Cycling UK Forum

Not much of a presumption.
I can not remember the actual numbers involved but the energy required to crack expanded polystyrene is merely that required to break all the chemical bonds on the area of fracture. To crush the material requires the compression of the material through a large volume, this is much larger.
This is supported by generally accepted theories and demonstrated in scientific research which is why the helmets are made to utilise this phenomena.

If you are disbelieving of the science you can demonstrate this to yourself very easily with any piece of expanded polystyrene. Just try snapping a piece hanging over the edge of a table and then punching a piece against a wall.

I'm not disbelieving on science - I have a Masters in Celestial Mechanics. I'm experienced in the mathematics governing the absorbtion and dissipation of terajoules of energy.

I'm suggesting that the gentleman has made a claim and that he may be wrong, and he may be right, but until he can reference his data as any good scientist would when presenting a written position, then its meaningless and I can neither support or oppose his claim.

Never said I was disbelieving, never said the gentleman was wrong or right. You might want to read the posts to which you respond.

What does snapping a piece of expanded polystyrene, or smashing it against a wall prove? Nothing without instruments, and even less in a helmet discussion unless it also is arranged in the form and structure of a cycle helmet. A pointless, rather emotional response to an empirical problem, and is nicely illustrative of why the helmet debate never advances in a constructive manner.

Cracking the polystyrene and crushing it in your own home doesnt prove anything, it merely demonstrates it. The proof has been done extensively in laboratories yet that seems to be disbelieved (people suggesting it is a presumption), hopefully a demonstration will help them to believe what has been scientifically accepted.

However if somebody then brings up the question, "what does the demonstration prove?" we have probably left the rational argument behind.

To be slightly more accurate then... A cracked *and only cracked* helmet has failed.
The amount of energy absorption is trivial compared with the energy absorption to deform a helmet.

But actually even more of the "benefit" of a helmet comes from the temporal smear of the impact - by deforming the helmet allows for the impact energy to be applied to head over a few milliseconds. Fast but a lot slower than a direct impact. If it hasn't crushed (and EPS has very low bounce back) then it hasn't done this temporal smearing.

Of course either way the brain is still going to hit the skull at pretty much the same rate because it doesn't have a helmet on inside - but it's already very well protected from linear impacts of this sort, it's rotational impacts that do the damage in there.... And what kind of impact do we explicitly /not/ test for when designing cycle helmets?


We don't need exact numbers to see that the requirements for a(n EPS based) helmet to work are that the EPS crushes rather than snaps - that's basic materials science and design.

I have no doubt that a cracked helmet has saved a person from a laceration or two - maybe even some soft tissue damage above the skull... But it hasn't done what most people think a helmet does (usually because incidents don't actually have enough damage causing potential to need a helmet)

I still cannot see how the few millimetres of compression (if any) of a helmet will prevent the brain from sloshing about and causing a TBI.
Re energy absorbtion, this was in 'cycle' in July 2005 http://www.cyclehelmets.org/papers/c2023.pdf
A materials expert ascertained that a cycle helmet with respect to a circa 70joule impact would have made no difference, nor prevented the head injury sustained.

Evans's you need to think in terms of 1-2 cm of compression. The brain hitting a still decelerating skull, not a stopped one.

Or you accept that they are laceration protection...

To be fair the human brain is remarkably well protected- and capable of sustaining ridiculous linear acceleration. American football is one of the key drivers of research, and they have regular hits over 400g (in helmet) without apparent ill effect. Add in a bit of rotation and everything goes to pot though, shear forces are the issue....

[XAP]Bob wrote:To be slightly more accurate then... A cracked *and only cracked* helmet has failed.
The amount of energy absorption is trivial compared with the energy absorption to deform a helmet.

<snip>
I have no doubt that a cracked helmet has saved a person from a laceration or two - maybe even some soft tissue damage above the skull... But it hasn't done what most people think a helmet does (usually because incidents don't actually have enough damage causing potential to need a helmet)

A cracked helmet has failed, end of.
However, looking at your failed, cracked helmet doesn't in itself tell you what happened in the very short space of time before it failed. It might have done some useful absorption work before it reached its limits and failed. Or not...

As is often the case, lots of assumptions are made and not many are safe. We're left with piles of conflicting evidence that don't tell us much that can be taken as a given. Certainly nothing good enough to properly underpin their unguarded recommendation as a public health and safety policy measure for utility cycling, which is hardly surprising for something developed as a "better hairnet" for sports riding.

Pete.

pjclinch wrote:A cracked helmet has failed, end of.

I'm afraid boldly proclaiming 'end of' doesn't make it so. We would be interested in seeing the technical and/or scientific evidence upon which you have based that statement.

You are complaining that "lots of assumptions are made" while doing exactly they same yourself to support your own favoured view.

PJ...

I'd suggest that a cracked helmet which has been held in place by the outer and subsequently crushed has down most of it's job.

Not sure I've ever seen such a thing, but it's theoretically possible..


Lance - you might want to read some of PJ's history before suggesting that he is making this stuff up. He has access to more research libraries than I do for certain.

But given the stats, 'doing its job' in best case scenario isn't anywhere close to being good enough to reduce head injuries thus helmets are a complete and utter failure in terms of being a 'safety' measure. Actually it's worse than that, it's turned people into victim blaming morons, deflecting horrendous and callous acts against people on bikes, helped to remove basic human rights, removed accessibility, removed inclusion at virtually all levels and putting a massive barrier to cycling to swathes, it also makes people feel like outcasts from an activity they love/once loved.

As i've said before, only the motorvehicle and its drivers present a greater level of harm than cycle helmets do to riding a bike and/or riding a bike safely.

Lance Dopestrong wrote:I'm afraid boldly proclaiming 'end of' doesn't make it so. We would be interested in seeing the technical and/or scientific evidence upon which you have based that statement.

I suspect semantics the main issue here. At some point it has had its limits exceeded and broken, nothing more, nothing less. "Failure" meaning "it has broken", not that it has failed to do anything useful. The trick is knowing whether the limits it reached were before, at, or beyond design spec, and I'd suggest that's non-trivial.

Technical evidence for failure, thus, a matter of seeing the crack in it and saying, "this has a crack, it is broken and no longer fit for purpose".

[XAP]Bob wrote:I'd suggest that a cracked helmet which has been held in place by the outer and subsequently crushed has down most of it's job.

Deciding the order of crushing/cracking would be an interesting game, I suspect. My guess is that typically it would crush, get to a limit point and then brittle fracture would be the failure point. Though only a guess. Whatever, it's no longer fit for purpose because it's failed (is borken).

Pete.

The fact that it has cracked will not greatly diminish its ability to keep absorbing energy through compression after that crack, so long as the polystyrene is kept in the correct place by some sort of casing. You could even theoretically make a helmet from polystyrene chips if you wanted to.

meic wrote:The fact that it has cracked will not greatly diminish its ability to keep absorbing energy through compression after that crack, so long as the polystyrene is kept in the correct place by some sort of casing.

That's a pretty big "if", especially given the shell typically has no rigidity to speak of and is on the outside only, and all those nice vent holes give the stuff somewhere to go when being pushed by an impact.

Pete.

The shell doesnt need rigidity, just to keep the padding between head and force. String would suffice in the total absence of shear forces.

The picture that sparked the discussion did have a cracked polystyrene inner still held in place by the outer. So even if it is a pretty rare case it is the one that we were talking about.

It probably was cracked by a pretty minor force and a slightly larger force could have torn it apart before it had the chance to do any more absorption but we didnt get that far.

meic wrote:The shell doesnt need rigidity, just to keep the padding between head and force. String would suffice in the total absence of shear forces.

Indeed. Remember the early efforts from the 1970s and 80s which were plain polystyrene shells with an removable nylon mesh cover. The cover was there to hold everything together in the event of the polystyrene fracturing.