Crash and helmet

For all discussions about this "lively" subject. All topics that are substantially about helmet usage will be moved here.
Steady rider
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Re: Crash and helmet

Post by Steady rider »

In 2015 Teschke et al reported:
Helmet legislation was not associated with reduced hospitalisation rates for brain, head, scalp, skull or face injuries, indicating that factors other than helmet laws have more influence on injury rates.
(hospital admissions data)
Teschke K, Koehoorn M, Shen H, et al. Bicycling injury hospitalisation rates in Canadian jurisdictions: analyses examining associations with helmet legislation and mode share. BMJ Open 2015;5:e008052. doi:10.1136/bmjopen-2015- 008052
https://bmjopen.bmj.com/content/5/11/e008052
Stevek76
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Re: Crash and helmet

Post by Stevek76 »

Really not sure why there's such a distraction on KE transfers when talking about concussions resulting from internal stresses on the brain.

The key thing here is how rapidly the skull decelerates. A helmet that only cracks isn't going to change this, if the thickness of the helmet has not changed then your head inside it has stopped just as quickly. That a small amount of KE transferred to the helmet to crack it doesn't change any of that.

If the helmet crushes, as it should do on faster impacts, then it has more feasibly reduced internal stress related TBI to some degree.

Also, given there's no real easy mechanism for a helmet to remove itself between the head and the ground then it strikes me that it's very likely a helmet that only cracks with no sign of crushing or similar deformation didn't take a great deal of force, else it would have crushed, even if it cracked during that process. It's more likely the helmet was either poorly manufactured, or, more probably old and not very well treated and thus had a few internal fractures already and cracked along one of those.
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dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

Stevek76 wrote: 4 Oct 2022, 6:32pm Really not sure why there's such a distraction on KE transfers when talking about concussions resulting from internal stresses on the brain.

The key thing here is how rapidly the skull decelerates. A helmet that only cracks isn't going to change this, if the thickness of the helmet has not changed then your head inside it has stopped just as quickly. That a small amount of KE transferred to the helmet to crack it doesn't change any of that.

If the helmet crushes, as it should do on faster impacts, then it has more feasibly reduced internal stress related TBI to some degree.

Also, given there's no real easy mechanism for a helmet to remove itself between the head and the ground then it strikes me that it's very likely a helmet that only cracks with no sign of crushing or similar deformation didn't take a great deal of force, else it would have crushed, even if it cracked during that process. It's more likely the helmet was either poorly manufactured, or, more probably old and not very well treated and thus had a few internal fractures already and cracked along one of those.
Most modern helmets are not solid and have gaps with a solid outer shell with polystyrene inside that has gaps. Cracks follow on from deformation. After the material has deformed past its limits it cracks and the outer shell keeps it in place. The freedom of movement from these cracks allow the head to decelerate further by moving out of the way and dispersing more energy. I want my helmet to crack. Polystyrene doesn’t cruch much, the cracking allows the whole head to move further and decelerate. As kinetic energy is related to the square of the velocity a small slowing of the head has a big difference to the reduction in KE.
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Chris Jeggo
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Re: Crash and helmet

Post by Chris Jeggo »

A note on momentum:
There have been some incorrect statements up thread.
In a collision between two bodies total momentum is conserved even though kinetic energy may not be. This follows directly from Newton's laws of motion. A fundamental difference is that momentum is a vector quantity whereas kinetic energy is a scalar. If two identical cars travelling at the same speed V collide head-on, before the collision one car has momentum mV whereas the other has momentum -mV, so the total momentum is zero, and it is still zero after the collision, whether they bounce or coalesce.
The same reasoning applies to the momentum of atoms in thermal vibration. The total momentum of the atoms within a rigid body is equal to the momentum of the body as a whole, because the summation is vectorial. If no external force acts on the body as it heats or cools, its total momentum does not change.
If a cyclist collides with Planet Earth, his momentum appears to have been entirely lost, but in fact it has been transferred, imperceptibly, to the much more massive Planet Earth.
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pjclinch
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Re: Crash and helmet

Post by pjclinch »

dmrcycle wrote: 4 Oct 2022, 10:11pm <snip>
Polystyrene doesn’t cruch much
Assuming you mean "crush", and the expanded polystyrene used in helmets, yes it does crush much. That's why it's so light (because most of the material is air, also why it's a good insulator), and more to the point is why it's used in cycle helmets.

I just dug out a polystyrene shock case for an ultrasound probe, it's about 3cm thick polystyrene (not exactly the same as used in a helmet, but the same basic stuff). I could easily compress it in my fingers to less than 1cm thick, and then it returned to pretty much its original shape. Had I really tired (e.g. by standing on a bit) I'm quite sure it would have compressed/crushed more. Why faff about with theory when you can do a simple experiment?

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dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

Chris Jeggo wrote: 5 Oct 2022, 1:36am
If a cyclist collides with Planet Earth, his momentum appears to have been entirely lost, but in fact it has been transferred, imperceptibly, to the much more massive Planet Earth.
Agreed. But in reality there can be a substantial amount of momentum transferred to vibration of molecules as heat from friction. That’s why you can drop two things of the same mass say one tennis ball and one ball of plasticine and they will bounce different heights. The masses at the start are the same and their velocities are the same (assuming similar air resistance) but after the collision the masss are the same but their velocities are different.
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 5 Oct 2022, 8:57am
Chris Jeggo wrote: 5 Oct 2022, 1:36am
If a cyclist collides with Planet Earth, his momentum appears to have been entirely lost, but in fact it has been transferred, imperceptibly, to the much more massive Planet Earth.
Agreed. But in reality there can be a substantial amount of momentum transferred to vibration of molecules as heat from friction*. That’s why you can drop two things of the same mass say one tennis ball and one ball of plasticine and they will bounce different heights. The masses at the start are the same and their velocities are the same (assuming similar air resistance) but after the collision the masss are the same but their velocities are different.
... but the total momentum** of the ball+planet system will be the same after both collisions. (The vibration of molecules doesn't affect that - they all cancel each-other out** ).

**momentum being a vector as explained by Chris, unlike energy.

*Edited by Mattheus
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

pjclinch wrote: 5 Oct 2022, 8:12am
dmrcycle wrote: 4 Oct 2022, 10:11pm <snip>
Polystyrene doesn’t cruch much
Assuming you mean "crush", and the expanded polystyrene used in helmets,

Pete.
Sorry. Typo. Yes it does crush it’s all relative. Helmets do however seem to be slightly denser than the packing type. What I was trying to say is that the polystyrene will compress say 5mm but then there will be a fracture. The fracture is not like a pane of glass and is a slow propagation and non linear and slows down the head further (more like a tear than a shatter so to speak. The resultant distance they head can move is say 10’s of mm slowed down by the slow fracture. My point is a cracked helmet is part of the design and a good thing not a failure of poor design or an old helmet.
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

mattheus wrote: 5 Oct 2022, 9:02am
dmrcycle wrote: 5 Oct 2022, 8:57am
Chris Jeggo wrote: 5 Oct 2022, 1:36am
If a cyclist collides with Planet Earth, his momentum appears to have been entirely lost, but in fact it has been transferred, imperceptibly, to the much more massive Planet Earth.
Agreed. But in reality there can be a substantial amount of momentum transferred to vibration of molecules as heat from friction*. That’s why you can drop two things of the same mass say one tennis ball and one ball of plasticine and they will bounce different heights. The masses at the start are the same and their velocities are the same (assuming similar air resistance) but after the collision the masss are the same but their velocities are different.
... but the total momentum** of the ball+planet system will be the same after both collisions. (The vibration of molecules doesn't affect that - they all cancel each-other out** ).

**momentum being a vector as explained by Chris, unlike energy.

*Edited by Mattheus
Agreed. Some are trying to argue that momentum conservation explains why helmets do not slow collisions down and the head and car conserve momentum. Yes agreed the total is zero. But that the system is more than the head and the car. It’s actually better to explain it as energy transfer.
mattheus
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Re: Crash and helmet

Post by mattheus »

Steady rider wrote: 4 Oct 2022, 4:46pm In 2015 Teschke et al reported:
Helmet legislation was not associated with reduced hospitalisation rates for brain, head, scalp, skull or face injuries, indicating that factors other than helmet laws have more influence on injury rates.
(hospital admissions data)
Teschke K, Koehoorn M, Shen H, et al. Bicycling injury hospitalisation rates in Canadian jurisdictions: analyses examining associations with helmet legislation and mode share. BMJ Open 2015;5:e008052. doi:10.1136/bmjopen-2015- 008052
https://bmjopen.bmj.com/content/5/11/e008052
Thank you 👍
Psamathe
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Re: Crash and helmet

Post by Psamathe »

Tiggertoo wrote: 12 Aug 2022, 9:56pm I crashed last Friday - wheels went out from under me on a tight/wet curve - and banged up my knee, arm and thigh (all mendable), but for the first time crashing - I have done it a few times (mostly by dogs) my head hit the pavement hard and set my head pounding.

The reason I am posting this thread is that my helmet broke along the side receiving the blow. If I had not been wearing the helmet, I would not be here today to write this. I am not going to preach, but I cannot imagine for one second riding without a helmet and I wouldn't allow anyone to ride with me who did not wear one.
Several years back I was turning at a junction and rear wheel slid out from under me and I went over. I was surprised at how fast I went from upright to horizontal. I did happen to be wearing a helmet and loud "crack" noise as it (with head inside) hit the road.

A few mins later after bending handlebars back straight my feeling was that without the helmet my head may easily not have even hit the road - helmet making my head heavier and larger.

After the incident I think I became "variable" on helmet wearing - some ride I would, others I wouldn't. Finally switched to no helmet at start of 2 months tour in EU - where I refused to wear a helmet cycling off the ferry because Brittany Ferries attitude/rules annoyed me and the helmet stayed bungee-corded to the rear rack for the entire tour and stayed on the shelf once home.

Maybe the daftest thing is these days when I (now rarely) ride my DF upright and I often do wear a helmet but leave the strap undone because if I have an accident I want it off my head without the additional weight and twisting torques. I would be far more sensible not to wear it atall.

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pjclinch
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Re: Crash and helmet

Post by pjclinch »

dmrcycle wrote: 5 Oct 2022, 9:04am My point is a cracked helmet is part of the design and a good thing not a failure of poor design or an old helmet.
As we see in Brian Walker's "Heads Up" from a few years ago (reproduced at https://ecf.com/sites/ecf.com/files/Sta ... ts-etc.pdf), shell breakage is indeed part of the total end-game in absorbing energy, but that's not necessarily the same thing as "a cracked helmet is part of the design and a good thing not a failure"

If I tell you my helmet cracked then you can't tell whether that's failed pathetically to do its job properly or whether it's absorbed every last joule it possibly could have, only fracturing when it could compact no more. The latter is what is meant to happen (assuming I'm at or above spec limit) and is a Good Thing, the former very much isn't. They can both result in a cracked helmet.

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dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

I guess that’s true that a crack could be a failure that occurs because of bad design or manufacturing or material faults. However a cracked helmet does not mean it didn’t do it’s job as a crack is part of the impact mitigation strategy of the designs. Bike helmets are not designed to take multiple impacts and are designed to be replaced after an impact. Some manufacturers even replace free if it’s had an impact. The one shot design is a reason why comparisons with American football helmets not helping concussion incidents is not comparable to sacrificial bike helmets.
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 5 Oct 2022, 4:02pm I guess that’s true that a crack could be a failure that occurs because of bad design or manufacturing or material faults. However a cracked helmet does not mean it didn’t do it’s job as a crack is part of the impact mitigation strategy of the designs.
Agreed. But if there is no sign of deformation, my money is that it's a failure and negligible energy has been absorbed.

Plus there's the statistical view:
the number of "Cracked helmet saved my life!" stories VASTLY outweighs the number of serious head injuries in the days before helmets.
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Chris Jeggo
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Re: Crash and helmet

Post by Chris Jeggo »

mattheus wrote: 5 Oct 2022, 4:26pm <snip>

Plus there's the statistical view:
the number of "Cracked helmet saved my life!" stories VASTLY outweighs the number of serious head injuries in the days before helmets.
Do you happen to have a reference for that statistic, please?
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