Helmets, Accidents and NOT Removing a Helmet

This sub-forum all discussions about this "lively" subject. All topics that are substantially about helmets will be moved here, if not placed here correctly in the first place.
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Cunobelin
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Re: Helmets, Accidents and NOT Removing a Helmet

Postby Cunobelin » 13 Jan 2016, 7:27pm

Slightly OT

This was unintentional

I keep my helmet on the stairs as it is handy when I wish to wear it

MiL decided that was unsafe and placed it on kitchen hob......

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Must admit to being very naughty as one of my acquaintances is a bot of a helmet evangelist and thinks that any helmet is better then none

Took great pleasure when riding one of the trikes to wear this as "it was only slighty damaged and I doubted it would affect its function!"

MartinC
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Re: Helmets, Accidents and NOT Removing a Helmet

Postby MartinC » 14 Jan 2016, 8:41am

Cunobelin wrote:..................."it was only slighty damaged and I doubted it would affect its function!"


Does anybody have a clear definition of what the manufacturer intended it's function to be?

kwackers
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Re: Helmets, Accidents and NOT Removing a Helmet

Postby kwackers » 14 Jan 2016, 9:10am

MartinC wrote:Does anybody have a clear definition of what the manufacturer intended it's function to be?

Yeah, they're good at having a 5kg mass placed inside and then dropped at 12 mph onto a 'grapefruit' sized solid metal object.

What they're less good at is having 80kg of mass doing the same thing. In other words, glancing blows are OK, but hitting something head first is a complete no no.

Vorpal
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Re: Helmets, Accidents and NOT Removing a Helmet

Postby Vorpal » 14 Jan 2016, 10:26am

MartinC wrote:
Cunobelin wrote:..................."it was only slighty damaged and I doubted it would affect its function!"


Does anybody have a clear definition of what the manufacturer intended it's function to be?

If you read their marketing material, helmets are meant to be aerodynamic and almost weightless.
“In some ways, it is easier to be a dissident, for then one is without responsibility.”
― Nelson Mandela, Long Walk to Freedom

TonyR
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Re: Helmets, Accidents and NOT Removing a Helmet

Postby TonyR » 14 Jan 2016, 3:46pm

kwackers wrote: In other words, glancing blows are OK


Unfortunately not. They are not tested at all for glancing blows and any rotational forces that might impart on the brain. They are only good, if at all, for a severed head hitting things head on but not too fast.

And there is evidence emerging that under impact the "rigid" skull actually deforms quite a bit to absorb the energy and strapping it into a rigid helmet prevents that, disabling natures mechanism for dealing with it.

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Cunobelin
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Re: Helmets, Accidents and NOT Removing a Helmet

Postby Cunobelin » 16 Jan 2016, 10:11am

[XAP]Bob wrote:
rfryer wrote:Thinking about this further, my issue is with the oft-stated opinion that cracked polystyrene means that the helmet "failed" and no impact was absorbed. In fact, surely a good deal of energy could have been absorbed before the point of cracking, and more may have been absorbed afterwards if the other structural parts of the helmet had held the cracked parts together.

If the now separate pieces show evidence of significant crushing, then yes - but I'm yet to see pictures of a cracked cycle helmet showing such deformation.

Next time you have some packaging try squashing the expanded polystyrene - it takes significant effort, this effort is applied across a distance equivalent to ~2/3rds (IIRC) of the thickness of the material - force * distance = energy absorbed.
Then try snapping it - almost no energy required - the force builds to a certain point, but the fracture is brittle, so that force gets applied across a negligible distance - that's why there is practically no energy absorbed by a fractured helmet.

It's also why there are often reinforcing threads through helmets, to try and hold the pieces together, but they are not there to absorb energy, just to hold the polystyrene packing material in place...

The human head is a marvel of design - and it can cope with far more than most people think.


One of the possible reasons for cracking is the polystyrene foam itself

The experiment with packaging is with soft foam, and early helmets used exactly this method

However as the helmet became lighter and requires more ventilation changes have been made. Firstly the foam is denser as there is less of it and it has to be denser to support the shape and form of the vents. Secondly there are now various stiff cages within the helmet to support the shape and vents

This results in helmets that are less able to absorb impact as there is less material, and the density means it compresses less

Secondly where you have a thin layer of foam over a strut of the "cage" it is a weak point that can only break in impact


THis is from a classic "Helmet saved my life" article

All the failures are the weak bridge point between vents!

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