[Lewton]

>But helmets are not supposed to shatter. When a helmet protects your head from a serious injury, the styrofoam inside will be compressed and stay that way

Is this correct? It sounds very wrong to me. I got the impression that shattering is a great way to redirect kinetic energy away from your head

[brwnll]

It doesn't appear anyone read the Author's linked article about the compression of the helmet and which forces it is designed to prevent. So here is a short snippet :

"If the styrofoam does not compress, it cannot reduce linear acceleration of the brain. The most protection that it can give to the wearer is to prevent focal damage of the skull and prevent minor wounds to the scalp. It is not likely to prevent serious brain injury.

This helmet has split along the ventilation slots, which is common. However, the thickness of the styrofoam has not been compressed. It most likely gave no more than superficial protection.

Some dissipation of impact force might occur from the action of a helmet breaking, but in most cases this is likely to be small. Helmet standards require the foam to start to compress at a level of force less than that which might be expected to lead to brain injury. While it is known that many helmets do not actually meet the standards to which they are supposed to be accredited (BHRF, 1081), it follows that if the styrofoam does not compress at all, the direct linear force on the helmet was minimal and it's quite possible that the cyclist would not have received any injury if the helmet had not been worn.

[tres]

Back when crushable foam helmets first came out, there was just an elastic cover wrapped around a foam helmet. The effect of the helmet impact was that it always broke apart; the helmet cover just kept the pieces together after impact.

Most helmets now have a plastic skin over the foam which provides some structural integrity while the foam is being crushed [1]. But you are correct, the helmet only works because the kinetic energy is being absorbed by a material whose composition (closed cells) will naturally break apart into smaller pieces when enough kinetic energy is applied.

[1] http://www.bhsi.org/general.htm

[arethuza]

About ~20 years ago I had a nasty accident that left my helmet as a cover containing small chunks of foam - given I was knocked out in the process I hate to think what would have happened if I hadn't been wearing a helmet.

[NB Accident was completely self inflicted on a forest track]

[zimpenfish]

Similar - two years ago, caught the edge of a gravel track going downhill, tried to steer out of it; woke up in the middle of the road with not-quite two piece helmet but no lasting damage (except a twingy knee).

[arethuza]

I was bombing down a track in the twilight, didn't see a gate that was basically a single post across the road until the last minute, jammed on my brakes and bike fell sideways, I skidded along on my side (still attached to bike by SPDs) and my head hit one of the gate posts.

I was out for quite bit, woke up cold and with trousers shredded on left side. Was in a lot of pain for a couple of days and ended up with a spectacular giant hematoma on my left hip.

My first reaction on coming round after being knocked out was, of course, "I hope nobody saw that" :-)

Edit: I kept the helmet (or remains of the helmet) for years to show other people.

[zimpenfish]

Hah, my first reaction was "damn, we don't have cameras!" - I wanted to see just how quick things happened and what I could/should have done differently.

Otherwise basically the same for me (I'm told) - bike slammed over onto the right, walloped me into the road, and then skidded along the ground with me still attached and out cold. Knee and right shoulder blade were skinned but that was it.

[mcv]

> I skidded along on my side (still attached to bike by SPDs)

You mean you can't use your legs to save you because they're attached to your pedals? That sounds incredibly dangerous.

[arethuza]

I didn't have time to react to get my shoes unclipped - SPD pedals are actually very easy to unclip from. I don't think it would have made any difference if I had unclipped - I was still going to be in trouble one way or another...

NB The accident was completely my own fault (tired, went cycling after a 250 mile drive, getting dark, out alone...)

[wolfgke]

As long as the styrofoam gets bent/compressed (but does not break), it takes a lot of energy from the fall. As soon as it breaks, it can't absorb much energy anymore, thus it impacts your head a lot more.

That's the reason, why after a crash the helmet should be replaced, whether there can be seen damages or not (the shell internally breaks when it absorbs energy this way, but this can hardly be seen (except when, say, you use X-ray analysis or something similar).

[maccard]

> s soon as it breaks, it can't absorb much energy anymore, thus it impacts your head a lot more.

But it has still absorbed the energy that it took to crack the helmet. If the strength of the impact is exactly the same as the strength of the helmet, then the helmet will simply crack, and leave your skull intact. However if you're not wearing the helmet, your head receives the full force of the impact.

[bweitzman]

Yeah this is mostly correct. Helmets should not shatter but they will deform during a crash. Often times you will see a flat part where the helmet hit and slid on the road.

[joshdance]

Shattering, bending, compressing, all these take energy, which reduced the energy your skull has to absorb.

[kolektiv]

Also, (can't read the article but assuming some kind of context), even if the styro compressed rather than shattering - that's still energy being used for something which isn't cracking your skull. It still seems like it would be beneficial.

If the author is claiming that once it's compressed it won't be as good next time, then that's insane - helmets aren't supposed to be re-used after accidents (whether bike, motorbike, or car racing - I've had to sadly replace my motor racing helmet after an incident, and it's a lot more than buying a new bike lid - but you still do it without flinching because it's your brain on the line here).

[omaranto]

If the author is claiming that once it's compressed it won't be as good next time, then that's insane - helmets aren't supposed to be re-used after accidents

Why is it insane to think it won't be as good next time? Isn't that exactly the reason you are not supposed to re-use them?

[Lewton]

Full paragraph:

"People will so often put up photos on social media of obliterated helmets and say, “Holy crap, look at my helmet! It saved my life!” But helmets are not supposed to shatter. When a helmet protects your head from a serious injury, the styrofoam inside will be compressed and stay that way. Most of the pictures I’ve seen are of helmets that have broken apart. It’s likely that the helmet did not protect someone from a severe injury."

[kolektiv]

In which case yes, just a very misinformed original author. Shattering, cracking, etc. are all known, planned features of a helmet impact - anything which can absorb energy in the helmet and not the head is a good thing (with some obvious caveats of course).

[moopling]

Depends, depends... Fracturing of a flaw requires much less energy than plastic deformation. So it is really a question of whether it deformed first and then cracked, or fractured early. In the latter case, possibly caused by a flaw or very sharp impact, not much energy would be absorbed at all!

[morsch]

He refers to this article in that paragraph: http://www.cyclehelmets.org/1209.html

[oldmanpants]

Yes the foam in the helmet is supposed to shatter. Like OP's other points, this one is made up.