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Discussion in 'Australia and New Zealand' started by Euan, Aug 21, 2005.

  1. ritcho

    ritcho New Member

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    This doesn't smell right - surely it is the distribution of the energy of an impact through time and across an area that determine the likelihood of damage. You have shown that the distribution of energy through time is little changed, especially for higher speed impacts, but not shown that energy is dissipated across a wider area of the head. For example, a 20kg plate can be supported by balancing it on your head, but put a nail in the centre of the plate and you'll pierce a nice hole in your head if you try to balance it in the same way. This example says nothing about velocity, but something about the distribution of force...

    Ritch
     


  2. Claes

    Claes New Member

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    You argue shite again I think.
    If you come to a dead stop a 50 km/h, the helmet will not save you, agreed. But that was never the case. You fronting with a car or truck, helmet will probably not save you either. The MAJORITY of accidents, in Sweden at least, are single accidents. The only force absorbed is head towards the ground. Lets assume head hits the ground at a VERTICAL speed of 20 km/h as stated before, and suddenly, the helmet makes sense.

    You weighing more than 10 kgs is not relevant either in most cases. Your body is not a stiff metal rod connected to your head. You will not transfer the energy the same way so your example is shite once again.

    Your head weighs about 5 kgs. We can argue for ever about how the kinetics work, but that will not help. From sweden there is statistics, and we do NOT have a helmet law, that 40 % of the cycling accidents are head injuries where helmets COULD help. The MAJORITY, of accidents are single accidents too. That is sweden, so it seems like helmets could help. No?
     
  3. Kim

    Kim Guest

    Euan wrote:
    >>>>>>"Bob" == Bob <[email protected]> writes:

    > Bob> Everyone do what they want, legally or otherwise, I will
    > Bob> continue to wear a helmet that may save my life.
    >
    > That's a very big may. I prefer not to entrust my safety to what is
    > essentially a piece of polystyrene designed to absorb the kinetic energy
    > of a fall from head height. That's all it does.


    all my helmet did for me when i was hit by a car was put a nasty big
    hole in a windscreen of a car that ran a stop sign and collected me...
    i was fine ;) the driver was furious. bike trashed the bumper on the
    car, bent the bonnet, hole in windscreen and i had to replace a
    derailuer. anyone who prattles on that helmets do nothing obviously are
    clinically insane. i'm quite willing to be unfashionable =P

    cheers,

    kim
     
  4. Claes

    Claes New Member

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    More stats from another source.
    Swedish statiscs from january 1997 - june 2000 shows that
    91 people died while cycling in southern sweden. Only 7 wore helmets.
    The statistics for helmet usage in the area around the period was 17-18 %.

    Assuming that the helmet had no effect, the number of dead with helmet should have been 16, we have less than half that. That indicates that helmet does indeed work.
     
  5. Euan

    Euan Guest

    >>>>> "Resound" == Resound <[email protected]> writes:

    >> Bicycle helmets absorb kinetic energy (KE). The formula for KE
    >> is:
    >>
    >> KE = 1/2 * M * V^2
    >>


    Resound> That does make a bit of difference, dunnit? I do wonder how
    Resound> constant the energy dispersion of a helmet relative to
    Resound> speed is though. Probably not a squared function though.

    No idea, I'm not an engineer. I've just got basic physics under my belt
    and I can remember some equations and Google what I can't :)
    --
    Cheers | ~~ [email protected]
    Euan | ~~ _-\<,
    Melbourne, Australia | ~ (*)/ (*)
     
  6. Euan

    Euan Guest

    >>>>> "Theo" == Theo Bekkers <[email protected]> writes:

    Theo> Resound wrote:
    >> And, importantly, it's only recently that we've been moving at
    >> greater than running speed. Hit the ground at 20kph and you're
    >> okelydokely. Hit the ground at 40kph and you're much more likely
    >> to break something important. Not always of course, but doubling
    >> impact speed is always going to skew your results more than a
    >> touch.


    Theo> Err, if you fall off your bike you will hit the ground at
    Theo> approx 20km/h regardless of the speed at which you are
    Theo> travelling. This is the design spec of bike helmets. Should
    Theo> you have a horizontal velocity of 40 km/h you will still hit
    Theo> the ground at 20km/h.

    I don't think that's correct.

    When there are two or more velocities what we have a vectors. We have
    the horizontal component (40km/h) and the vertical component. The
    vector simplistically is the root of the sum of the horizontal squared
    and the vertical squared.

    For the cited figures that gives a velocity of 44km/h on point of
    impact.

    A combination of kinetic absorption and friction dissipates the
    velocity.
    --
    Cheers | ~~ [email protected]
    Euan | ~~ _-\<,
    Melbourne, Australia | ~ (*)/ (*)
     
  7. Euan

    Euan Guest

    >>>>> "ritcho" == ritcho <[email protected]> writes:

    ritcho> Euan Wrote:
    >> Bicycle helmets absorb kinetic energy (KE). The formula for KE
    >> is:
    >>
    >> KE = 1/2 * M * V^2


    ritcho> This doesn't smell right - surely it is the distribution of
    ritcho> the energy of an impact through time and across an area that
    ritcho> determine the likelihood of damage. You have shown that the
    ritcho> distribution of energy through time is little changed,
    ritcho> especially for higher speed impacts, but not shown that
    ritcho> energy is dissipated across a wider area of the head.

    It's kinetic energy. Area is not a factor in kinetic energy. It's an
    absolute figure. A helmet has X kinetic energy absorption capacity.

    ritcho> For example, a 20kg plate can be supported by balancing it
    ritcho> on your head, but put a nail in the centre of the plate and
    ritcho> you'll pierce a nice hole in your head if you try to balance
    ritcho> it in the same way. This example says nothing about
    ritcho> velocity, but something about the distribution of force...

    That's correct, force. That's different from kinetic energy and
    depending what you're trying to calculate there are many different
    equations.
    --
    Cheers | ~~ [email protected]
    Euan | ~~ _-\<,
    Melbourne, Australia | ~ (*)/ (*)
     
  8. Euan

    Euan Guest

    >>>>> "Claes" == Claes <[email protected]> writes:

    Claes> Euan Wrote:
    >> >>>>> "Claes" == Claes

    >> <[email protected]> writes:
    >>
    >>

    Claes> It seems that statistics can not solve this one. How about a
    Claes> simple test. You wear nothing on your head, I smack a
    Claes> baseball bat on your head, just hard enough to crack you
    Claes> scull, then we do a test with your head again, healed up and
    Claes> all, and smack at the same force, you think you head would
    Claes> not crack this time?
    >> You would have to have a very fine gradient in the velocity of
    >> the baseball bat.
    >>
    >> Bicycle helmets absorb kinetic energy (KE). The formula for KE
    >> is:
    >>
    >> KE = 1/2 * M * V^2
    >>
    >> It's tempting to think that a bicycle helmet that's rated for a
    >> 19 km/h impact will take 19km/h off of any impact speed and make
    >> a difference. This isn't the case.
    >>
    >> Let's say the mass is 10kg and the velocity is 19km/h. The
    >> kinetic energy is 1805.
    >>
    >> Now let's take an impact at 40km/h. The kinetic energy is 8,000.
    >>
    >> So we take away the 1805 from the 8,000 which leaves 6,195.
    >>
    >> Re-arranging the equation a bit we can find out how much speed
    >> the helmet's taken off the impact. The effective speed of the
    >> impact is 35.2km/h.
    >>
    >> The higher the impact speed, the more ineffective the helmet is
    >> and it's an exponential curve. At 60km/h the effective speed of
    >> impact is 56.9km/h. At 80km/h the effective speed of impact is
    >> 77.7km/h
    >>
    >> I ride consistently at speeds over 35km/h. A collision at that
    >> speed whilst wearing a helmet would make the collision speed
    >> 29.39km/h. I don't think that's going to make a huge difference
    >> to the extent of a head injury incurred, but that's a personal
    >> judgement.
    >>
    >> Add in the fact that I weigh considerably more than 10kg and that
    >> makes a helmet almost irrelevant. -- Cheers | ~~ [email protected] Euan | ~~
    >> _-\<, Melbourne, Australia | ~ (*)/ (*)


    Claes> You argue shite again I think. If you come to a dead stop a
    Claes> 50 km/h, the helmet will not save you, agreed. But that was
    Claes> never the case. You fronting with a car or truck, helmet will
    Claes> probably not save you either. The MAJORITY of accidents, in
    Claes> Sweden at least, are single accidents. The only force
    Claes> absorbed is head towards the ground. Lets assume head hits
    Claes> the ground at a VERTICAL speed of 20 km/h as stated before,
    Claes> and suddenly, the helmet makes sense.

    You forgot the horizontal component, that can be significant. See
    earlier post about vectors.

    Claes> You weighing more than 10 kgs is not relevant either in most
    Claes> cases. Your body is not a stiff metal rod connected to your
    Claes> head. You will not transfer the energy the same way so your
    Claes> example is shite once again.

    If I land head first on my head then a significant amount of my mass
    will be transmitted through my head. Tell you what, drop yourself off
    a ladder head first from two meters wearing a helmet.

    Claes> Your head weighs about 5 kgs. We can argue for ever about how
    Claes> the kinetics work, but that will not help. From sweden there
    Claes> is statistics, and we do NOT have a helmet law, that 40 % of
    Claes> the cycling accidents are head injuries where helmets COULD
    Claes> help. The MAJORITY, of accidents are single accidents
    Claes> too. That is sweden, so it seems like helmets could help. No?

    Could of, would of. Those are not facts.
    --
    Cheers | ~~ [email protected]
    Euan | ~~ _-\<,
    Melbourne, Australia | ~ (*)/ (*)
     
  9. Euan

    Euan Guest

    >>>>> "Claes" == Claes <[email protected]> writes:

    Claes> More stats from another source. Swedish statiscs from
    Claes> january 1997 - june 2000 shows that 91 people died while
    Claes> cycling in southern sweden. Only 7 wore helmets. The
    Claes> statistics for helmet usage in the area around the period was
    Claes> 17-18 %.

    Claes> Assuming that the helmet had no effect, the number of dead
    Claes> with helmet should have been 16, we have less than half
    Claes> that. That indicates that helmet does indeed work.

    Maybe it's because it's late, but I'm not following your math. Please
    elaborate.
    --
    Cheers | ~~ [email protected]
    Euan | ~~ _-\<,
    Melbourne, Australia | ~ (*)/ (*)
     
  10. Euan

    Euan Guest

    >>>>> "Claes" == Claes <[email protected]> writes:

    >> The human brain is not a vegetable. It's a highly sophisticated
    >> organ which is highly protected by a thick skull and in-built
    >> shock absorption. Comparing the two is like comparing apples and
    >> oranges.


    Claes> He he, it was an example, nothing else, read it and apply
    Claes> "critical thinking" to it.

    >> Please read http://www.cyclehelmets.org/papers/c2022.pdf
    >>
    >> Then come back to me and explain to me the case for helmet
    >> compulsion when it's proved beyond all doubt that helmet
    >> compulsion discourages cycling and therefore increases the risk
    >> per kilometer cycled because there are less cyclists on the road.


    Claes> Ehh, what that does that prove? You can not prove what would
    Claes> have happened without the helmets. Too many other variables
    Claes> change, and many are not included. That report is total BS.

    You can't dismiss a report as total BS without substantiating that
    claim. Which facts in the report do you question?
    --
    Cheers | ~~ [email protected]
    Euan | ~~ _-\<,
    Melbourne, Australia | ~ (*)/ (*)
     
  11. Claes

    Claes New Member

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    Why do you get in to vectors when you do not know what they mean?
    The vertical component of it, is what give you impact against the ground, that is what the helmet should absorb. The horizontal component gives rotation, you could argue that the helmet makes that worse, since the radius of the helmet is bigger than the head. You could also argue that the friction of the helmet against the road is lower, and that helps to minimise the rotation. It also gives road rash, where the helmet does help. Again, if your horizontal component is 50 km/h and you hit a boulder straight on, well, helmet or not, you die.
     
  12. aeek

    aeek New Member

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    Vertical? I clipped a protruding building site stake and went straight over the bars with half a twist. Landed flat. The impact was 98% horizontal. That's what cut the back of my head.

    Now, when I am crawling along on a stinking hot summer day, having stripped of my gloves for the cooling, should I protect my hands with my head?
    After all, my helmet will protect me!
     
  13. Zoom

    Zoom Guest

    Euan wrote:
    >>>>>>"ritcho" == ritcho <[email protected]> writes:

    >
    >
    > ritcho> Euan Wrote:
    > >> Bicycle helmets absorb kinetic energy (KE). The formula for KE
    > >> is:
    > >>
    > >> KE = 1/2 * M * V^2

    >
    > ritcho> This doesn't smell right - surely it is the distribution of
    > ritcho> the energy of an impact through time and across an area that
    > ritcho> determine the likelihood of damage. You have shown that the
    > ritcho> distribution of energy through time is little changed,
    > ritcho> especially for higher speed impacts, but not shown that
    > ritcho> energy is dissipated across a wider area of the head.
    >
    > It's kinetic energy. Area is not a factor in kinetic energy. It's an
    > absolute figure. A helmet has X kinetic energy absorption capacity.
    >
    > ritcho> For example, a 20kg plate can be supported by balancing it
    > ritcho> on your head, but put a nail in the centre of the plate and
    > ritcho> you'll pierce a nice hole in your head if you try to balance
    > ritcho> it in the same way. This example says nothing about
    > ritcho> velocity, but something about the distribution of force...
    >
    > That's correct, force. That's different from kinetic energy and
    > depending what you're trying to calculate there are many different
    > equations.


    You're thinking of pressure. Pressure is force over an area.
    If you hit your head on the pavement without a helmet your head cracks
    open because a high pressure is applied to a small area of the skull,
    but with a helmet the force is spread over a larger area and the
    pressure is reduced, and that's why you only end up with a bit of a
    headache.

    Zoom

    Zoom
     
  14. Claes

    Claes New Member

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    Again, if you hit something at high speed, horizontal speed, the impact speed will be roughly the same as the speed that you are travelling. Now, when you just plainly fall, or go over the hanldbars on a flat road, your horizontal speed will not be as bad as the vertical speed at which you hit the ground, that has got nothing to do with how fast you go. And again, a single accident is not that interesting.
     
  15. ritcho

    ritcho New Member

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    The point being that damage is a function of the rate at which energy is absorbed and the area over which it is absorbed. The helmet doesn't have to absorb all of the energy of an impact - it just has to spread it around space and time.

    Ritch
     
  16. aeek

    aeek New Member

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    My impact was almost all horizontal, which meant it was glancing, so minor.
    I think your horizontal/vertical distinction is meaningless. What matters is direct/glancing.

    ===

    Helmets in cars - I have about 1cm ceiling clearance. A helmet is going to wedge my head into the roof. Seems like a bad idea.
     
  17. Bleve

    Bleve Guest

    Euan wrote:
    > >>>>> "Resound" == Resound <[email protected]> writes:

    >
    > >> Bicycle helmets absorb kinetic energy (KE). The formula for KE
    > >> is:
    > >>
    > >> KE = 1/2 * M * V^2
    > >>

    >
    > Resound> That does make a bit of difference, dunnit? I do wonder how
    > Resound> constant the energy dispersion of a helmet relative to
    > Resound> speed is though. Probably not a squared function though.
    >
    > No idea, I'm not an engineer. I've just got basic physics under my belt
    > and I can remember some equations and Google what I can't :)


    You also forget that forces work in directions. 35km/h horizontally
    is mostly irrelevant* when you fall down from 2m under the influence
    of gravity. A bike helmet won't do squat at 35km/h to dead stop,
    but that's not the point.

    Back to your sums, Euan :)

    * yes, rolling adds repeated impacts, all the more reason to have a
    helmet on ...
     
  18. Bleve

    Bleve Guest

    Euan wrote:
    > >>>>> "ritcho" == ritcho <[email protected]> writes:

    >
    > ritcho> Euan Wrote:
    > >> Bicycle helmets absorb kinetic energy (KE). The formula for KE
    > >> is:
    > >>
    > >> KE = 1/2 * M * V^2

    >
    > ritcho> This doesn't smell right - surely it is the distribution of
    > ritcho> the energy of an impact through time and across an area that
    > ritcho> determine the likelihood of damage. You have shown that the
    > ritcho> distribution of energy through time is little changed,
    > ritcho> especially for higher speed impacts, but not shown that
    > ritcho> energy is dissipated across a wider area of the head.
    >
    > It's kinetic energy. Area is not a factor in kinetic energy. It's an
    > absolute figure. A helmet has X kinetic energy absorption capacity.
    >
    > ritcho> For example, a 20kg plate can be supported by balancing it
    > ritcho> on your head, but put a nail in the centre of the plate and
    > ritcho> you'll pierce a nice hole in your head if you try to balance
    > ritcho> it in the same way. This example says nothing about
    > ritcho> velocity, but something about the distribution of force...
    >
    > That's correct, force. That's different from kinetic energy and
    > depending what you're trying to calculate there are many different
    > equations.


    One of the advantages of a helmet (or any device designed to lessen
    point impacts) is that pressure (which does a lot of damage, eg nails
    cv dinner plates) is reduced. That square law you're thinking about
    wrt KE, well, pressure = force/area, and area is a square function
    also. The rest is left as an exercise to the reader.
     
  19. Bleve

    Bleve Guest

    David Trudgett wrote:

    > Option 1: Compulsory xyz
    > Option 2: Refuse healthcare
    >
    > Unfortunately, both your options are unChristian.


    I am not a christian.

    This is aus.bicyles, religious argument really doesn't
    belong, eh?
     
  20. geoffs

    geoffs New Member

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    So if you don't like to wear a helmet don't wear one! hey it only affects you and it's your decision to make and they can do terrible things to your hairstyle!
    Wrong
    My better half is an emergency staff specialist at one of the main teaching hospitals in Sydney.
    If you get permanent brain damage you are fscked. You will be a burden on the community and your family for the rest of your life. The tax payers will be paying to suport you through your long rehab, if you are able to be rehabilitated. This does happen!!
    She has seen numerous instances wear someone has completely smashed their helmet but they are OK.
    So it wont happen to you, it'll happen to someone else. You might be the someone else.

    Cheers

    Geoff
     
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