On 7/2/04 9:36 pm, in article
[email protected],
"David Kerber" <ns_dkerber@ns_ids.net> wrote:
> In article <[email protected]>,
> [email protected] says...
>> On Sat, 7 Feb 2004 13:15:06 -0500, David Kerber
>> <ns_dkerber@ns_ids.net> wrote in message
>> <[email protected]>:
>>
>>> If you hit a rock or tree at 60mph, it's not
>>> going to matter one bit whether you have that helmet on or not. At 20
>>> or 25, it probably would because the helmet will absorb a significant
>>> portion of the impact energy.
>>
>> Up to a point, Lord Copper. Actually a helmet will absorb the energy
>> of a fall onto a flat surface from a standing start at about 5'4"
>> height.
>
> Exactly. And that leaves that much less energy to be transmitted to my
> skull. It won't absorb it all, but it will absorb whatever it can,
> reducing what hits my scull.
Do the math. E=1/2 mv^2 vs E= 1/2 m(v)^2 - (1/2 x 7 (12)^2)
(design spec for a helmet is to reduce an impact at 12mph of just the head
to a just non-lethal for 50% estimated decelleration on a linear impact)
Using rather nasty non SI units (mph and kilograms)
I get the following equivalent speeds:
Speed just head full bodyweight
5 0 3.52
10 0 9.35
15 9 14.6
20 16 19.7
25 21.9 24.75
30 27.5 29.8
Let me explain these. The first column is the speed at impact. 5 and 10 are
equivalent to falls from a seated position on floor or chair, well below the
height of a straight fall on a bike.
The second column is the equivalent speed of a head impact if the only
momentum the helmet is protecting against is the standard 7kg head.
The third column is the effective speed of an impact if the helmet is having
to decelerate the whole body (assuming an 80kg rigid body.) (bear in mind
that the bodyweight has no effect on the unhelmeted head)
These numbers are not terribly convincing for the utility of helmets.
Bear in mind that these assume certain things.
1. a rigid body
2. a linear impact. If there is a rotational component all bets are off. It
takes 1/50th the force to provide damage with rotational impacts than it
does with linear impacts. Does the extra bulk and nature of construction of
a helmet contribute to rotational injuries?
Given the lack of effectiveness of helmets in preventing serious head
injuries (including fatalities) the answer may well be yes.
Personally I wear a helmet when there is a reasonable risk of falling off at
slow speeds with linear impacts, ie when I am riding off road (I am quite
cautions so am not a downhill MTBer, just someone who likes to get out for a
pleasant ride.) or in the winter when there is ice on the ground. My
children wear helmets as they are still likely to fall off at slow speeds (7
and 5 years old) but I will not force them to wear them when they are
confident on two wheels and reasonably aware of the risks of crashes.
I don't wear a helmet most of the time I ride the bike. I do not 'just fall
off' on the road. If I do have a collision it will be one where the speed
will be relatively high and the energies involved will make a bike helmet as
useful as a teacup for bailing the titanic. Given my accident record and
riding style, I don't see that as a problem to be concerned with. I have
bumped my head more times just walking than cycling.
Might a helmet make a difference in the unlikely event of a serious
accident? Possibly, but whether it would be a positive or a negative
difference is very hard to predict. I'll quite happily trade off a few
scrapes and bruises versus serious brain injury.
...d
I nominate this for post of the
