Once again I’ll start with a disclaimer: I’m not saying don’t wear a helmet. If you want the protection that a helmet gives then buy and wear a certified helmet. Obviously. Moving on…
Over the years I’ve attempted to use many dodgy helmet ‘alternatives’. I wrote a piece in Issue 8 of The Albion about a test I did on some of them, with a follow up article on their website which you can find here. In both of these bits of writing I was rightly critical of my dodgy test methods and I said the only way to be sure about how well something protects your head is to test it in the same way that helmets are tested. Practically speaking, this means using an accelerometer. Since I am currently bored out of my mind, and have access to an accelerometer, I decided to finish what I’d started.
After the Albion article, someone got in touch to tell me about G-Form pads. A bit of digging about on the website revealed that they were made out of something called Poron XRD. I requested some free samples and they sent me tonnes of the stuff in a range of thicknesses and styles, which I used to make some knee pads and then forgot about.
After stumbling across them recently I decided to borrow some stuff to build a proper helmet test rig in my garage, as you do. I basically wanted to prove to myself that wearing a little bit of padding, while not being as good as a helmet, is definitely better than nothing. Read on…
I started by trepanning a hole through the concrete head that I used in my previous tests, and then threading a gear cable from the top of my garage through the hole and into a bit of leftover wood. This ensures that the head drops vertically, meaning that the results from the accelerometer are accurate. Using a metal plate instead of the wood at the bottom would be better, but I wasn’t convinced that the head wouldn’t shatter if I used metal! However, the wood is sturdy stuff and was hardly dented at all by the head being dropped on it from 1.5m.
I drilled another hole in the head and jammed a bit of plastic sawn off my workshop toothbrush into it. I drilled a hole in the toothbrush fragment which I used to mount the accelerometer. The combination of my gear cable slide and the hole angle meant that the accelerometer could not be more than 15° from vertical, which could cause an error of less than 4%. Not ideal but not too bad!
The accelerometer is piezo-electric, which means that it gives off a charge when it is accelerated. This charge is converted to a voltage that can be read with the oscilloscope using a charge amplifier. The oscilloscope has a floppy disk drive which allowed me to get the results onto an old laptop, from where I could get them onto a new laptop to do stuff with them. Floppy disks are a massive ball-ache to deal with when you’ve got used to modern stuff…
The full test rig is shown below….
I did the tests with the head being dropped from 0.5m, 1m and 1.5m. I’d have liked to do 2m as well but my garage isn’t tall enough! I had samples of 6mm hexagonal pads, 9mm foam and 12.5mm foam, so I tested all of those. I also did the tests without any padding for comparison.
The oscilloscope samples the voltage from the accelerometer 10,000 times per second, allowing you to get a readout of the results throughout the impact. The graph below shows a comparison of the acceleration with and without padding. The limit for a helmet to pass ‘real’ tests is 300g, which is thought to be the difference between a nasty concussion and a serious head injury. You can see that the Poron extends the duration of the impact and greatly reduces the peak acceleration. What this says to me is that the padding is definitely doing its job!
Watching the tests, you can see it is working because the head doesn’t bounce at all when dropping onto the Poron.
A comparison of the results for all the different scenarios is given below. Each test was performed three times and the peak acceleration was averaged. Poron XRD is rate dependent, meaning if you knock your head a little bit then it is softer, but if you really smash it will harden up more. This means that for small knocks you could end up being less injured than if you’d worn a helmet, which is designed to protect you from the serious bangs. It looks like the 9mm padding might be a good combination of ‘wearability’ and protection to me, although the 6mm stuff also makes you a lot safer.
So that’s it… not a very exciting read if you came here looking for BMX videos but I’ve got a posting account so I can do what I like within reason (I probably couldn’t slander H Man but I wasn’t going to anyway).
Now you can make an informed decision about getting some free samples of Poron XRD and putting them in a hat. Awesome.
EDIT: I’ve just remembered this video…
The legit helmet (which looks to be made of expanded polystyrene much thicker than half an inch) results in an acceleration of 159g for a 1m drop height. The 12.5mm Poron XRD resulted in an acceleration of 186g for a 1m drop height in my tests. They obviously can’t be compared directly (they tested the helmet in a lab and I did my tests in the garage…) but I’d say Poron XRD looks pretty good for use in helmets. Sort it out helmet manufacturers!