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It concerns me that there's so many stories of bike helmets cracking in two and allegedly having saved someone's life. Bike helmets are designed to compress und

It concerns me that there's so many stories of bike helmets cracking in two and allegedly having saved someone's life. Bike helmets are designed to compress under impact and as such, they are very weak under tension, so when you see a helmet split into two, it indicates that it wasn't working as designed. Compressed polystyrene in the helmet would indicate that it was doing its job.

altcognito 4y ago
Basically, crumple zones for heads. If you could demonstrate that the helmet was splitting and taking energy with it maybe you could make an argument for splitting, but it seems unlikely that this is a mode of operation.
- hgomersall 4y ago
  Why not? Given every helmet I've seen in a significant crash was split somehow, my assumption is that splitting is an important part of the energy absorption.
  - ndsipa_pomu 4y ago
    Polystyrene is very weak in tension (can be broken by hand) so won't be deflecting much energy. The principle is to slow the deceleration of the head by the polystyrene compressing and thus reducing the g-forces to the skull (not so much the brain which tends to slosh in the skull and cause concussion). Some motorbike helmets use materials such as polycarbonate which are intended to provide protection by breaking - quite different to bicycle helmets.
taeric 4y ago
Could be both? Usually it is the outer shell that is cracked and described as split. I could see that happening more on the road style helmets, due to their shapes.
- ndsipa_pomu 4y ago
  The pictures that I've seen of the various "saved my life" destroyed helmets have the polystyrene split apart - it's usually quite easy to split polystyrene if the force is applied the right (wrong) way.
  - taeric 4y ago
    Interesting and scary. Curious what could cause the wrong behavior.
    - ndsipa_pomu 4y ago
      I'd guess impacts at different angles. AFAIK bike helmets are tested for direct impacts via drop tests, so manufacturers may not care so much about how the helmet performs under different conditions - it may even be a good marketing gimmick to have helmets destroy themselves dramatically as people are more likely to share a picture along with a "saved my life" anecdote.
323 4y ago
It's not the polystyrene which cracks, but the plastic shell holding it in place. The separation will happen after the compression force is removed, because the compression will keep the split parts close together. Meaning that the helmet falls apart after it's done the job. Also, there are various degrees of helmets, from a simple polystyrene to a mountain bike one to a motorcycle helmet. You can always go to the next level if you want more protection. The polystyrene one is not supposed to be the end all of protection, just to be better than nothing with minimal inconvenience.
- ndsipa_pomu 4y ago
  The pictures that I've seen of the various "saved my life" destroyed helmets have the polystyrene split apart - it's usually quite easy to split polystyrene if the force is applied the right (wrong) way. I don't like the concept of requiring PPE for a relatively safe activity as cycling as it makes cycling seem like a far more risky activity than it is and there's also the problem of "helmet hair" which can dissuade commuters. It's telling that countries where bike helmets were mandated had a sharp downturn in the numbers of cyclists.
  - 323 4y ago
    Cycling is risky. I've stopped cycling on roads with cars because I realized it's just a matter of time until I get in a serious accident. https://www.washingtonpost.com/express/wp/2016/05/12/how-safe-is-bike-commuting-perhaps-less-than-you-think/ https://www.washingtonpost.com/express/wp/2016/05/12/how-saf... I'm with you on the mandatory helmet issue, I don't like it either, but mostly because I don't like needlessly restricting ones freedoms, if you want to risk your life, that's fine with me as long as you don't risk others lives too.
    - bluejekyll 4y ago
      Cycling is only risky due to the shared infrastructure with cars. The thing that raises the risk is cars. We should all be demanding that our streets are made safe for all users. We already have sidewalks in many places for this reason, we just need to extend similar safe infrastructure to bicycles and other road users too. Just throwing your hands up and not asking for change is a sure way to not improve things.
Kaibeezy 4y ago
I have worn a bike helmet without fail starting in the early 80s. I have always understood that they are disposable after any hard hit. Visible or not, polystyrene compresses, cracks, crumbles, etc. The shell or skin of the helmet never seemed to matter much, so whether it splits or shreds, doesn't matter. Maybe I'll have a look for a source on this.
- usrusr 4y ago
  Yeah, it's a crumble zone that might add a few precious millimeters to the very short deceleration path of the brain if limbs and reflexes fail to do that job completely. Disintegration means that it's doing its job. That's a completely different story from the primary task of the helmets for rock climbers, construction workers or soldiers, which is distributing a small, concentrated impact (a rock or a dropped tool or random debris) to a wider area.
  - hgomersall 4y ago
    Rock climbing helmets differ in design substantially. Loads of modern ones are more like bike helmets - recognising, I suggest, that most head injuries climbing are head hitting the crag rather than rocks falling, making it more like cycle impacts. That said, I've no real insight into what makes one design better over another.
hutzlibu 4y ago
Well, when I was hit by a car and hitting the asphalt head first as a teenager, I found it adequate, that the helmet was cracked to pieces afterward. Did it save my life? I don't know, but with the helmet I only had a light concussion (and broken leg) compared to very possible skull crack. Compressed polystyrene I know only from light accidents, but it has been a while and I suppose todays helmets are a bit more durable. (But luckily never had to find out, if they fare better nowdays. Also I learned to fall and only rarely wear a helmet nowdays)
msie 4y ago
Famous last words: "Well, that wasn't supposed to happen!"
kevin_thibedeau 4y ago
I had a helmet damaged by a roommate who knocked down a bike on a stand. The polystyrene was cracked completely through but remained bonded to the shell. They are supposed to break like that. Older styles of construction are going to be less durable but if it keeps the structure constrained to your head it will to a better job than nothing.
- Drunk_Engineer 4y ago
  LOL...they are not supposed to break like that.

altcognito 4y ago

Basically, crumple zones for heads. If you could demonstrate that the helmet was splitting and taking energy with it maybe you could make an argument for splitting, but it seems unlikely that this is a mode of operation.

hgomersall 4y ago
Why not? Given every helmet I've seen in a significant crash was split somehow, my assumption is that splitting is an important part of the energy absorption.
- ndsipa_pomu 4y ago
  Polystyrene is very weak in tension (can be broken by hand) so won't be deflecting much energy. The principle is to slow the deceleration of the head by the polystyrene compressing and thus reducing the g-forces to the skull (not so much the brain which tends to slosh in the skull and cause concussion). Some motorbike helmets use materials such as polycarbonate which are intended to provide protection by breaking - quite different to bicycle helmets.

taeric 4y ago

Could be both? Usually it is the outer shell that is cracked and described as split. I could see that happening more on the road style helmets, due to their shapes.

ndsipa_pomu 4y ago
The pictures that I've seen of the various "saved my life" destroyed helmets have the polystyrene split apart - it's usually quite easy to split polystyrene if the force is applied the right (wrong) way.
- taeric 4y ago
  Interesting and scary. Curious what could cause the wrong behavior.
  - ndsipa_pomu 4y ago
    I'd guess impacts at different angles. AFAIK bike helmets are tested for direct impacts via drop tests, so manufacturers may not care so much about how the helmet performs under different conditions - it may even be a good marketing gimmick to have helmets destroy themselves dramatically as people are more likely to share a picture along with a "saved my life" anecdote.

323 4y ago

It's not the polystyrene which cracks, but the plastic shell holding it in place. The separation will happen after the compression force is removed, because the compression will keep the split parts close together. Meaning that the helmet falls apart after it's done the job. Also, there are various degrees of helmets, from a simple polystyrene to a mountain bike one to a motorcycle helmet. You can always go to the next level if you want more protection. The polystyrene one is not supposed to be the end all of protection, just to be better than nothing with minimal inconvenience.

ndsipa_pomu 4y ago
The pictures that I've seen of the various "saved my life" destroyed helmets have the polystyrene split apart - it's usually quite easy to split polystyrene if the force is applied the right (wrong) way. I don't like the concept of requiring PPE for a relatively safe activity as cycling as it makes cycling seem like a far more risky activity than it is and there's also the problem of "helmet hair" which can dissuade commuters. It's telling that countries where bike helmets were mandated had a sharp downturn in the numbers of cyclists.
- 323 4y ago
  Cycling is risky. I've stopped cycling on roads with cars because I realized it's just a matter of time until I get in a serious accident. https://www.washingtonpost.com/express/wp/2016/05/12/how-safe-is-bike-commuting-perhaps-less-than-you-think/ https://www.washingtonpost.com/express/wp/2016/05/12/how-saf... I'm with you on the mandatory helmet issue, I don't like it either, but mostly because I don't like needlessly restricting ones freedoms, if you want to risk your life, that's fine with me as long as you don't risk others lives too.
  - bluejekyll 4y ago
    Cycling is only risky due to the shared infrastructure with cars. The thing that raises the risk is cars. We should all be demanding that our streets are made safe for all users. We already have sidewalks in many places for this reason, we just need to extend similar safe infrastructure to bicycles and other road users too. Just throwing your hands up and not asking for change is a sure way to not improve things.

Kaibeezy 4y ago

I have worn a bike helmet without fail starting in the early 80s. I have always understood that they are disposable after any hard hit. Visible or not, polystyrene compresses, cracks, crumbles, etc. The shell or skin of the helmet never seemed to matter much, so whether it splits or shreds, doesn't matter. Maybe I'll have a look for a source on this.

usrusr 4y ago
Yeah, it's a crumble zone that might add a few precious millimeters to the very short deceleration path of the brain if limbs and reflexes fail to do that job completely. Disintegration means that it's doing its job. That's a completely different story from the primary task of the helmets for rock climbers, construction workers or soldiers, which is distributing a small, concentrated impact (a rock or a dropped tool or random debris) to a wider area.
- hgomersall 4y ago
  Rock climbing helmets differ in design substantially. Loads of modern ones are more like bike helmets - recognising, I suggest, that most head injuries climbing are head hitting the crag rather than rocks falling, making it more like cycle impacts. That said, I've no real insight into what makes one design better over another.

hutzlibu 4y ago

Well, when I was hit by a car and hitting the asphalt head first as a teenager, I found it adequate, that the helmet was cracked to pieces afterward. Did it save my life? I don't know, but with the helmet I only had a light concussion (and broken leg) compared to very possible skull crack. Compressed polystyrene I know only from light accidents, but it has been a while and I suppose todays helmets are a bit more durable. (But luckily never had to find out, if they fare better nowdays. Also I learned to fall and only rarely wear a helmet nowdays)

msie 4y ago

Famous last words: "Well, that wasn't supposed to happen!"

kevin_thibedeau 4y ago

I had a helmet damaged by a roommate who knocked down a bike on a stand. The polystyrene was cracked completely through but remained bonded to the shell. They are supposed to break like that. Older styles of construction are going to be less durable but if it keeps the structure constrained to your head it will to a better job than nothing.

Drunk_Engineer 4y ago
LOL...they are not supposed to break like that.