be enough to kill you. If it turns on your head you may suffer facial damage. If it comes off you may not survive the next impact your head takes.

[bert_is_evil]

The couple of mm that's needed for the impact momentum to increase so it's not a controlled deceleration anymore. Hmmm, no thanks!
Head injuries suck!

But he's not travelling 2mm further. What if he was 1 foot shorter than Bert - his head would travel 1 foot further so is a couple of mm's really relevant?

[bert_is_evil]

Yes. The lining needs to be in contact with as much of your skull as possible to absorb the energy in an impact. If it isn't touching your head and the helmet will move relative to each other and the velocity difference between head and helmet will add to the force that skull will be absorbing. This differential may be enough to kill you. If it turns on your head you may suffer facial damage. If it comes off you may not survive the next impact your head takes.

So if a helmet is a few mm too big would adding a liner like a balaclava for example remedy the problem? I'm asking in theory - my head is snug as a bug in my helmet

[CookMySock]

I bought a helmet two months ago, and now it doesn't fit. It wobbles around and I constantly have to grab it and push it back into place. Not nice.

It might be something to do with the 1kg of fat I lost off my head, or whatever, but the thing is, it must fit or it will be very distracting and you don't need this shit while ur riding.

DB

[James Deuce]

So if a helmet is a few mm too big would adding a liner like a balaclava for example remedy the problem? I'm asking in theory - my head is snug as a bug in my helmet

No. A balaclava will only add microns, if silk or polyester, or maybe 0.5mm if wool.

A "few" mm sounds insignificant. It's not.

[James Deuce]

But he's not travelling 2mm further. What if he was 1 foot shorter than Bert - his head would travel 1 foot further so is a couple of mm's really relevant?

Yes. F=ma. Your head is accelerating relative to the helmet after the helmet has hit something. Nasty. Instead of absorbing the impact and spreading the load over the helmet shell and compressing the lining to protect your skull, the helmet potentially smashes on impact because there's nothing supporting the shell from inside, leaving it unable to spread the impact shock when your head catches up. Probably better off with no helmet than one too big. Too small is infintely preferable in the short term. Just right is best.

[bert_is_evil]

Yes. F=ma. Your head is accelerating relative to the helmet after the helmet has hit something. Nasty. Instead of absorbing the impact and spreading the load over the helmet shell and compressing the lining to protect your skull, the helmet potentially smashes on impact because there's nothing supporting the shell from inside, leaving it unable to spread the impact shock when your head catches up. Probably better off with no helmet than one too big. Too small is infintely preferable in the short term. Just right is best.

Ah right, now I see the difference - thanks! < We need a smiley with a lightbulb going on over its head

[007XX]

Heck yes!

Having had an off as a teenager, with a head contact with the rear bumper of a car, I cannot stress enough how important a well fitting helmet is.

It's one of these things where trying to cut corners will cut your lifespan real short, real quick.