Skiing vs cycling

[Wobblers]

Define 'brittle' .... without cut and paste ?

More evasion perchance? I'm very glad the the Institute of Mechanical Engineers adopts the very sensible precaution of not awarding CEng status to just anyone with a pajero...

A brittle material is one which does not have any mechanism of crack stopping. Which means, once a crack is initiated, it will continue to grow - unless all stress is removed. A tough material does possess a crack stopping mechanism - in metals, this is because dislocations have mobility in all six degrees of freedom (though admittedly I only add that as I know it will annoy Sazzaa).

Now let's revisit this little gem:

The crack is indicator that the force on the styrene has exceeded its shear strength along that particular stress line . That does not mean that the styrene stops deforming through the rest of it's structure away from that point. The thing about a fall is it is normally associated with a tumble and that means that the lid will also continue to give a benefit with secondary impacts.

Firstly, your chemistry: styrene is a rather gloopy liquid at room temperature. Its polymer, polystyrene is a solid. It's important to know the difference, isn't it? Because it would take a rather gifted muppet to try and make a helmet out of styrene...

Polystyrene is a brittle material. In other words, it behaves in an elastic fashion until its strength (tensile, shear or compressive, depending on the applied stress) is exceeded when it will crack. Chemical bonds are broken in this process, and new surfaces created: both absorb energy. The word "deform" unless qualified, means elastic strain. Elastic deformation absorbs almost no energy whatsoever, so will not provide any protection whatsoever. Your claim that it will is wrong. (Tip: accurate definitions are everything in science.)

The expanded polystyrene used in lids can fail in one of two modes. It can crush, where a lot of very small cracks appear within the bulk of the structure, or it can crack, where a few large cracks propagate thoughout the bulk of the structure. Bulk cracks such as those absorb little energy so do little to mitigate impact forces. The aim of the helmet designer is to ensure that the helmet will crush rather than crack. Worse, failure in brittle materials cannot be predicted accurately. If you don't believe me, try to guess how many peices will a glass shatter into when you drop it? This unpredictability means it is impossible to guarantee that a helmet will provide any more protection once it has failed. The fundamentals of materials science contradict your claims.

 

[Wobblers]

Still waiting on this one

I didn't realise I wasn't allowed to have any sleep or indeed a life...

Any chance of you showing evidence that you actually understand the concept of "torque"? (Hint, as swansonj has already pointed out pounds per foot is wrong)

 

[Linford]

More evasion perchance? I'm very glad the the Institute of Mechanical Engineers adopts the very sensible precaution of not awarding CEng status to just anyone with a pajero...

A brittle material is one which does not have any mechanism of crack stopping. Which means, once a crack is initiated, it will continue to grow - unless all stress is removed. A tough material does possess a crack stopping mechanism - in metals, this is because dislocations have mobility in all six degrees of freedom (though admittedly I only add that as I know it will annoy Sazzaa).

Now let's revisit this little gem:


Firstly, your chemistry: styrene is a rather gloopy liquid at room temperature. Its polymer, polystyrene is a solid. It's important to know the difference, isn't it? Because it would take a rather gifted muppet to try and make a helmet out of styrene...

Polystyrene is a brittle material. In other words, it behaves in an elastic fashion until its strength (tensile, shear or compressive, depending on the applied stress) is exceeded when it will crack. Chemical bonds are broken in this process, and new surfaces created: both absorb energy. The word "deform" unless qualified, means elastic strain. Elastic deformation absorbs almost no energy whatsoever, so will not provide any protection whatsoever. Your claim that it will is wrong. (Tip: accurate definitions are everything in science.)

The expanded polystyrene used in lids can fail in one of two modes. It can crush, where a lot of very small cracks appear within the bulk of the structure, or it can crack, where a few large cracks propagate thoughout the bulk of the structure. Bulk cracks such as those absorb little energy so do little to mitigate impact forces. The aim of the helmet designer is to ensure that the helmet will crush rather than crack. Worse, failure in brittle materials cannot be predicted accurately. If you don't believe me, try to guess how many peices will a glass shatter into when you drop it? This unpredictability means it is impossible to guarantee that a helmet will provide any more protection once it has failed. The fundamentals of materials science contradict your claims.

Good to see you appreciate the difference between polystyrene and expanded polystyrene. I used the term (expanded poly)'styrene' as an abbreviation for the sake of the debate (just so you know for future reference)....I thought you were intelligent enough to understand what exactly I was referring too in relation to the expanded foam used in all safety headgear as they pretty much all use the same stuff for the task of energy absorption.

You are implying now that the (expanded poly)styrene will not crush before reaching its shear point now, or that the foam in the shell will not only lose all of its structural integrity as a component, but also the foam pieces once divided inside the shell will also lose all of their mechanical properties ?

Now I put it to you again....if you feel that styrene is inadequate for the task of energy absorption in impacts, what is your alternative?.... in fact you must be in the wrong business because the safety headgear industry is worth billions each year....you need to educate the manufacturers that they don't know what they are doing....why not start with HJC...they are the worlds largest lid maker and manufacture millions of them each year ?

In fact why not approach the FIA and tell them what you think....and that all their drivers should abandon the use of crash helmets because you can prove through your peer reviewed pingpong ball lab test that when you exceed the shear point of a material that it does indeed shear.

Whilst we are about it, metal is a fairly rubbish absorber of kinetic energy, but is a very good translator of it....perhaps you might try this with a newtons cradle one day ?

 

[Linford]

And you don't feel that there would be any value in a walking, driving, using the stairs helmet?

Erm in the same way as putting on my biker leathers to sit in the theatre or go bowling ?

 

[Linford]

I didn't realise I wasn't allowed to have any sleep or indeed a life...

Any chance of you showing evidence that you actually understand the concept of "torque"? (Hint, as swansonj has already pointed out pounds per foot is wrong)

You mean in respect to how much force is required to move an object like a drive shaft or bolt around its Z axis....or in practical terms if it requires 1ft lb of torque to turn a nut to its required setting then the easiest way to do that would be to apply that 1lb of force to the end of a lever or fulcrum which is 1ft long.
I live in the real world mister and I solve problems on a daily basis which you might only ever read about in your books.

 

[PK99]

The expanded polystyrene used in lids can fail in one of two modes. It can crush, where a lot of very small cracks appear within the bulk of the structure, or it can crack, where a few large cracks propagate thoughout the bulk of the structure. Bulk cracks such as those absorb little energy so do little to mitigate impact forces. The aim of the helmet designer is to ensure that the helmet will crush rather than crack. .

But even if it cracks and therefore absorbs less energy: Do i want forces from striking the sharp edge of a kerb (of rock) directly impinging on my skull or do i prefer them spread over a longer time and wider area?

 

[theclaud]

I think if kids can get by wearing a helmet without whinging like little bitches about it, then grown ups should be able to. You are setting an example after all. Cyclists need to just get over it already, moany c****.

I reckon I'm setting a much better example by just cycling without a load of entirely unnecessary paraphernalia and obviously enjoying it. The only ones moaning are those trying to project their fears onto others. Wear a helmet (or a St Christopher) if it makes you feel safer, and leave the rest of us alone.

 

[theclaud]

Come on Linf, are you going to prove TMN wrong here?

New Year's resolution involving Hope?

 

[Linford]

That selfie @Linford, is that how you normally wear your helmet?

No it was taken off half way up a climb to cool off before the pic was taken

 

[theclaud]

Yep, it is to master riding the Brompton no hands.

Good luck with that. They are twitchy buggers.

 

[PK99]

Errr... no - the link you link to is simply saying look elsewhere for more information.

Care to point me at any documented case of rotational injury caused by a cycle helmet? Or study demonstrating the fact of same rather than speculation as to it being a possible a possible mechanism

The link says :

some doctors have expressed concern that cycle helmets might make some injuries worse by converting direct (linear) forces to rotational ones

I have searched and asked many times, but on one has ever been able to point to case studies which say any more than that.

 

[PK99]

Follow the links and read the research. It's all in there.

I have done - all the research is desk analysis or crash dummy testing. no documented cases from life.

 

[PK99]

You obviously haven't read all the evidence then, as as it isn't all desk analysis or crash dummy testing...

Care to point me to any documented case(s) of cyclists demonstrably having sustained rotational injuries as a result of wearing a helmet.

A simple enough request. A simple link will suffice as an answer.

 

[PK99]

I've pointed you to the links. They're in the cases cited and linked to on the very page you linked to.

If you're going to pronounce on the quality and quantity of evidence then it's up to you to prove your point - not me to do otherwise.

You are dissembling.

If there are links to medical cases, I have clearly missed them. Care to do me the courtesy of pointing out my omission? I would be very grateful.

 

[srw]

I've just said, because those activities feel safer than cycling and boarding.

Stuff "feel safer". What's the reality? There's oodles of statistics out there, and oodles of links on this forum alone proving (without a shadow of a doubt) that cycling is a very low risk activity.
[edit]
Sod that. I'm not giving you a get-out. Here's an excellent place to start:
http://www.cyclechat.net/threads/the-risks-of-cycling.146101/#post-2849822