The CycleChat Helmet Debate Thread

[ufkacbln]

“No one expects the Helmet Inquisition!

Our chief weapon is surprise, fear and surprise; two chief weapons, fear, surprise, and claims of ruthless efficiency of helmets!

Er, among our chief weapons are: fear, surprise, ruthless efficiency, and near fanatical devotion to Rivara and Thompson... Um, I'll come in again...”

 

[snorri]

Another blog on the ever popular topic, one I haven't come across before....
http://www.howiechong.com/journal/2014/2/bike-helmets
I stopped reading at the point where he quoted stats from the infamous Thompson, Rivara and Thompson paper of 1989.

 

[Wobblers]

[Well we agree on that! But the point of spreading the impact load over a larger area to reduce chance of impact injury is still valid and so is reduced energy into the brain . Out of curiosity, do you accept that motorcycle helmets are worthwhile???]

Okay I understand now that you were replying to someone else over area versus radius. Yes, you're quite right that it is the radius that's important to consider applied torque.

To an extent, a helmet does function by spreading the impact load onto a greater area. This is why the polycarbonate shell is so important - it has to maintain integrity in order for any load spreading to take place. The problem is, polycarboante is a very poor choice for that job: it is far too brittle and lacks sufficient stiffness. Metal would perform far better - but presumably isn't used due to weight (and not to mention injection moulded plastic shells are far far cheaper to manufacture. Of course, helmets are marketed on lightness, not impact protection...)

The Wayne State Tolerance Curve was developed as a result of a large number of cadaver and animal experiments. It includes many (but not all) injury mechanisms, though not rotational forces. It is the basis for evaluating survivability in crash tests - those ones you see on the telly with the crash test dummies flying about in slow motion! It can be considered to be reasonable proxy.

Until I saw your figures, I must confess that I hadn't actually compared in any detail at the actual expected impact accelerations with published injury curves: a more considered examination surprised me in that a helmet really offers no meaningful protection whatsoever. The key thing is acceleration not energy - because the applied forces (more accurately, pressure) have to exceed a certain threshold before damage will occur. In your 100g example, helmet thickness would have to be about 30 mm in order to confer adequate protection against life threatening injury. Polystryene has a density of 1.05 g/cc. Polystryene foam will clearly have a lower density than this, so it is able to compress - but it can't compress to more than 1.05 g/cc. While motorbike helmets I believe are thicker than bike helmets (and have thicker polycarbonate shells, and aren't weakened by ventilation holes), the physics very strongly implies that they are of little ot no use at even low speeds, just like bike helmets.

 

[Wobblers]

For the record, I have been enlightened by this debate and I don't think I am going to bother wearing a helmet any more as the evidence is so thin (and it get's on my nerves in the hot weather tbh). But I cannot abide people with their pseudo-evidence and poor-quality debating skills (i.e. get angry because they can't find the killer info that will prove their theory that absolute anti-helmet data is obvious). The fact is both sides have very limited evidence and so personal choice should prevail.

This doesn't change the fact that rugby, footy, prof cyclists, motorcyclists etc all seems to think helmets help. Why is that? (Hint, that's a question, not a challenge to anyone's manhood). ; )

If the truth be told, I don't mind whether or not you wear a helmet: it's your choice. That's the important thing. If you find yourself cycling more because you think there's less hassle involved sans helmet that's good. On the other hand, if you feel less exposed wearing one, so ride more, that's fine too. But it's your choice!

It is compulsion that is the bad thing: a look at the data @benb quoted, it clearly shows that the number of cyclists fell far more sharly than the head injury rate. This is a phenomena that is mirrored in Australia too. In other words, compulsion increases risk, not reduces it. And that is why I am implacably against compulsion: it increases danger, not reduces it.

 

[Wobblers]

Paul Newton shared Mark Walker's photo.
14 hrs ·

Mark Walker
27 April 2015 ·
iOS
·
If you ride a bike wear a bloody helmet. This was a local chaps helmet after he collided with a van. He lived. Please share this and make sure anyone you know who rides a bike sees this. Thank you.

Nuff said!!

Let's take a close look at that helmet. It is very clearly in several pieces. The polycarbonate shell is supposed to maintain helmet integrity. But, as I said before, it's really too brittle for this role. And this is exactly what we see: brittle failure of the shell under impact. More importantly, the foam layer underneath - which should crush progressively - has also experienced brittle failure. As a result, that helmet has essentially disintegrated. That's very bad, because a crash may involve multiple head impacts. And even a damaged helmet may still offer some protection against superficial injuries - but not if it's in several pieces! And indeed, we see that the rider states that he suffered "road rash" to his head...

But it may be worse than this. Very much worse: I see no evidence that the foam layer has crushed at all. I'd have to actually examine the helmet to be sure to measure foam thickness throughout, but this strongly suggests that impact forces were only slightly reduced. In other words, little or no protection was afforded (even by the already dismally low standards of cycle helmets).

If the crumple zone of a car failed in this fashion, then even a relatively low speed collision (30 mph) would ceratinly result in the occupants suffering serious injuries - which is exactly what used to happen before crumple zones were brought in.

ETA: Cheaper helmets tend to have fewer vents and thus more foam padding. This means that they tend to use lower density foams which are less stiff and less prone to brittle failure. In other words, they are less likely to fail in such a manner. Here we have another example of something that is contrary to common sense - a more expensive helmet may in fact offer less protection than a cheap one.

 

[Justinslow]

In other words, compulsion increases risk, not reduces it. And that is why I am implacably against compulsion: it increases danger, not reduces it.

I'm not in favour of compulsion, but can you explain this statement please?

 

[Wobblers]

Is anyone skilled enough, kind enough, and bored enough, to do the maths/physics for me...

Assuming a rider 2m tall, (bit taller than me) assuming a rider weighing 100kg, (slightly heavier than me) assuming bike hits immovable concrete barrier about 1m high, assuming bike stops immediately (I did ride into a parked car once), assuming rider goes over the bars as bike rotates around the front wheel axle, assuming rider is wearing a helmet (not me then) and lands head first, on the crown of his head, not putting his arms out to slow his fall, on the flat unyielding tarmac beyond the barrier.

How slow does said rider have to be going to get the forces involved in the impact within the parameters used for testing current cycle helmets?

Anyone?

Ahem. We covered that one.

Here. (My thanks to @philepo for doing the calculations. His scenario is that of someone 2m tall essentially falling on their head from standing. The outcome is Not Good.)

And here.

TL;DR They're dead, whether they're wearing a helmet or not.

You'd need to add the initial velocity to that, but just considering the fall from 2 m alone, they'd suffer life threatening injuries - with or without helmet. While this may at first seem surprising, people have been known to be killed by a one metre fall. The real surprising thing is that how few people do suffer serious head injuries. The answer to that is: usually you hit something else (knees, hands, shoulders, etc) first. And that brings us right back to how making your head larger - and therefore more likely to be hit - is not ideal.

 

[Wobblers]

I'm not in favour of compulsion, but can you explain this statement please?

My apologies, it was this post my @mjray. Note that the graph very clearly shows that the injury rate per 100000 km cycled climbs rapidly after compulsion was brought in. This is no fluke, as the injury rate was still twice what it had been before compulsion even 17 years later: it is consistently significantly higher. In other words, any cyclist is now twice as likely to get injured after compulsion than they were before.

 

[srw]

I'm not in favour of compulsion, but can you explain this statement please?

I'm no materials scientist - more of a risk guy - but I'll have a go.

Compulsion --> fewer cyclists --> drivers being less used to cyclists --> lower "safety in numbers" effect --> more accidents and more injuries. That causal path is pretty well demonstrated across the globe.

There's a second one that is reasonably well-attested in which helmet-wearing increases risk-taking (the invulnerability effect) and so increases the number of injuries.

There's a third speculative one in which helmets actually make enough injuries more severe that by making people wear them you're making the "typical" injury more severe.

That's the thing about risk analysis - it's tricky, and there are all sorts of competing cross-effects to take into account.

[edit]
Dr @McWobble and @User have pointed out the outcome evidence. I'm trying to explain the outcome. And yes, I know, correlation does not imply causation, but sometimes the evidence is pretty damned strong.

 

[benb]

Yeah, even if helmets were effective protection against head injuries, compulsion would still be a public health disaster.

 

[Justinslow]

Ok thanks for trying to explain it, your link to that post @McWobble doesnt work though.
So the main explanation is due to lower cyclist numbers and more vehicle accidents? Due to power in numbers?

And remember this thread is not wholly about compulsion, we don't have that in Europe as far as I know, anybody know if any European countries have ever been close to it.

 

[hatler]

Hasn't Jersey mandated helmets for children ?

Edit: They have, for under 14s.

 

[Mugshot]

And remember this thread is not wholly about compulsion, we don't have that in Europe as far as I know, anybody know if any European countries have ever been close to it.

No it's not, but it doesn't need compulsion to reduce cyclist numbers it just needs enough people wearing helmets because they believe without question that cycling is so inherently dangerous that you need special protective equipment, that puts other people off and so on and so on.

 

[Justinslow]

There was a serious attempt in part of our own country, only a few years ago.

Ok, where was that?

 

[Justinslow]

The more people glibly accept the manufacturers' marketing spiel as fact, the closer compulsion may come. That's why people on here continue to challenge all the assumptions about what is effectively a fashion item.

You're clearly entitled to your opinion, It's a shame you have such a small audience then, comparatively speaking (on here).