Warning: H*lm*t content

Euan Wrote:
"Theo" == Theo Bekkers 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 | ~~ __@
Euan | ~~ _-\,
Melbourne, Australia | ~ (*)/ (*)
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 th
ground, that is what the helmet should absorb. The horizontal componen
gives rotation, you could argue that the helmet makes that worse, sinc
the radius of the helmet is bigger than the head. You could also argu
that the friction of the helmet against the road is lower, and tha
helps to minimise the rotation. It also gives road rash, where th
helmet does help. Again, if your horizontal component is 50 km/h an
you hit a boulder straight on, well, helmet or not, you die

--
Claes

Claes Wrote:
The MAJORITY of accidents, in Sweden at least, are single accidents. Th
only force absorbed is head towards the ground. Lets assume head hit
the ground at a VERTICAL speed of 20 km/h as stated before, an
suddenly, the helmet makes sense.


Vertical? I clipped a protruding building site stake and went straigh
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, havin
stripped of my gloves for the cooling, should I protect my hands wit
my head?
After all, my helmet will protect me

--
aeek

Euan wrote:
"ritcho" == ritcho 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

aeek Wrote:
Vertical? I clipped a protruding building site stake and went straigh
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, havin
stripped of my gloves for the cooling, should I protect my hands wit
my head?
After all, my helmet will protect me!
Again, if you hit something at high speed, horizontal speed, the impac
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 fla
road, your horizontal speed will not be as bad as the vertical speed a
which you hit the ground, that has got nothing to do with how fast yo
go. And again, a single accident is not that interesting

--
Claes

Euan Wrote:
"ritcho" == ritcho
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 | ~~ __@
Euan | ~~ _-\,
Melbourne, Australia | ~ (*)/ (*)

The point being that damage is a function of the rate at which energ
is absorbed and the area over which it is absorbed. The helmet doesn'
have to absorb all of the energy of an impact - it just has to sprea
it around space and time.

Ritc

--
ritcho

Euan wrote:
"Resound" == Resound 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 ...

Euan wrote:
"ritcho" == ritcho 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.

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?

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

Cheers

Geof

--
geoffs

geoffs Wrote:

She has seen numerous instances wear someone has completely smashe
their helmet but they are OK.
So it wont happen to you, it'll happen to someone else. You might b
the someone else.


so, in summer, I SHOULD protect my bare hands with my helmetted head

--
aeek