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Helmet - Impact

Old 11-21-19, 10:52 AM
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wphamilton
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Helmet - Impact

Understanding one basic, core concept of how bike helmets protect us bypasses several mistakes that we hear repeated often. Fortunately it's very simple to grasp and I'll only address this one concept.


The helmet reduces the forces felt by our skull by stretching out the time of the impact. It takes a certain amount of time to crush the foam, depending on how fast the helmet is going, and how massive your head is of course. Simple as that, and that's all of it.


It does not "spread out the force" very much. It does not "dissipate energy". Energy is absorbed by crushing. And it's basically linear in the crush range - ie, twice as thick gives you twice as much protection. No amount of shaping and mysterious designs change that.


Check it yourself: hit a styrofoam sheet with a hammer and look at the damage. There will be a dent where the hammer hit, and maybe just a little slope in the surface closely surrounding the round hole. You can see that there is little if any "spreading out" of the force, because there would be visible damage if there were. Now crack it in half at an undamaged section, push the edges together and hit it with a hammer again right on the crack. You'll see the same damage as before, some on each half. As long as the crack is held together, it does not harm the impact protection. Surface (UV) damage of a mm thick or less is insignificant, because the only affect is reducing the crush by a mm.


That's all, valid corrections are welcome.
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Old 11-21-19, 11:16 AM
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You are right the basic design of the helmets is to reduce acceleration forces on the head and BRAIN by increasing the time of impact.

There are hardshell and softshell helmets. Generally the harder helmets are designed for kids, or perhaps some sports like BMX. In that case, the thick shell may spread the force further around the skull. I presume the foam is also different, so from your example, foam designed for a 1" impact radius from the hammer may be different from foam that spreads the impact out to 6".

The helmet industry has been studying different types of impacts from a direct impact flat surface to an impact on a wedge shaped surface. And, the need for one helmet for all potential impacts.

The net over foam helmets from a few years ago apparently seemed like a good idea, but had a side-effect of being more sticky on some surfaces causing torsion stress. Fixed by both a slick surface over the helmet, as well as the MIPS and WaveCell helmets.
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Old 11-21-19, 11:28 AM
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A helmet can make a blow to your head that would otherwise be harmless MUCH more comfortable to endure. If only because it will probably prevent scalp laceration and/or bruising. If the blow is serious enough to kill or seriously damage your brain then a helmet cannot save you. Do not spend more than $80 for a helmet, ever. There is nothing that a $300 helmet can do that the $80 (or $30) can't. Styrofoam is the active ingredient in all helmets and the ONLY way (as outlined above) to make a 1.5" thick liner twice as effective is to make the liner 3" thick! Would that be practical? Helmets keep us safe because most of us don't ride enough to run up against the law of averages of serious helmet failure. With all due respect to the o.p. I don't think I have ever read a more pointless exercise in mansplaining. WTF? I have NEVER seen anyone supply an explanation about how helmets work that was incorrect or correct for that matter. The intuitive deduction is that the material of the helmet liner absorbs the blow. Somehow. No one cares how. This is America. How many people could correctly explain how a lightbulb works. The old kind before LED's. Again, WTF? My admittedly harsh reaction comes mainly from the unsolicited nature of the o.p. I'm not understanding why we needed this.
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Old 11-21-19, 12:57 PM
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Originally Posted by Leisesturm View Post
..With all due respect to the o.p. I don't think I have ever read a more pointless exercise in mansplaining. WTF? I have NEVER seen anyone supply an explanation about how helmets work that was incorrect or correct for that matter.
Geeksplaining not mansplaining. It's just an FYI because I'm constantly seeing among other things:

1. Replace your helmet if it's dropped, it might be cracked!
2. Sunlight degraded your helmet after x years.
3. This new design improves your impact protection

... followed by incorrect "explanations" about why. And how many people know that the crush is linear with respect to thickness (within parameters I'm not going into now), when you'd intuitively expect it to get harder to crush the deeper you went? That's a uncommon material property after all.

If my explanation seems obvious, then it means I did a good job with it so I'm happy on that score.
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Old 11-21-19, 01:00 PM
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Originally Posted by wphamilton View Post
Understanding one basic, core concept of how bike helmets protect us bypasses several mistakes that we hear repeated often. Fortunately it's very simple to grasp and I'll only address this one concept.


The helmet reduces the forces felt by our skull by stretching out the time of the impact. It takes a certain amount of time to crush the foam, depending on how fast the helmet is going, and how massive your head is of course. Simple as that, and that's all of it.


It does not "spread out the force" very much. It does not "dissipate energy". Energy is absorbed by crushing. And it's basically linear in the crush range - ie, twice as thick gives you twice as much protection. No amount of shaping and mysterious designs change that.


Check it yourself: hit a styrofoam sheet with a hammer and look at the damage. There will be a dent where the hammer hit, and maybe just a little slope in the surface closely surrounding the round hole. You can see that there is little if any "spreading out" of the force, because there would be visible damage if there were. Now crack it in half at an undamaged section, push the edges together and hit it with a hammer again right on the crack. You'll see the same damage as before, some on each half. As long as the crack is held together, it does not harm the impact protection. Surface (UV) damage of a mm thick or less is insignificant, because the only affect is reducing the crush by a mm.


That's all, valid corrections are welcome.
Is the mechanism the same for gel helmets? Having absolutely no technical knowledge, I would have assumed that a gel would act more as a fluid, with the shockwave of the impact spreading out like a ripple around the helmet instead of inward towards the brain.

You also don't mention rotational forces, which are explicitly redirected per the claims of the manufacturers. MIPS actually allows the helmet to rotate around the head, correct? That's redirecting the rotational forces that otherwise would be transmitted to your head. I know that WaveCel is also supposed to redirect those rotational forces, but I'm unclear on the supposed mechanism. I'm guessing it's the helmet riding on the ripples, but that's an uneducated guess at best. I have a MIPS helmet, the mechanism for rotational redirection is obvious when you see it.
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Old 11-21-19, 01:23 PM
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Fluids work entirely different from foam structures. Rotational forces may be an even more important causal factor in TBI than are linear impacts, and the standard plastic shell makes a slicker surface to mitigate that (but doesn't help much for impacts). Rotational forces arise from the initial and subsequent impacts and from friction forces.

These are interesting topics but I'm personally not going off on tangents on this thread. Anyone else feel free.
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Old 11-21-19, 01:40 PM
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There is no such thing as a "gel helmet". All helmets are (still) styrofoam helmets. Some styrofoam helmets have gel pads inside that are primarily to provide a low friction slip between the skull and the helmet liner and secondarily to increase comfort. It is NOT to provide any additional shock reduction. Fluids, being incompressible are very poor 'shock absorbers'. That becomes even worse when constrained by 'capsules' or other containment as must be the case in helmet applications.
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Old 11-21-19, 02:05 PM
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Originally Posted by Leisesturm View Post
There is no such thing as a "gel helmet". All helmets are (still) styrofoam helmets. Some styrofoam helmets have gel pads inside that are primarily to provide a low friction slip between the skull and the helmet liner and secondarily to increase comfort. It is NOT to provide any additional shock reduction. Fluids, being incompressible are very poor 'shock absorbers'. That becomes even worse when constrained by 'capsules' or other containment as must be the case in helmet applications.
Pretty much explains the concept and function of hydraulics.
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Old 11-21-19, 02:22 PM
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Originally Posted by Leisesturm View Post
There is no such thing as a "gel helmet". All helmets are (still) styrofoam helmets. Some styrofoam helmets have gel pads inside that are primarily to provide a low friction slip between the skull and the helmet liner and secondarily to increase comfort. It is NOT to provide any additional shock reduction. Fluids, being incompressible are very poor 'shock absorbers'. That becomes even worse when constrained by 'capsules' or other containment as must be the case in helmet applications.
I see what you're talking about.

So is the Bontrager claim that the CELL technology produces more protection from both "linear and rotational forces" nonsense? Honest question.

Looking at it a little closer, the liner in a WaveCel is basically a different kind of foam--the gel material is arranged in a lattice with lots of air spaces, and I think the air spaces collapse on impact both linearly and rotationally then bounce back. Definitely contained in an EPS shell, so there is a layer of the conventional.
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Old 11-21-19, 02:47 PM
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Originally Posted by wphamilton View Post
Geeksplaining not mansplaining. It's just an FYI because I'm constantly seeing among other things:

1. Replace your helmet if it's dropped, it might be cracked!
2. Sunlight degraded your helmet after x years.
3. This new design improves your impact protection

... followed by incorrect "explanations" about why. And how many people know that the crush is linear with respect to thickness (within parameters I'm not going into now), when you'd intuitively expect it to get harder to crush the deeper you went? That's a uncommon material property after all.

If my explanation seems obvious, then it means I did a good job with it so I'm happy on that score.
I had not heard about UV light degrading helmets. How does that work?
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Old 11-21-19, 03:09 PM
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Originally Posted by livedarklions View Post
So is the Bontrager claim that the CELL technology produces more protection from both "linear and rotational forces" nonsense? Honest question.
I haven't studied their helmets enough but the physics and properties of liquids and gels is enough to inform an opinion that the 'gel' technology helmets are using the gel mainly as friction reduction to isolate rotational forces from the head and neck. That could be done entirely with a lower density foam than the main liner but gels are hot right now. Gels are in your saddle, your insole, your shorts, your snacks ... why not your helmet? If I was late to the Patent Office for a filing on a pure hybrid gel/EPS shell liner I may well throw in some lower density foam with the gel pads in some kind of proprietary matrix, give it a catchy name, trademark it, and go catch customers with it.
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Old 11-21-19, 03:36 PM
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Friends, I don't care how good helmets get. I don't want or need a better one. What I want is never to have an unplanned dismount off my bike for any reason! The best helmet in the world can't save you from a broken neck! You must not come off that bike! Tell yourself that at the start of every ride. Believe it. Do all in your power NOT to need that helmet to keep your brains from being scrambled. Your brains are no good to you if you are in a power-chair. Well, some people make VERY good use of their brains despite some pretty horrific injuries or congenital disablements to their bodies but, notwithstanding that, I'd rather spend the money saved not buying the best helmet sold on reliable tires, top of the line brake pads, things that can make a difference to how a bike handles and operates.

I smile when people say "ride like they (the cagers) can't see you" and then they add 10lbs. of lights and passive signaling to their bike. I'd like to suggest that people ride like they have NO helmet because when all is really said and done you don't really have much going on up there. The riders who split their helmets in half and credit it with saving their lives don't really know that it in fact did! What it probably did was make what might have been a mighty painful injury, maybe even a serious concussion, the helmet made it much less serious. Any decent helmet can do that. Anything given a CSPC rating and legally allowed to be sold in the U.S. can do that job.

Do you know why air travel is so safe? Because airplanes simply must not be allowed to crash with hundreds of people on board. That's why the NTSB is going to be up Boeing's hoo hoo for the rest of their existence. They allowed the unthinkable to happen. If the kind of international rigor towards airline safety was extended to road vehicle operations the reduction in car, motorcycle, bicycle and anything else that moves on roads would become as safe as or safer than for airlines. It starts with attitude. If you think that you simply cannot ever allow yourself to crash then you might crash once in a lifetime. Once. One too many but ... there you are. Is that what we have? Nope. Lots of you have crashed several times. Real crashes with broken bones and lasting injuries. Some of you have crashed so many times your spouses no longer allow you to ride. Don't be that guy. Take every ride seriously. Don't worry about what device or piece of clothing or brand of helmet can keep you safe. Worry about how YOU can keep YOURSELF safe. You have a lot more influence over the outcome than you think.
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Old 11-21-19, 04:37 PM
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Well, I agree with a lot of that. The problem is, some things are either out of the cyclist's control, or, they are just infinitesimally, barely, not out of his or her control. Example. Coming home this morning in the dark from my early ride. My apartment building is at the end of the street I am on, so all I have to do is just ride straight on to its driveway and dismount, then, I am home. Apartment buildings on either side of me. Half a block to go. I'm the only person on the street. Quarter of a block to go. Almost there. Nice ride. Great start to the day.

Then, suddenly, out of the ether, a silver midsize sedan pulls out onto the street from a driveway on my right. Not only does the driver not see me, but he is actually looking the complete other way. While not looking, he continues to pull out fully into the street, turning my direction. I am doing maybe 15-16 mph. As he appears, he is maybe eighteen feet ahead of me. Something like that. No time to think. Just a survival reaction. I immediately jerk the bars to the left. It's way too sharp and too quick and I immediately go to the pavement. The driver finally turns and sees what has happened and stops his car. I did not make contact with his vehicle. I have some scrapes and no doubt bruises. Did not lose consciousness, though I most definitely did strike the pavement with my right parietal scalp. Pretty hard. My helmet saved me from a trip to the Neuro ICU, I have no doubt.

The point of telling this is that we as cyclists are actually not in control of everything that may happen to us on the road. This guys just wasn't thinking and pulled right out into me. Had I been going any faster, I would not have been able to react at all. Totally his fault. His decision. I have great gear and lights and a terrific bike that is well-maintained and I am extremely experienced. And I did a good job of trying to save myself as best I could in the 2 seconds I had to act. No matter how well prepared we are, sometimes stuff just happens. It's life. We can try to stack the odds in our favor, but at the end of the day, we aren't responsible for other people's stupidity. And no amount of science is ever going to change that.

And since the Boeing situation was brought up, I would recommend anyone interested to find the story in the NYTimes Sunday edition from the weekend of Sept. 20ish. There is in the magazine section a superb analysis of those crashes by a very experienced 737 Max pilot. By far, the best reporting I have seen on this story. Anyhoo, on the cycling topic of being prepared for anything at any time, this pilot's argument is that the Lion and Ethiopian crashes would never happen in the U.S. because the quality of the preparation and training our pilots receive prepares them for essentially anything. They would have handled it with the same professionalism they manage everything else. Including an MCAS system that is causing headaches and acting weird. There was a simple, basic solution to that MCAS problem for the Lion and Ethiopian crews. Any competent 737 Max pilot knows this. But they didn't use it. They completely screwed it up, God rest their souls. They had no clue what they were doing. Those crashes were pilot error. Yes, Boeing screwed up. But those crashes never, ever should have happened, even with the Boeing mess-up.

Life is Brownian motion and crazy things just happen all the time, sometimes, to cyclists. Prepare, practice, be paranoid, but ultimately, when that inattentive guy just pulls right out into you, it's just you and that car and a fraction of a second. And you, unfortunately, can't always control how that interaction is going to turn out.
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Old 11-21-19, 05:05 PM
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Originally Posted by bpcyclist View Post
I had not heard about UV light degrading helmets. How does that work?
If you leave styrofoam out in the sun, eventually the exposed parts will get brown/yellow and dusty. The dust is surface damage - however much turned to dust is how much the helmet is degraded. One millimeter is an exaggeration.
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Old 11-22-19, 01:42 AM
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Originally Posted by Leisesturm View Post
Friends, I don't care how good helmets get. I don't want or need a better one. What I want is never to have an unplanned dismount off my bike for any reason! The best helmet in the world can't save you from a broken neck! You must not come off that bike! Tell yourself that at the start of every ride. Believe it. Do all in your power NOT to need that helmet to keep your brains from being scrambled. Your brains are no good to you if you are in a power-chair. Well, some people make VERY good use of their brains despite some pretty horrific injuries or congenital disablements to their bodies but, notwithstanding that, I'd rather spend the money saved not buying the best helmet sold on reliable tires, top of the line brake pads, things that can make a difference to how a bike handles and operates.

I smile when people say "ride like they (the cagers) can't see you" and then they add 10lbs. of lights and passive signaling to their bike. I'd like to suggest that people ride like they have NO helmet because when all is really said and done you don't really have much going on up there. The riders who split their helmets in half and credit it with saving their lives don't really know that it in fact did! What it probably did was make what might have been a mighty painful injury, maybe even a serious concussion, the helmet made it much less serious. Any decent helmet can do that. Anything given a CSPC rating and legally allowed to be sold in the U.S. can do that job.

Do you know why air travel is so safe? Because airplanes simply must not be allowed to crash with hundreds of people on board. That's why the NTSB is going to be up Boeing's hoo hoo for the rest of their existence. They allowed the unthinkable to happen. If the kind of international rigor towards airline safety was extended to road vehicle operations the reduction in car, motorcycle, bicycle and anything else that moves on roads would become as safe as or safer than for airlines. It starts with attitude. If you think that you simply cannot ever allow yourself to crash then you might crash once in a lifetime. Once. One too many but ... there you are. Is that what we have? Nope. Lots of you have crashed several times. Real crashes with broken bones and lasting injuries. Some of you have crashed so many times your spouses no longer allow you to ride. Don't be that guy. Take every ride seriously. Don't worry about what device or piece of clothing or brand of helmet can keep you safe. Worry about how YOU can keep YOURSELF safe. You have a lot more influence over the outcome than you think.
Now that's some first-class mansplaining.

How arrogant does someone have to be before they think it's not obvious to people that safety devices designed to reduce brain injuries and lights designed to increase visibility aren't some sort of total protection systems?

Good luck instituting air traffic control on roads, btw.
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Old 11-22-19, 07:42 AM
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Originally Posted by wphamilton View Post
It does not "dissipate energy". Energy is absorbed by crushing.
Expanded Polystyrene (EPS) helmets act in three ways during a collision.

Primarily, in bicycle helmets EPS is an isotropic cellular hard foam structure, composed of air filled "bubbles" of styrene.

Mechanism one - as pneumatic springs. Here, you are correct, if we ignore very small losses during deformation of the styrene it does not "dissipate energy" it stores it. The air inside the EPS cellular structure is compressed, when the force is removed, it decompresses. Again, if we ignore the very small losses during air compression/decompression, it does not "dissipate energy" it simply stores it. This is a minor mechanism in bicycle collisions.

Mechanism two - as failed pneumatic springs. Continue compressing the air, and some of the polystyrene bubbles will exceed yield strain. This does "dissipate energy." This is a less minor mechanism in bicycle collisions. (This mechanism is a common way to turn an isotropic EPS structure into an anisotropic EPS structure, under controlled conditions compress an EPS sheet to half it's thickness, remove compression, and it will slowly "spring back" to only 80% of it's original thickness.)

Mechanism three - as compressors. In a collision the air in the polystyrene bubbles are rapidly compressed to a relatively high pressure such that some of the polystyrene bubbles will burst. This releases the compressed air. (Think of it as a "jack brake".) This *DOES* dissipate energy - ONCE per burst polystyrene bubble. This is the dominant mechanism in many bicycle collisions. It is why bicycle helmets are one collison only personal safety devices. It does dissipate energy.

These mechanisms also have the effect of REDUCING the peak decelerations both by dissipating the energy in two cases AND increasing the time of the deceleration.


Amateur physicists can quickly learn about this in your home kitchen. (Maybe even some professional physicists.)


Make yourself the equivalent of an EPS foam - Italian Meringue. (It is air inside denatured protein bubbles.)

After making the meringue let it cool in the bowl. Now drop a whole raw uncracked egg into the center of the bowl. The air in the meringue will compress, and many of the denatured protein bubbles will burst. Take the egg out. Note the "divot" in the center of the the bowl of meringue. Drop the egg again in the same spot. This is why bicycle helments are one collision only devices.

An alternative more advanced study, preheat two ovens just over 100C/212F. Place the meringue in three oven safe containers which are large enough to easily drop one whole uncracked egg into the center.

Keep one on the counter. Place the other two in the ovens. After 30 minutes, turn off one oven. After 60 minutes, turn off the other oven. After another 60 minutes, remove both from the oven.

You now have three foam structures with different properties to experiment with. Time to break some eggs.


-mr. bill

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Old 11-22-19, 07:44 AM
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Originally Posted by livedarklions View Post
I see what you're talking about.

So is the Bontrager claim that the CELL technology produces more protection from both "linear and rotational forces" nonsense? Honest question.

Looking at it a little closer, the liner in a WaveCel is basically a different kind of foam--the gel material is arranged in a lattice with lots of air spaces, and I think the air spaces collapse on impact both linearly and rotationally then bounce back. Definitely contained in an EPS shell, so there is a layer of the conventional.
The claim isn't nonsense, but it's theoretical and based on bench testing. I can say pretty confidently that there will never be prospective clinical or animal data on the subject. I think my next helmet will be something of the kind, but, having some knowledge and experience in this area, I have little faith in the ability of bicycle helmets to mitigate anything but skull fractures and brain contusions, both of which are related to direct, linear, forces. They are both potentially catastrophic and completely worthy mitigating, but not nearly as common as concussion.

On the gel issue, fluids are incompressible, but fluids in stretchy spaces, constrained to flow through little holes, or with lots of intrinsic viscosity, absorb energy by their resistance to flowing away from the area of impact.
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Old 11-22-19, 08:12 AM
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Originally Posted by livedarklions View Post
Looking at it a little closer, the liner in a WaveCel is basically a different kind of foam--the gel material is arranged in a lattice with lots of air spaces, and I think the air spaces collapse on impact both linearly and rotationally then bounce back. Definitely contained in an EPS shell, so there is a layer of the conventional.
Now, it's not a foam. It's a collapsible deformable plastic structure, and while they call it cellular, it is cellular as in honeycomb structure rather than a foam structure. Yes, it has an EPS shell that is thinner than a conventional bicycle helmets EPS shell.

-mr. bill
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Old 11-22-19, 08:23 AM
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all good info, but what puzzles me, is why are we still using Styrofoam? (maybe this was addressed)

look at modern football helmets, which are designed for collisions and impacts. I get that bike helmets need to be lighter, but surely they can be more advanced than foam and a shell
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Old 11-22-19, 08:25 AM
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Riddell SpeedFlex Helmet - Helmets - On-Field Equipment - Shop

here's the specs for the Riddell speed flex which you see in every football game you watch
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Old 11-22-19, 08:45 AM
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Originally Posted by Flip Flop Rider View Post
all good info, but what puzzles me, is why are we still using Styrofoam? (maybe this was addressed)

look at modern football helmets, which are designed for collisions and impacts. I get that bike helmets need to be lighter, but surely they can be more advanced than foam and a shell
Football helmets are wildly overbuilt for bicycle use. They're designed to survive literally dozens (hundreds?) of impacts per season, while bike helmets hopefully won't see any impact during their life-span. They also cost several times the price of your average bike helmet. Foam is not just weight effective, it's extremely cost-effective. Other than the fact that people associate foam with cheap and disposable items, why would you want to make bike helmets hotter, heavier and more expensive?

Virginia Tech first got into the helmet testing business on football, nowhere have they suggested that redesigning bike helmets to be built more like football helmets is ideal.
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Old 11-22-19, 08:48 AM
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Originally Posted by mr_bill View Post
Now, it's not a foam. It's a collapsible deformable plastic structure, and while they call it cellular, it is cellular as in honeycomb structure rather than a foam structure. Yes, it has an EPS shell that is thinner than a conventional bicycle helmets EPS shell.

-mr. bill
Yeah, I should have been clearer that I meant foam in a figurative sense, not literal or technical--all I meant was that the protective mechanism is still at least in part an air space collapsing.
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Old 11-22-19, 08:57 AM
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Originally Posted by livedarklions View Post
Yeah, I should have been clearer that I meant foam in a figurative sense, not literal or technical--all I meant was that the protective mechanism is still at least in part an air space collapsing.
In the case of the WaveCell, you should think of that space just as clearance space between plastic members. It also happens to provide ventilation space. (There is no air compression going on in the WaveCell portion of the helmet.)

-mr. bill
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Old 11-22-19, 09:00 AM
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It's true that there is a contribution from air compression in constrained foam structure, in addition to elastic deflection and buckling of cell walls. You can go as deep as you wish into this Deformation mechanisms and energy absorption of polystyrene foams for protective helmets for example.

Any physical process involves exchanging energy - don't anyone get too worked up by "absorbs" vs "dissipate". Generally speaking with classical mechanics, you usually have a choice of approaches based on conservation of momentum or conservation of energy, without mixing them. It's not invalid to use both, but generally unnecessary. I focused on a momentum approach and didn't mean to imply any particular energy equations.

The intended takeaway there is that the protective mechanism is reducing peak impact by extending the time.
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Old 11-22-19, 09:17 AM
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Originally Posted by mr_bill View Post
In the case of the WaveCell, you should think of that space just as clearance space between plastic members. It also happens to provide ventilation space. (There is no air compression going on in the WaveCell portion of the helmet.)

-mr. bill
I defer--I know I'm talking through my helmet.

So the real action is the flexing of the plastic members? I've been to gyms like that.
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