Would a military helmet make a safe alternative to a bicycle helmet?

Question

I am in need of a new bicycle helmet and have been exploring some alternative options. One possibility I am considering is visiting my local Army Surplus store and getting a helmet there. I only use my bike for commuting, 3-4 miles in a single trip, tops. So the weight or lack of ventilation won't be a problem for me. Additionally, having a helmet durable enough to not need replacing after a minor bump or contact with the ground seems appealing (and cost-effective).
Right now, I am thinking something along the lines of a vintage WWII helmet.

So my question is would this be a viable and safe option for me or do I need some sense slapped in to me? I figure if it's meant to deflect bullets or shrapnel, then it should work fine for a fall or if I get hit by a car.

Obviously, I'll have look into the legality of this for my state on my own. But I am wondering if anyone out there has any experience or advice for me on this topic.

Thanks.

Rider_X · Accepted Answer

Bicycle helmets contain crushable foam that works to extend the duration of impact by about 6 ms (milliseconds).   This doesn't make the impact force disappear, instead it extends the duration of force experienced by the brain.  By extending the duration, you reduce the peak force.  The brain can withstand impacts to some degree, however if the impact is too forceful over too short a period of time (like your head hitting asphalt), the peak force experienced by your brain becomes too high resulting in brain damage or even death.  While 6 ms may seem  small, it can be enough to reduce the peak force experience from lethal and/or damaging to survivable and/or reduced damage (see Figure 1).

A)  B)  
Figure 1.  Impact force by time a brain sees with a helmet (A) and without a helmet (B).  The total energy is the same (area under the curve), but the peak force is lower for the brain with a helmet.  (Source: http://www.bhsi.org/)

Modern military helmet are designed to protect against things like shrapnel, not absorb impacts (although newer research may change that).  As a result your peak impact force in a crash will not substantially change and you will be under a similar risk for brain injury as being helmet less.

Also you mentioned having to replace bike helmets that take an impact as a downside. If you wonder why bicycle helmets are not made with foam that "bounces back" (e.g. hockey helmets) this is because it can cause more damage in an impact.  With this type of foam your brain receives force in the initial hit and then it experiences a secondary "hit" as the foam returns (often these non-crushable foams return rapidly).  Crushable foam however does not rebound as it remains crushed. The downside is that once it is crushed it no longer provides any protective benefit and needs to be replaced. By extension when the crushable foam degrades it also needs to be replaced.  This is why it is also suggested that old bicycle helmets also need to be replaced (what constitutes old is up for debate).  In this case the crushable foam has degraded over time and its impact absorption properties change for the worse and will not provide the same benefit in an impact as a newer helmet with foam that has not degraded.

Finally, I do remember hearing about a new type of foam that behaves similar to the crushable foam used in helmets, but rebounds slowly.  I will have to work to find that reference. But if they are in the marketplace they will be more robust to hits and repeat impacts.

mkoryak · Answer

I'm not an expert, but I would say it is probably not safe.

Things that are good at deflecting small fast moving objects aren't always good at absorbing crushing force. Bike helmets are designed to absorb most of the force from the impact as to protect your dome from it. I am not sure how well an army helmet will do in this regard.

A good analogy might be using a bullet vest to stop a bullet vs using a bullet vest to protect yourself from a sledge hammer wielding psycho. It may work really well for one but not the other.

joelmdev · Answer

No. Military helmets are designed to protect your head against flying shrapnel, not absorb blunt force trauma. It's better than nothing, but that's not saying much.

Sean · Answer

It depends, if you get a WWII surplus helmet, then no, definitely not. They do not meet any safety regulations and have no cushioning to absorb the impact of a crash.

A modern combat helmet (i.e. MICH 2000s, PASGTs) work fine because they have a cushioned lair that can absorb the impact. In fact, many special forces units still wear them when they use vehicles like motorcycles to move around the battlefield.

Criggie · Answer

In my country, cycle helmets are mandated by law for use on the road.  It is illegal to sell a non-certified helmet for the purposes of riding on the road.

So any accident where you're not wearing an approved and certified helmet could result in insurance cover being refused, regardless of whether the helmet is a cause or a mitigator.

Like having your car stopped and parked on the road, being struck, but because you have an expired Warrant of Fitness, the insurer refuses to cover your damage because the car shouldn't have been on the road.

Its petty, but that's how insurance companies roll.  Why risk it?

Combat veteran · Answer

Old WWII helmets were basically the original motorcycle helmet. You might want to consider a newer military helmet if you got one, look fort the Kevlar helmets as they are designed to absorb the impact of small rounds and I know 9 mm rounds produce 467 joules of energy on impact and is designed to penetrate, a helmet actually has to be stronger to stop small objects from penetrating it because the impact is in a smaller and more localized area on the helmet's surface than if it were to be dropped or be on the wearer's head when they fell. This whole logic that 'it's designed to protect against sharp objects but not impacts' is kind of flawed. Modern military helmets have adjustable foam pads inside and exceed DOT standards for even a motorcycle helmet when looking at the numbers. The main reason most people don't use military helmets on motorcycles, except for military personnel, is because the modern helmet is what you'd want and it's expensive. military helmets have been being designed to take blows from blunt objects, sharp objects, and falls for years and have evolved since the first helmets were worn into battle over 3,000 years ago. They normally have a space around the head and a padding inside to absorb impacts and have been designed that way for over 1,000 years. They are really your best choice of helmet for pretty much anything since they are tested more rigorously and have to protect a soldiers head in the event of a fall, something falling on their head, and more. They're just expensive, soldiers only tend to use them on bicycles and motorcycles because they are issued to them and therefore it's cheaper for them, most other people use motorcycle helmets or bicycle helmets because they're considerably cheaper than military helmets.

Answered by Combat veteran on January 1, 2021

Would a military helmet make a safe alternative to a bicycle helmet?

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