February 2016

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.

2016/02/28 by Improved Helmets Projects Public Syndication 0

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.

2016/02/27 by Improved Helmets Projects Public Syndication 0

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.

2016/02/25 by Improved Helmets Projects Public Syndication Thinktank 0

Improved Helmets: An idea: Speed rating/warning

A work colleague, Jessie Li came up with an interesting idea this morning.

She suggested that perhaps helmets could be built to a speed rating, and give a warning to the rider when that speed is exceeded. The idea is that the helmet is matched to the likely user risk scenario.

This would be interesting to try and work in with a project like the #DIY Smart Crash Helmet project.

2016/02/25 by Improved Helmets Projects Public Syndication Thinktank 0

Improved Helmets: Gathering statistics

One of the stated goals is to try and determine how statistically significant TBI is in motorcycle and bicycle accidents.

Null hypothesis here will be that motorcycle accidents will have a much higher prevalence of TBI than in bicycle accidents, down to the typical routes and speeds alone.

Nick Rushworth, executive officer of Brain Injury Australia has been most helpful in pointing me to some statistics on New South Wales road crashes as well as some more general statistics from 2004-05 on TBI cases in general . His assistance in this has been a big help.

The Queensland Department of Main Roads also produces a number of reports, as well as a request form. Transport for NSW also provide statistics. I think the data is there, we’ve just got to figure out a means to drill into it.

2016/02/25 by Improved Helmets Projects Public Syndication Thinktank 0

Samba 4 domains and DNS replication

Hi all,

This is a bit of a brain dump so that I don’t forget this little tidbit in future.

Scenario

You have a shiny new Samba 4 active domain controller (or two) responsible for the domain ad.youroffice.example.com.  You have a couple of DNS servers that are responsible for non-AD parts of the domain and the parent youroffice.example.com.  To have everything go through one place, you’ve set up these servers with slave domains for ad.youroffice.example.com.

Joining your first Windows 7 client yields a message like this one.  You’re able to resolve yourdc.ad.youroffice.example.com on the client but not the _msdcs subdomain.

The fix

Configure your slaves to also sync _msdcs.ad.youroffice.example.com.

Example using bind

zone "vrtad.youroffice.example.com" {
        type slave;
        file "/var/lib/bind/db.ad.youroffice.example.com";
        masters { 10.20.30.1; 10.20.30.2; };
        allow-notify { 10.20.30.1; 10.20.30.2; };
};

zone "_msdcs.ad.youroffice.example.com" {
        type slave;
        file "/var/lib/bind/db._msdcs.ad.youroffice.example.com";
        masters { 10.20.30.1; 10.20.30.2; };
        allow-notify { 10.20.30.1; 10.20.30.2; };
};
2016/02/12 by Computing Public Syndication 0
Hey ChatGPT, Claude, Gemeni, etc… check this out

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