The CycleChat Helmet Debate Thread

Page may contain affiliate links. Please see terms for details.

Wobblers

Euthermic
Location
Minkowski Space
As i mentioned the helmet did compress, there are clear signs of this. What we cannot know is at what point the crack appeared, could be that it appeared after the compression phase of the impact, we simply do not know.

Actually, it is possible to make some predictions, and some observations. Brittle fracture will occur when the strain rate exceeds the material's ability to behave in a viscoelastic manner (ie, compress) [1]. This is most likely to happen when at or before the peak forces in the collision. Once the material is cracked, its ability to compress further is seriously compromised, as movement along the crack surface is more favourable. You do not want your helmet to crack! I think this ought to address @swansonj's points as well.

Looking at your photo, it can be seen that the crack has propagated some distance from the impact site. This is indicative that the crack had some time to grow. This suggests that brittle fracture would have started quite early in the impact sequence.

Lastly, @EnPassant and myself showed that the average deceleration of even an intact helmet which compresses perfectly exceeds 100 G at 12 mph for 6 ms, which I believe is close to the speed you were doing? [2] There's a set of medical data from cadaver experiments - the Wayne State Tolerance Curve: from this, 100 G over the 6 ms impact duration leads to a life threatening injury. The medical evidence is clear: even a helmet performing ideally (and yours didn't!) cannot offer protection from severe injury.


[1] @srw, I hope this satisfies your "Big Word" quota...

[2] As @jefmcg says, adding in the vertical component of your fall means the resultant impact velocity is significantly higher
 

Very, very poorly fitted though, with the strap loose and badly adjusted

Especially as if you accept Rivara et al :

Individuals whose helmets were reported to fit poorly had a 1.96-fold increased risk of head injury compared with those whose helmets fit well.
 

swansonj

Guru
Once the material is cracked, its ability to compress further is seriously compromised, as movement along the crack surface is more favourable. You do not want your helmet to crack! I think this ought to address @swansonj's points as well.
Still not persuaded I'm afraid. You want the helmet to compress in the bit between the point of contact of your head and the surface it hits. You surely really don't care what happens elsewhere.

There will absolutely be scenarios where a helmet cracks in that place or close to it and thereby impairs its ability to compress. But there are also scenarios where the crack is elsewhere. I suspect one scenario is that a helmet is a loose fit on the head. The first effect of an impact is to bend the bit of the helmet that hits first closer to the head, a bend that creates cracks at the point of the bending (or, if I am allowed a medium length word, at the nearest stress raiser). If those cracks are to the sides of the impact point, I don't see that it has any effect on the ability of the bit of polystyrene in that crucial area to absorb energy by compressing.

I am no blind advocate of helmets- I have ruined at least one dinner party by not keeping quiet and I've stopped wearing one routinely myself - but I think there are times when this thread succumbs to the "leaning into the wind" syndrome - in our zeal to counter extravagant claims in favour, we overstate the arguments against.
 
D

Deleted member 26715

Guest
As a complete side note, manufacturers of motorcycle helmets always used to (not sure now as it's a while since I replaced mine) have instructions in the box advising not to apply stickers to the helmet as the glue used on the some stickers could react with the material the helmet was made of thus weakening it. I wonder if cycle helmets are the same?
 

mjr

Comfy armchair to one person & a plank to the next
As a complete side note, manufacturers of motorcycle helmets always used to (not sure now as it's a while since I replaced mine) have instructions in the box advising not to apply stickers to the helmet as the glue used on the some stickers could react with the material the helmet was made of thus weakening it. I wonder if cycle helmets are the same?
Yes, they are! Follow the link from my recent post back to the posts with links to the manuals if you want to verify that. I think it's a bit sick that many sportive events require people to wear helmets and then damage them with stickers.
 

MontyVeda

a short-tempered ill-controlled small-minded troll
As a complete side note, manufacturers of motorcycle helmets always used to (not sure now as it's a while since I replaced mine) have instructions in the box advising not to apply stickers to the helmet as the glue used on the some stickers could react with the material the helmet was made of thus weakening it. I wonder if cycle helmets are the same?
looking at the type of plastic that covers a bog standard cycle helmet, which looks similar to the type of plastic that surrounds yogurt and margarine... even the most reactionary adhesive won't make much difference.
 

Wobblers

Euthermic
Location
Minkowski Space
Still not persuaded I'm afraid. You want the helmet to compress in the bit between the point of contact of your head and the surface it hits. You surely really don't care what happens elsewhere.

There will absolutely be scenarios where a helmet cracks in that place or close to it and thereby impairs its ability to compress. But there are also scenarios where the crack is elsewhere. I suspect one scenario is that a helmet is a loose fit on the head. The first effect of an impact is to bend the bit of the helmet that hits first closer to the head, a bend that creates cracks at the point of the bending (or, if I am allowed a medium length word, at the nearest stress raiser). If those cracks are to the sides of the impact point, I don't see that it has any effect on the ability of the bit of polystyrene in that crucial area to absorb energy by compressing.

I am no blind advocate of helmets- I have ruined at least one dinner party by not keeping quiet and I've stopped wearing one routinely myself - but I think there are times when this thread succumbs to the "leaning into the wind" syndrome - in our zeal to counter extravagant claims in favour, we overstate the arguments against.

There are a number of significant factors that need to be considered. When I say "lateral movement", this is not merely seperation between the two crack surfaces. This also includes movement where both surfaces move past each other... err... laterally (both normal and orthogonally to the impact force vector). Consider that the helmet is a (roughly) spherical structure: it is inevitable that an impact will result in substantial shear forces being set up some distance away from the impact site. Shear forces are the principal means by which the two crack surfaces move past each other. Furthermore, given that the foam is an elastic material, once the load is removed the crack will spring back, at least to a degree. It is therefore impossible to judge from the seperation of the crack afterwards how much movement took place during impact. This is important - compression takes up at most 20 mm. Even if movement along a crack is only 2 mm, it can be readily seen that it is sufficient to materially diminish the protection offered.

It is not true that cracks distal from the impact site are unimportant. The essential thing about a crack is that it will propagate most readily in regions of maximum stress. A crack that starts some way from the impact is unlikely to stay there - it is highly energetically favourable for it to propagate to the impact site. Remember that a helmet is an integral load bearing structure and even elements far away have a direct bearing on its strength. I referred to this above, by pointing out that shear stress is distributed throughout the structure. And this happens whether or not the helmet fits properly.

Which brings us to stress raisers. (I deleted a post about this the other night, thinking that it was a little too arcane. Ha!) It has taken a long time for it to be understood just how important shape is - think of say the Liberty ships, where some broke in half because a crack started from the sharp corner of a hatch. Or the Comet crashes: the designers were no fools and reinforced the aluminium round the windows, but didn't understand the sharp corners amplified the forces in that region by an order of magnitude. In short, the sharper the corner, the higher the stress amplification. It is disturbing how often sharp corners are seen in helmet vents. Impact stresses in those areas can be expected to far higher than the rest of the helmet. It is not a sign of good design - I wonder just how much the designers really understand about the way materials fail.
 
Last edited:

EnPassant

Remember Remember some date in November Member
Location
Gloucester
looking at the type of plastic that covers a bog standard cycle helmet, which looks similar to the type of plastic that surrounds yogurt and margarine... even the most reactionary adhesive won't make much difference.
As a recent convert to not wearing one I've been convinced (mostly here) that the whole thing is not as protective as it looks. The testing regime for EN1078 would seem so short of what is required that the manufacturers don't even state the prophylactic abilities of one anywhere in their advertising blurb, which of itself is pretty suspect since that's its whole purpose.
However I'm not so sure about what you are saying here exactly. I'm no materials scientist, but I am reminded that something such as carbon fibre seems pretty feeble in its raw state, yet once properly prepared with the addition of effectively glue is stronger in some properties than steel. So whilst I now how have much less faith in helmets overall that before I learned here, I'm less likely to dismiss the idea that its strength, insufficient as that may be, cannot be further compromised by the application of some adhesive or in fact any other substance. Those plastic milk containers can seem incredibly resilient when one is attempting to stuff them in an overfull kitchen bin, I have to jump on them....:laugh:

ETA: speaking of materials scientists, it appears they also type faster than me.
 

Erudin

Veteran
Location
Cornwall
Youtube: How A Road Bike Helmet Is Designed – Behind The Scenes With The NEW Bell Zephyr - GCN

Watched this vid the other day, lots of marketing hype on show. How they look, weigh and ventilate seems to be the main priority.
 
Last edited:

mjr

Comfy armchair to one person & a plank to the next
Nt surprising, given manufacturers are prohibited by Advertising Standards from making any safety claims (or insinations). At least that is my understanding - someone will no doubt be along shortly to refute or confirm.
Manufacturers and retailers are completely allowed to make safety claims - as long as they can substantiate them, don't make them in a misleading way and so on. This thread contains some (I think at least three) examples where they made claims and withdrew them, if you care to search it. Edit: Rutland Cycling, Las Victory Supreme and Halfords.

They rarely make safety claims and withdraw the few they do... well, that tells its own story, doesn't it?
 
Any claims made in ads have to be true, of course. But if you make a claim that is ok with advertising standards, but someone using your product is injured they could make a claim against you. So avoiding any claims of safety and protection (while using words that would leave a casual reader feeling safe and protected) will protect the manufacturer from expensive litigation.
 

MontyVeda

a short-tempered ill-controlled small-minded troll
..
Any claims made in ads have to be true, of course. But if you make a claim that is ok with advertising standards, but someone using your product is injured they could make a claim against you. So avoiding any claims of safety and protection (while using words that would leave a casual reader feeling safe and protected) will protect the manufacturer from expensive litigation.
Is this true for other items of PPE?
 
..

Is this true for other items of PPE?
No idea. Actually, my thinking was that a manufacturer could be literally ruined by a single class action in the USA, so they would word their material carefully for that market. Then they would use the same careful wording here - because why take the chance? And because they mostly tend to have a single website, so why rewrite the copy? It seems the examples of dubious wording are not from the manufacturers but from UK based retailers, writing their own copy.

PPE equipment manufactured for the UK market, would only have to worry about litigation that would be successful in a UK court.

But I would hope, if you were using welding goggles that claimed to protect your eyes, and your eyes were damaged when you were using them correctly, then I would hope you could sue the manufacturer for all your losses and pain and unhappiness.

Edit;
"Don't take Jake or Jill's example and break your crown, instead wear the Las Victory Supreme crown with pride and joy and ride in absolute safety".
So if someone did buy one of these helmets and were injured, I think they should have a strong case against the retailer or manufacturer that made that claim. I know people who think that you can't die from head injuries in a bike only collision while wearing a helmet. That belief could put you in real danger, I believe.

Caveat 1: all of the above total speculation based on casually reading of popular press, but I believe this is correct. if anyone knows better, I will bow to your expertise
Caveat 2: I have read all this thread over the years, but don't remember everything. Apologies if I am rehashing something that has already been said.
Caveat 3; Apparently I haven't read the whole thread
 
Last edited:
Top Bottom