"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.

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

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

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.

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.

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

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

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).

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.

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).

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?

"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."