What helmet foam gives the best protection?
September 15, 2010 1:15 PM   Subscribe

I want to understand the science of different kinds of helmet foam and how it protects brains. Specifically: is a helmet with hard (EPS) foam always preferable, or is there scientific evidence that in some situations soft foam may provide better protection?

There is some discussion on skateboarding forums about the best kind of helmet (for street skateboarding where falls are relatively rare - not skatepark skateboarding where you are expected to fall often). There are basically 2 kinds of helmets used for skateboarding: one with hard foam (EPS) that is also used in bicycle helmets. This is a one-use-only helmet, if it is in a crash it has to be replaced. It is, however, believed to protect better against serious crashes than a helmet with soft foam (EVA, Brock Foam). However, some people argue that if you engage in sports where the helmet is not likely to endure that much force (skating 15mph and falling on the ground is totally different from snowboarding into a tree at 60mph), a soft foam helmet is actually better. A (sic) quote from a forum about this:

The test results on the brock foam actually prove that in a low to mid speed crashes on a concrete surface the spongy construction of the brock foam is better for your head and neck protection than the industry standard EPS liner!! Mad hey!!! The force is absorbed by the foam and not by the bones in your neck!!

That sounds great, but surely there must be some real, independent research about this? Is this something that you can calculate? Like: a person with weight x, length y travels with speed z and falls on the ground, foam a protects against b force, so you need c?

I am looking for independent science, not info from helmet manufacturers but also not just opinions of pro or anti helmet groups.
posted by davar to Science & Nature (6 answers total) 3 users marked this as a favorite
This (slightly dated) article may provide useful information for you.
posted by spacewrench at 1:30 PM on September 15, 2010

previously, which links to this story about motorcycle helmets which is a bit long. tl;dr = EPS w/ softer shell is better than hard shell with soft foam.
posted by ecco at 1:32 PM on September 15, 2010

Doh! beaten by spacewrench (same article).
posted by ecco at 1:33 PM on September 15, 2010

In an accident, roughly, the speed of the impact and the distance travelled on impact determines the deceleration of your brain in your skull:
a = --------
The thicker the helmet foam (d) the lower the deceleration. If your head is your helmet, the d is much smaller, while the speed v is still the same. Thicker foam is better, all else the same.

There are other innovations that help quickly redistribute energy through the helmet foam matrix and away from your skull. People test different designs and measure force that manages to get through to a fake head. Google Scholar could help you find specific research papers and patent applications.
posted by Blazecock Pileon at 1:50 PM on September 15, 2010

There's a lot of physics hiding in this and a number of tradeoffs between each of these factors.

You have something at some velocity v0 which will end up at a velocity 0. The longer the distance this occurs over, the better, as this means a lower acceleration. That's why pads for stuntfolk are big and fluffy — the falling stuntperson accelerates against the direction of travel (deceleration, colloquially) over a great distance and so it isn't like slamming into the nice hard, mostly inflexible earth. Of course, we can't wander about with helmets that are a meter thick so ...

Kinetic energy — you want this to dissipate. One of the handy ways to do this is deformation of some sacrificial element (not your head). Consider cars, your bumper takes the hit rather than the frame, and then finally you want the frame to crumple before the passengers and driver do. Here, a soft shell is better than a completely rigid shell because some of the kinetic energy goes into deforming the shell and you have less kinetic energy to dissipate in your noggin. The natural limit of this is something where some impact edge tears through the shell and hits your skull: you don't want that but short of that is fine.

In a complex system like this, you have a series of impacts. Consider your helmeted head against a rail.

1) The helmet shell hits the rail.
2) The helmet shell begins to deform. Simultaneously with this, your head is still moving. But your head isn't a unit in and of itself. Model it as skull, fluid, and brain. It's more complicated than this, but let's subsume hair, scalp, and on through to the bone as "skull."
3) Your skull thocks against the foam and begins to deform the foam.
4) Your brain has some inertia of its own and begins to swim through that fluid towards the skull boundary.
5) The foam's compressibility varies, obviously, and so the force pushing back on the skull goes up as the foam is compressed. Foam isn't quite a spring but at some point the shoveback means ...
6) If things are going badly, your brain contacts the interior of your skull as it has run through all of the fluid slowing down its motion and that's when you can get some damage.
7) Hopefully you do not get to the point where the brain now bounces back against the other side of the skull.

Everything is about your brain not hitting your skull. Scalps can be stitched, fractures can knit, but the melon is fragile.

The same thing happens with seatbelts and internal organs in car crashes. Your ribcage might be stopped but your guts continue to move and slam up against them.

For the brain, linear motion is better than rotational motion, if you had to pick between them, which is why knockouts and the associated concussions are about twirling someone's head rather than just snapping it in one direction. There's probably some very advanced modeling done on trading off the rotational damage for the more palatable linear damage going on in helmet design.

There are probably reams and reams of complicated equations and heavy simulations involved in this once you get past the simple algebra.
posted by adipocere at 2:01 PM on September 15, 2010

Thank you! This is all great. I'll have to brush op un my highschool physics and try to understand it better. I don't know why I missed the FPP about motorcycle helmets (I think I mistakenly limited my search to Ask), that linked article is very interesting.

I found it really puzzling why helmet manufacturers do not give more information about the safety of their helmets. It is apparently totally normal to answer questions about that with something like "famous person xyz uses our helmets so it should be good enough for you too". I found an explanation for that that makes some sense on the Bicycle Helmet Safety Institute site, in their article on the ideal helmet: Manufacturers are not able to advertise a helmet model as safer than their other models, or safer than their models from last year, because they will be sued when someone is hurt in the less safe ones. For that reason you will not see any helmet ads claiming that the helmet will protect you against injury. That removes some of the incentive to make helmets more protective. Marketing a helmet is tricky business.
posted by davar at 1:47 AM on September 16, 2010

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