The CycleChat Helmet Debate Thread

[Justinslow]

I didn't see that one coming. 8/10 for unpredictably. Sadly only 3/10 for content.

Like most posts on here! Touché! (The last bit anyway)

 

[Fab Foodie]

Just a thought ... Are there stats or can you compare the injuries per time walking or cycling. The use of injuries over the same distance covered seems somehow wrong to me.

 

[benb]

Why do I get the feeling that you wouldn't have said a murmur had I said that the threshold of fatal injury was 300g's?
And to answer your question, I know because the Wayne State Tolerance Curve says so. In fact, for your hypothetical 100g example, impact duration is 6 ms and the threshold of life threatening injury is 90-95g for this duration.. And this assumes that the helmet not merely performs perfectly (does not fail under load by brittle fracture as opposed to crushing) but compresses to a density significantly higher than that of bulk polystyrene. This simply is not possible. And I'm neglecting the (incompressible) polycarbonate shell! Under the most favourable conditions, with the most benign of assumptions, helmet wearing fails to prevent fatal injury. [1]



Incorrect. Collisions take place at surfaces, not radii. Which is why physicists talk about "collision cross sections", measured in units of area. It is not the length of the barn door, nor its height - or even its volume, but its frontal area which determines your chances of hitting it at 6 paces. Your 28% increase in radius (assuming circular geometry, because I CBA to work out a more complex model) means a collision cross section increase of over 60%. And how do you know that the probability increase will "be tiny"? Kindly show your workings.



Absolutely not. You cannot simply discount lateral velocity: momentum does not disappear! Hint: conservation of momentum. One of the more fundamental postulates in physics. Lateral velocity simply cannot be ignored as you claim.


[1] For any innocent bystander - if there's anyone left after all this time! - this actually is the perfect example of why common sense breaks down in situations like this. The human body is surprisingly resilient to extremely high accelerations - for 2-3 ms duration a acceleration of 200g's or more can be tolerated without fatal injury. But it's time dependent! The longer the duration of the acceleration load, the less can be endured. A 75g load will induce a life threatening injury if it lasts for more than 10 ms.

In other words, it is not sufficient merely to reduce impact loads, as common sense suggests. This does not reduce impulse (that pesky conservation of momentum again) any decrease in the collision forces must necessarily be accompanied with a increase in impact duration. Since acceleration tolerance falls substantially as duration increases, this means any protective headgear must act to reduce the experienced force by a greater magnitude than is commonly realised. In fact, given the shape of the Wayne State Tolerance Curve (negative exponential), it is perfectly possible for an inadequate helmet to simply increase the impact duration such that a very short duration but high force impulse which will not cause potentially fatal injuries is extended such that the duration enters the lethal zone simply because it cannot reduce the impact acceleration sufficiently. This worse outcome due to protective head gear is not one that "common sense" would ever predict...

I wish there were more posts like this (& I include myself in that criticism): clear, factual and thoughtful.

 

[Fab Foodie]

It is all comparing apples, pears, and ferrets.

Like compare the Meerkat dot com?

 

[Profpointy]

Just a thought ... Are there stats or can you compare the injuries per time walking or cycling. The use of injuries over the same distance covered seems somehow wrong to me.

yebbut surely the purpose of tranport is to get somewhere rather than spending a certain amount ot time travelling. If we used your proposed measure you could make a car journey safer, let's say in thick fog, simply by driving faster thus reducing the exposure.

Pish-taking aside I do see what you're getting at if you see walking and cycling as mere leasure activities which you do for an amount of time, but still not a good way of looking at it

 

[srw]

Just a thought ... Are there stats or can you compare the injuries per time walking or cycling. The use of injuries over the same distance covered seems somehow wrong to me.

The thing is that for different purposes, comparing distance, time and number of journeys might all be valid. Unsurprisingly they give different answers, and even less surprisingly people with an emotional axe to grind find reasons why their favourite measure is the only true measure and the others are completely and utterly useless.

I prefer to focus on the very simple fact that, however you look at it, road cycling and walking are both extremely safe, as is driving in a car.

It might be time for @jo from the other place and his article again....
http://www.gicentre.net/blog/2013/11/24/risk-cycling-and-denominator-neglect

 

[MontyVeda]

...

I prefer to focus on the very simple fact that, however you look at it, road cycling and walking are both extremely safe, as is driving in a car.

...

Let's all focus in this.

 

[ufkacbln]

Let's all focus in this.

A helmet can only work if you are wearing it when your head hits the ground..... that is the only time they will work, and if you believe that they are effective this should surely be you aim

So what people should be asking is :

Do I want to protect my head if it hits the ground?

The answer is then very simple - wear one so that when your head hits the ground it can be effective (or not)

 

[MontyVeda]

A helmet can only work if you are wearing it when your head hits the ground..... that is the only time they will work, and if you believe that they are effective this should surely be you aim

So what people should be asking is :

Do I want to protect my head if it hits the ground?

The answer is then very simple - wear one so that when your head hits the ground it can be effective (or not)

i think i'll stick with the cycling being safe angle, thanks.

 

[ufkacbln]

i think i'll stick with the cycling being safe angle, thanks.

Which is the "or not" option

 

[MontyVeda]

I really don't know what you're driving at Cunners.

 

[ufkacbln]

I really don't know what you're driving at Cunners.

I am trying not to drive at anything, I prefer to miss.......

 

[MontyVeda]

hilarious.

 

[mjr]

i think i'll stick with the cycling being safe angle, thanks.

I'll stick with the helmets somehow causing about as many injuries as they prevent angle. I'd be quite happy to protect my head when it hit the ground if only the protection didn't seem to cause more injuries and more crashes in my experience wearing one for fifteen years.

 

[classic33]

Given out by the person who was checking passports and tickets on Sunday.

View attachment 127129

Picture used is from a charity event http://www.loophead.ie/events/hoppers-way/