The CycleChat Helmet Debate Thread

[Justinslow]

Here's a pic of the incident

 

[swansonj]

A failed helmet means it failed in its intended purpose - to absorb the energy of impact and reduce the impact forces (or more strictly, impact acceleration) to your head. The only mechanism by which impact forces can be reduced is by compression in the helmet. That your helmet cracked suggests that it failed in a brittle manner and did not compress. A "failed helmet" as you put it, does not offer any meaningful protection against serious injury whatsoever. In fact, by making it more likely that your head will collide with another object, it makes injury more likely.

ETA: It would be churlish not say "I'm glad you weren't badly hurt", so I'll just say I'm glad you're still here posting (and not drinking through a straw!) You helmet at least saved you from an unpleasant scalp injury so it would be wrong to suggest that it was useless.

A helmet that cracks will has a diminished ability to compress. A crack is not a good thing to see in a structure that is designed to deform in a controlled manner. Brittle fracture is in no way controlled.

I was taking a bit of a punt. But my reading of what @McWobble says is that it was a reasonable punt. Despite a bit of energy absorption (polystyrene foam doesn't need a lot of energy to deform slightly) enough energy made its way through to cause a crack.

I take @McWobble's point that cracking reduces the control of any crushing so is not helpful. But that seems a second-order issue and nor do I think I agree that cracking is necessarily evidence of helmet failure.

Consider a though experiment of a helmet comprised of multiple small balls of expanded polystyrene, held in place around the head by a flexible membrane with sufficient strength and compartments to ensure that there is always a layer of balls over every part of the skull. Cracking would then not be an option but the helmet would perform (or not) just as well on the basis of the same thickness of polystyrene to absorb energy.

It seems to me there are only two things a helmet can do to protect your head, which are to absorb energy, or to spread a point impact over a larger area. (From a physics PoV, you can always describe any problem equivalently in terms of energy or forces - talking about a helmet absorbing energy is not something different to talking about it reducing (linear) accelerations, it's the same mechanism just described in different ways). Creating cracks takes trivial energy in expanded polystyrene, as we have said many times and as is obvious to us all from the analogy of snapping a kitchen tile, so significant energy absorption will come only from crushing. When we criticise helmets for showing evidence of cracking not crushing, as we frequently and correctly do, the primary point is surely not that they did crack but that they didn't crush. I don't agree that cracking is evidence of failure - I do agree quite strongly that absence of crushing is evidence of failure.

Incidentally, nor do I necessarily agree with @Cunobelin that the need for bigger air vents and hence narrower ribs and hence denser foam is in itself detrimental - you could, in principle, get exactly the same energy-absorbed-per-unit-deformation that way by appropriate choice of material.

 

[srw]

This post sums it up for me, utter refusal to accept that a helmet can sometimes prevent more serious injury.
I'm big enough to admit helmets have their shortcomings, that they have limited protection abilities, that I don't always wear a helmet. But @Big Andy has explained the situation regarding his accident shown us pics of his helmet, told us that he hit his helmeted head on the Tarmac, explained how his helmet prevented more serious injury yet @srw will not accept that. There is no way in this case that it can ever be "evidenced" or "proved" that the helmet did some good, that is the problem here. Some people will be gracious enough to admit helmets can help, some won't.

I'm certainly gracious enough to say that I might be mistaken - the evidence is incomplete either way, although it's weaker in the direction it's often claimed to be irresistible.

I'm also going to be gracious enough to assume that you're misinterpreting what I'm saying through accidental mistake rather than deliberate provocation.

Now that I've pointed out that you're misinterpreting, are you now going to be gracious enough to accept it?

 

[Mugshot]

Im not saying that at all, you questioned the occurrence of a collision with a Deer, what does it matter how the accident happened, it's one of many hazards on our roads and trails.
For me when I'm on my road bike or mtb my helmet doesn't bother me so I'll wear it regardless, during Ride London for instance, it just doesn't affect my enjoyment.

Do you not think that it's reasonable to consider the likelyhood of an incident occuring when deciding whether to wear protective equipment or not. The reason you posted that link and the reason that Doog quoted it is that you both believe it somehow adds weight to your argument, you are implying that because an incident occurred it is worth considering a helmet just in case it happens again on the off chance that the helmet may offer some level of protection, this despite the fact that the incident is so phenominally rare it actually made headline news,

 

[srw]

I take @McWobble's point that cracking reduces the control of any crushing so is not helpful. But that seems a second-order issue and nor do I think I agree that cracking is necessarily evidence of helmet failure.

Consider a though experiment of a helmet comprised of multiple small balls of expanded polystyrene, held in place around the head by a flexible membrane with sufficient strength and compartments to ensure that there is always a layer of balls over every part of the skull. Cracking would then not be an option but the helmet would perform (or not) just as well on the basis of the same thickness of polystyrene to absorb energy.

It seems to me there are only two things a helmet can do to protect your head, which are to absorb energy, or to spread a point impact over a larger area. (From a physics PoV, you can always describe any problem equivalently in terms of energy or forces - talking about a helmet absorbing energy is not something different to talking about it reducing (linear) accelerations, it's the same mechanism just described in different ways). Creating cracks takes trivial energy in expanded polystyrene, as we have said many times and as is obvious to us all from the analogy of snapping a kitchen tile, so significant energy absorption will come only from crushing. When we criticise helmets for showing evidence of cracking not crushing, as we frequently and correctly do, the primary point is surely not that they did crack but that they didn't crush. I don't agree that cracking is evidence of failure - I do agree quite strongly that absence of crushing is evidence of failure.

Incidentally, nor do I necessarily agree with @Cunobelin that the need for bigger air vents and hence narrower ribs and hence denser foam is in itself detrimental - you could, in principle, get exactly the same energy-absorbed-per-unit-deformation that way by appropriate choice of material.

Let me find my physics brain and get back to you!

My immediate issue with your elegant thought experiment is that I don't see why your flexible package shouldn't crack. Unlike a fixed package, however, if it does crack no evidence of the fact would be preserved.

 

[srw]

Yes but this a "helmet" thread not a "ribs" thread.

Which raises one rather obvious question: why is there no "Cyclechat Rib Protection Debate"? And no "Cyclechat Clavicle Protection Debate"? Since the classic pro injury has always been a broken clavicle isn't it time to campaign for effective shoulder pads?

 

[ufkacbln]

Still shows both hands and a foot contacting before the helmet

Here's a pic of the incident
View attachment 141867

 

[ufkacbln]

The other question with the deer incident is that if eye witness accounts are to be believed the animals were visible for some time

The photographer had enough time to get out his camera, set it up predict and plan the shot

Rather than relying on a helmet, perhaps adequate observation of one's surroundings, slowing down and taking avoiding action would be a better option for longer term safety

 

[Deleted member 26715]

Hypothetical question.

Cyclist standing near some scaffolding with his bicycle about to cross a road, when one of the workmen on the scaffolding looses the grip on a 2lb hammer, it falls off the scaffolding falling 20 feet & striking the cyclist on the head. Would the potential injury the cyclist received be greater or less if he was wearing a cycling helmet?

 

[Mugshot]

Hypothetical question.

Cyclist standing near some scaffolding with his bicycle about to cross a road, when one of the workmen on the scaffolding looses the grip on a 2lb hammer, it falls off the scaffolding falling 20 feet & striking the cyclist on the head. Would the potential injury the cyclist received be greater or less if he was wearing a cycling helmet?

Ah, a cunning version of the "would you rather hit your head on the kerb wearing a helmet or not wearing a helmet," or the "Let me hit you on the head with a baseball bat" conundrum. Your clever ruse almost had me fooled there, as it is, perhaps you could have a look through the thread where many such hypertheticals have been covered.

 

[Deleted member 26715]

Ah, a cunning version of the "would you rather hit your head on the kerb wearing a helmet or not wearing a helmet," or the "Let me hit you on the head with a baseball bat" conundrum. Your clever ruse almost had me fooled there, as it is, perhaps you could have a look through the thread where many such hypertheticals have been covered.

Clearly too difficult a question, or one that would not give you the required answer, I suspect.

 

[Mugshot]

Clearly too difficult a question, or one that would not give you the required answer, I suspect.

Ok, lets try this for starters. If I may assume as the cyclist is waiting to cross the road he is likely to be standing on a pavement where it is likely that the amount of pedestrians is by some magnitude greater, are you suggesting, hypertheticaly of course, that the peds need no helmet to potentially protect them from your hyperthetical hammer?

 

[Deleted member 26715]

Ok, lets try this for starters. If I may assume as the cyclist is waiting to cross the road he is likely to be standing on a pavement where it is likely that the amount of pedestrians is by some magnitude greater, are you suggesting, hypertheticaly of course, that the peds need no helmet to potentially protect them from your hyperthetical hammer?

I never mentioned said peds, only you did that

 

[Tim Hall]

I never mentioned said peds, only you did that

So hypothetical hammers dropped from height are OK, but hypothetical pedestrians standing nearby aren't?

 

[Deleted member 26715]

So hypothetical hammers dropped from height are OK, but hypothetical pedestrians standing nearby aren't?

They have no relevance to the original question