This site may be of interest. I hadn’t visited this area of science for many years.
https://helmets.org/liners.htm
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Thanks for the link Scott.
Interesting that they are mostly trying to make the ideal bike helmet with essentially one material/layer, whereas I think it probably needs 2, as I have done with mine. I guess it comes down to cost… but there must be a huge profit margin, where $1? worth of foam turns into a $300 helmet with the addition of some straps and a plastic cover.
Does anyone have a spare head I can use? I’ve got some old helmets and my phone has an accelerometer, and app to record it, so I can do my own testing!
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Make one out of ballistic jelly, have a pocket inside to insert your phone 
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The Mythbusters favourite 
I’d hope most peoples’ skull is a bit more substantial than jelly though!
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Being a bike rider for many years i had 2 accidents in a very short space of time.The first time was on a walk/bike path clipped the bump on the track and went flying got a broken collar bone.Then the 2nd time i was going over a patch work quilt road and basically just loss control.Puts you off bike riding when these things happen but anyway my point is i’m glad i was wearing my helmet i know i would have been heaps worse if i decided not to wear one.Mine also carries the sticker on it.Even says on mine don’t remove the sticker
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My understanding is that the bits of foam you have replaced, are only there for comfort to help with fit.
The idea is that you need to maximise the scull and polystyrene contact area, and the small foam parts help you do that.
By replacing those with extra think foam you are increasing the gap between your scull and the polystyrene. In doing so you increase the likelihood of the polystyrene breaking, rather than compressing under the impact load.
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The original foam bits are so flimsy that they don’t serve any useful purpose in my experience. The foam strips I’ve added are only about 10mm thick, and compress quite easily down to ~1mm.
The chances of generically shaped helmets fitting anyones head with no to minimal clearance across a large area are, IMHO, not great. No foam helmet I’ved used- and that’s quite a few over the past 40 years 
has fitted my head very well. Maybe I have a weird shaped head [1] They rely on the rather flimsy plastic adjuster strap around the back of the head to obtain fit plus a few other small contact points, no doubt varying dependent on the shape of your head. The best fitting one I’ve used, but probabaly least protective, was the old leather one in the late 70’s, I think filled with horse hair, that had 5 or 6 longitudinal ~20mm thick sections, plus a couple of bits of foam that went behind the ears.
[1] it certainly did change shape a bit after my 5 skull fractures with a standard helmet!
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Similar with motorcycle helmets - people often say a specific brand is ‘their head shape’. Makes you wonder why they use foam and not some kind of auto-profiling gel cushion insert … maybe that would fit ‘too well’ - airflow might be an issue, but you could imagine a tube-web structure that might do both …
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I think you are setting yourself up for higher risk with 10mm high density foam. You are creating the classic karate chop breaking the brick scenario Instead of allowing the very soft 3mm foam designed to merely smooth out the contours and reduce discomfort.
I agree the modern helmets with essentially one adult size fits all, doesn’t seem to maximise the fit, but, that’s no reason to bastardise the design. That said, the rear adjuster does create a much better fit at the front of the helmet, ie, compressing the soft 3mm foam at the front, and the other foam is designed to reduce lateral and depth movement combined with the straps either side of each ear. It is the lateral and overall depth fit, where choosing a specific model seems to make the most impact.
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Helmets are generally helpful in the event of a spill…however they should NOT be mandatory.
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It isn’t high density, it easily compresses to a couple of millimetres between 2 fingers, as I said previously
The original pieces of foam are of such low density that they are useless in improving fit or spreading load.
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It’s an interesting question about ‘spills’ - I’d imagine my kids riding to school, ambling along the footpath and having ‘an off’ because they were also trying to text (no I don’t condone that, and I don’t think they do it - fool me eh? 
) might be slightly different to @gordon doing 60 km/h down a 45 degree rocky incline, coming a cropper and landing 100 metres down the track. When my kids did BMX they had lightweight full face helmets, mandatory for competition. Of course I wear full face helmets riding a powered bike - also mandatory. I’ve heard it said that motorbike helmets are tested for impact for fall from height (something under 25 km/h) not for hitting a rock at 130 km/h while sliding …
Some US states don’t require helmets on motorcycles. Here we require them on pushies - although here in the NT the law is vague and depends on age and whether you are on the road or footpath. How much should the government protect us from ourselves? How much should we let ‘natural selection’ determine the outcome? there is certainly an argument for helmet use for people pushing the limit, but kids in the park or on the footpath perhaps not so much …
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Three years ago I was found on the dirt track where I had landed on my head, and had a small brain injury. There was slight damage to my helmet and I binned it.
I spent the next 6 months grounded by my brain specialist, during this time I researched helmets as I was sure that there must be some helmets better than others although, I was being told all the time that they were all passed by the same code.
After a while I found the Swedish Insurance company (Folksam https://www.folksam.se/tester-och-goda-rad/vara-tester/cykelhjalmar-for-vuxna) who did their own testing and then rated each helmet 1-4.
This site is in Swedish but there is an English version of the same if you fiddle around you should find it.
With the group I ride with 30% now have the MIPS helmets that Folksam passed with higher scores. http://mipsprotection.com/technology/
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A more direct link to a pdf, unfortunately having a limited representation.
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It seems that the experts are digging in about the efficacy of helmets, but the Bicycle Network is questioning the overall impact. I doubt anyone thinks or implies that helmets will not reduce trauma in some situations, but everyone will not crash, everyone does not ride on streets, everyone does not ride ‘mountaineering style’, and for a large segment of us riders the laws as they are seem gratuitous overkill. Penalties we face for daily activity, legal in the rest of the western world, often attract fines beyond violent offenders, thieves, and robbers as well as those faced for bringing in consciously undeclared and environmentally dangerous items through customs.
This proposal seems a touch of sanity from my perspective and could go a small way to attracting more social/exercise riders, and others who chose not to wear helmets on trails relieved of relatively draconian consequences.
A new plan to scrap laws forcing Australians to wear helmets when riding bikes is being pushed by a cycling organisation.
People over the age of 17 should be able to choose if they want to wear a helmet when riding on cycling paths or trails, the Bicycle Network said in a report released on Wednesday.
https://au.news.yahoo.com/calls-compulsory-rule-aussie-cyclist-scrapped-041651312.html
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I don’t disagree where one is responsible enough to make an informed decision of whether to or not wear a helmet. Also, if one choses not to wear a helmet, they possibly also need to accept the responsibility/liability should one have a head injury through not wearing a helmet. The government/broader community should not be responsible for risk an individual takes on when the same individuals makes a decision not to wear one.
Notwithstanding this, children (or their parents/guardian) most likely will chose hot to wear helmets for reasons other than for head protection…is it cool not to wear one, it saves money, prevents arguments when making a child wear one etc. I possibly am correct saying children are more likely to have bike accidents as they are risk takers and sometimes don’t understand the potential outcomes of the risk.
If there is a change to the status quo, children (those who can’t make responsible decision under the law) need to be protected as one may not be able to rely on a parent/adult doing such on their behalf.
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Assuming the law was changed, our systems would still treat equally someone with severe head trauma or brain injury even when it was a consequence of not wearing a helmet. Where there are other parties involved will insurers see it the same way if they are being asked to pay out or will they see lack of caution and care by the rider as a way to reduce liability?
While more mature riders are likely more capable, if they do have accidents it is also more likely at higher speed. The older we get the less well we bounce.
The everyday crop of protective devices is far from inspiring, and as previously mentioned by others, varying in effectiveness. I still wear one, more out of spite than concern for falling off.
If there is any logic to not wearing a helmet - it may be that there are low risk low exposure bike activities where falling over while stationary and hitting ones head on the ground is nearly impossible. We do have an indoor trainer for one of our bikes which seems to qualify! 
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That’s the law in the NT.
Interestingly it seems common for states and territories to still require the Australian Standards certification - where most places now have woken up that re-testing helmets tested overseas to perfectly acceptable standards is a waste of everyone’s time and money.
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