Improved Helmets

Improved Helmets: Statistics trickle in

Well, today I received an email from the Queensland Department of Main Roads with some statistical data relating to bicycle, motorcycle and moped accidents. I thank them for their contribution.

The information is basic, but it covers the period from 2006 through to 2015, whether or not a helmet was worn, and whether the person died, was hospitalised, just received medical treatment or just had a minor injury.

Sadly, not information on whether there was TBI involved, but maybe Queensland Health could help me there. Similarly for emergency departments elsewhere in the world, I doubt the problem is just in Queensland.

I also have a fear that the information may not be tracked. I hope I’m wrong in that fear.

It’d be useful to know about upper-spinal injuries (neck vertebrae) since the weight of the helmet would be a contributor there. Basically injuries from the neck up.

I haven’t analysed the data as yet, but it’s right here if anyone wanted to look themselves.

Improved Helmets: An alternate test rig

I was doing some thinking last night, then it occurred to me. We are trying to do simulations of crashes using linear motion. Dropping a helmet vertically. That’s linear.

For sure, it’s a good-enough approximation when you hit something head-on… or is it? If you come off and fly through the air, then maybe, you’ll strike something dead-level.

More probable though, is you’ll follow an arc, under projectile motion. The most likely scenario is that as the bicycle/motorcycle tilts over, you follow it. It’s not going to be a direct-to-the-ground vertical drop of your head, but rather, a circular arc.

So how do we test for it? I suppose like this:

Improved Helmets: Criteria ideas

So, having done a little research, I’m starting to come up with some criteria about desirable features.

I had contacted the Bicycle Helmet Safety Institute , Randy Swart was able to provide some guidance on this. Specifically, he pointed to some research they did on the slip resistance of the helmet against various surfaces . Apparently this is a contributor to angular rotation inside the skull.

I’m not sure what the exact contribution is here, whether it’s torque applied to the skull and brain (which bigger diameters will also have an impact on) or something else at play.

He also pointed me to Smith Optics and 6D Helmets . The latter link, describes a system very similar to what I saw in the Vicis Zero1. That is, they’re relying on a filament that’s bonded at both ends, one end to the inner liner, the other to the outer shell, that’s permitted to deform and shear to absorb force.

I wonder if 6D Helmets is licensing Vicis’ patent.

The search query, ” coup contrecoup injury helmet ” yielded some interesting articles. Among them, was this article by Dewsnup, King & Olsen. Interestingly, when I try to access that article directly, I get an “IP Address Blocked” message, however, it can be accessed via Google’s cache .

Now, they’re in the business of accident litigation, and here they’re suggesting people take action against the helmet manufacturers. Ignoring that aspect, they link to some intriguing research. One such article being ” A New Biomechanical Predictor for Mild Traumatic Brain Injury “. There, they discuss angular acceleration as being of particular interest and the one that seems to be the least addressed in current designs.

MIPS is focussed on preventing that angular rotation. The Dewsnup, King & Olsen article also makes reference to another system, Angular Impact Mitigation (AIM) , which uses a honeycomb of aluminium that deforms. It’d be more like a heatsink and less an insulator too. Bike Magazine Australia mention some test results in their article ” Lifting the Lid “.

Based on the above, I’m starting to formulate some further ideas.

I had thought of a honeycomb of silicone rubber as an alternative to anything that looked like filaments. The fact that the AIM system uses a honeycomb structure, with a different material, is encouraging.

Either way, there would still need to be some sort of low-resistance shell. A lot of bicycle helmets have a very flimsy, similar to mylar, shell that if removed from the helmet, can be bent with finger-pressure. The purpose is to reduce friction with the road surface when sliding. This only needs hard-bonding at the edge — one of my old helmets uses electrical-style tape to achieve this.

An alternative might be a cloth with perforations, so that it tears away.

Under this, my initial idea of spines might work. The spines would deform on a direct impact and work to slow-down deceleration of the head. In a sliding type accident, when covered with a cloth cover, the cover would get stripped away to reveal the spines, which would then start to bend over, hopefully slowing down the angular deceleration in the process.

Then a traditional foam liner (or maybe an AIM-style one) can do its job.

This will need some analysis, I’m not sure how exactly to go about modelling this, finite element analysis seems the obvious tool and there are a few to choose from. I guess now is time to start reading up on how this stuff works.

Improved Helmets: A high-level injury overview

Well, having learned about the technical names for some of these injuries, I stumbled upon this article which describes, in lay terms, the various injury types.

Interestingly, they make reference to the cranial vault-brain dimensions that I was trying to figure out earlier:

In the subdural space is the cerebro spinal fluid. This fluid performs a number of functions, the main one being to protect the brain from impact concussion. The brain “floats” in the cerebro spinal fluid, lessening the weight on the base, and is supported at the base by the spinal cord. The brain, buoyant in the cerebro spinal fluid, can move within the cranium to a very limited degree. The space between the temporal bone and the brain is only one millimeter.

My earlier back-of-the-envelope calculation which yielded a gap of 700µm wasn’t far from the truth!

Improved Helmets: Analysing a simple case

Update 2016-03-19: Mia Culpa! I’ve been doing some revision of my physics since it’s been a good 12 years since I looked at this closely. The text I’ve been reading through for this is Physics for Scientists and Engineers with Modern Physics, Serway and Jewett, 6th Edition (International student edition), ISBN 0-534-40949-0 .

Seems in that time my physics has gone a bit rusty. The actual position with respect to time is given by:

h(t)={1 \over 2} at^2 + v_0t + h_0 So the results below are not quite correct. I shall re-do the calculations shortly.

Well, as a starting point, I figured I’d look at what happens in the current tests that are performed. I’ll have to dig up the relevant Australian Standards to see how they do things, but thanks to the Bicycle Helmet Safety Institute, we can have a look at the rig used in American CPSC labs.

They also make some interesting remarks about MIPS .

I’ll put some diagrams up, but for now bear with me. The typical test apparatus basically tries to measure the acceleration of the “headform” as it strikes a shaped anvil from some fixed height. In AS/NZS 2068, this height is 1.5m.

There’s a couple of different headforms they use, they’re basically a head-shaped block of wood, metal or plastic with an embedded accelerometer, with a known fixed mass and fixed dimensions. They strap the helmet under test to the headform, raise it to a fixed height (1.5m) and let it drop.

So let’s model this.

The following are our initial constants:

Variable Symbol used Value Notes
Free-fall height h_0 1.5 m From AS/NZS:2068
Free-fall acceleration a 9.8 m/s² Gravitational acceleration constant
Headform+helmet mass m 5 kg Educated guess here.
Initial Velocity v_0 0 m/s

We’ll start by trying to figure out the flight time, or time to impact. We’ll ignore wind resistance and the test apparatus, the height of the headform at any given time prior to impact is given by the equation:

h(t) = at^2 + v_0t + h_0 We simply solve this for h(t_I)=0 .

Most of the terms disappear, since we’re starting at rest and have a known starting height. We wind up with:

0 = -9.8t_I^2 + 0t_I + 1.5 We re-arrange this to find that the time to impact was 391.230 msec.

t_I=\sqrt{-1.5 \over -9.8}=0.391230

We can also determine that during this time, the headform accelerated to a velocity of -3.834 m/s. v_I=at_I=-9.8\times0.391230 = -3.834 This is at the point when helmet (or headform) meets anvil. The intention of the helmet is to absorb as much of the momentum as possible, so the worst thing that could happen here is a perfectly elastic collision.

The momentum at impact is given by the equation:

p_I=mv_I=5 \times -3.834=-19.170 The worst case is all of this momentum is reflected back to the headform itself. Let’s assume that happened over the course of 1 msec. So momentum after impact:

p_A=19.170 and the change in momentum:

\Delta p = 19.170 - (-19.170) = 38.340 which over 1msec, gives us a force of:

F_A={\Delta p \over \Delta t}={38.340 \over 10^-3}=38340 So 38.34kN, and what about the acceleration?

F_A=ma_A a_A={38340\over5}=7668

7668m/s² is 782g. Our cyclist would be dead.

Suppose the helmet did its job, and over 3 msec, managed to attenuate that to the 200g as specified in AS/NZS 2068. This equates to -1960m/s² acceleration, or a downward force of -9.8kN. What would the change in momentum need to be?

-9800 = {p_A - p_I} \over {3\times 10^-3} Re-arranging, we get an after-collision momentum of 1.534 Ns. So to meet the standard, the helmet has to attenuate that momentum. So what happens to the brain inside all this? Suppose we had a headform that modelled this.

The human brain is around 1.5kg on average, and has an approximate volume of 1130 cm³. It resides in the cranial vault, which has an approximate volume of 1170 cm³. These differ between males and females, and can vary wildly from this. For simplicity’s sake, let’s assume both are spherical. We can work out how much space there is around the brain using the following equation to calculate the radius of brain and vault:

V={4 \over 3}\pi r^3 Plugging these values in, we get a brain that has a radius of 6.461 cm, and a cranial vault of 6.537 cm. This leaves a gap of about 700um around the brain in which it can move. This is less than I expected, but let’s see what happens.

We know the headform was travelling at -3.834 m/s just before striking the anvil, and at this point, the brain is still moving at about that speed. We know it’ll continue to move forward that 700um before it hits the cranial vault, but how long do we have? About 183 microseconds.

Given the such small gap and time window involved, we could possibly consider the cranial vault in a simulated headform as being a gel with similar properties to the brain. It’ll deform as it hits the vault walls and “bounce” back, possibly causing it to ricochet into the opposing wall.

If we’re to have any hope in preventing this, we need to start speed reduction much earlier.

Improved Helmets: Developing the idea

Okay, so this is an area that’s big business, and with big business, come patents. This is not surprising.

This page links to quite a few. Some describe a crumple zone, but achieve it by means of sliding parts (sounds awfully like MIPS).

This appears to be the Zero1 patent. If Vicis come to the party and decide to adapt their design to motorcycle/bicycle applications (and they are most welcome to), that would provide the marketplace with an alternative solution. I’ve attempted to make contact with them, they may be interested.

As for this project, well, one of the goals is to come up with a model that can test the effectiveness and to push for updates in the standards that take this model into consideration.

That said, we can probably get by without the use of “filaments”. Could the cones on the design I scrawled on the home whiteboard be considered “filaments”? Possibly. We can also forget panels that slide around. That’s been done.

A honeycomb-like structure of soft-rubber is another option. It could cover a conventional helmet, and be covered itself by some sort of shell for aerodynamic properties.

The walls of the cells would deform when struck, and so would cushion the blow, and should survive multiple impacts. The questions are, how thick and how stiff?

Something the size of a beach ball isn’t going to fly no matter how good it is.

Pressure on the brain isn’t in itself the damaging bit. Scuba divers and free divers regularly submit themselves to as much as 6 atmospheres of pressure. This is uniformly applied to the entire body — brain damage doesn’t seem to result, nitrogen in the blood is a bigger problem. The key thing this pressure is applied gradually . It is not an impulse.

We need to dampen that impulse. Critical damping, so that the brain doesn’t bounce hard.

Time for some analysis.

Improved Helmets: A possible contender

The picture I uploaded on this project was an idea that came to me as I was lying in bed thinking about the problem.

The thought was inspired by a helmet I saw for sale in my local bike shop.

It had a single row of spines for “decorative” purposes. The thought occurred to me, what would happen if those spines covered the entire surface of the helmet? Hence, I came up with this.

Seems Vicis has had a similar idea, their Zero1 helmet incorporates a crumple zone not unlike the one I’ve drawn on the whiteboard here. Theirs is focussed solely on “football” applications (grid iron).

Now, patents are really what could throw a spanner in the works for this project. I’m looking to see if there’s a patent on this, and if so, how broad that patent is. It would be a big tragedy if other helmet users such as cyclists and motorcyclists, were denied access because of this , I must tread carefully as they are bigger than I am.