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EXTRACT
D.
For the table of contents of the
EXTRACT of the compendium.
Return to EXTRACT C
Part B: The influence of the
impact on a body inside the struck car. Continues.
Chapter B.3 The "Whiplash" mechanism.
If there is a difference in the velocity of the
car and the person sitting in a seat inside, an impact between
the car and the person must occur.
During an acceleration there will be a delay between
the start of acceleration of the car and the seated body inside
the car. The delay must occur due to the flexibility of the seat
structure as well as the body itself and as a result of how the
person is seated. The longer distance the car is allowed to move
until the acceleration of the body starts, the larger will the
difference of the velocities be.
The loads and the change of velocity of the body
inside the car depends both on the impact between the two cars
involved and how the person inside the hit car are seated as well
as the mechanical properties of the car seat.
In an impact between a car and a person the mass
of the person will be very low compared to the mass of the car
making the probability of significant change of the velocity of
the person even at low velocities of the car.
 
Fig. B.5 a-d shows the motion of a modeled body
caused by acting forces from the seat towards the back of the
model. The impulses towards the body starts with the section in
the closest contact with the seat, section a. Section b and c
will get a delay in time and they will together with the rest
of the "body" for a short time have a lower velocity
than section a. They are accelerated by a force acting from the
seat as the seat reaches the back and is deflected, and by longitudinal
forces acting between the sections of the "body", or
a combination of these forces.
In the beginning of this document I made a definition
of the "whiplash-mechanism" affecting a head to be a
mechanism involving a free movement of the head related to the
body. This free movement might be caused by an impact acting onto
the head giving it a velocity higher than the rest of the body
or it might be caused as the rest of the body is affected by an
impact giving a difference in velocity between the head and the
body.
1. A series of impacts.
I find it reasonable to describe the whiplash
mechanism in a rear-end collision as a mechanism where the body
is affected by a pressure from the back of seat towards the body.
This pressure moves upwards as the back of seat comes in touch
with higher elements of the body. In a rear- end collision, except
in a collision at an extremely low velocity, this acceleration
of the body will take place at the same time as the car it self
is increasing it's velocity. Because of the different weights
of the body elements, but first of all because of a higher velocity
of car when the latest elements are affected by the accelerating
forces, the different elements of body will reach different forward
velocities. The term "Whiplash-mechanism" is an appropriate
description of this kind of mechanism.
Then the body is slowed down in it's forward movement
when the safety belts are tightened or the body meets parts of
the car as the steering wheel or the dashboard etc. At the end
of this impact the velocity of the body is below the velocity
of the car and it will once more be picked up by the seat causing
another chain of impacts. This series of impacts will go on until
the velocity of the body and the car are equalized.
The number of relative forth and back throws by
which the body are affected during the collision depends on factors
as the velocity of the striking car and the ratio of mass of the
cars involved, the elastic abilities of the seat and how the person
is seated and how the car is moving.
Though the stiffness of cars does not influence
on the change of their velocities, an increasing structural strength
reducing the period of time the impact between the cars lasts,
will influence on the velocity of struck car as the body is accelerated
causing an increasing probability of high acceleration loads affecting
the body as the strength of cars increase.
If the car is moving freely with no brakes on
or the gear is in free and no chain collision into another car
in front happens, the difference in the velocity of the car and
the person seated as the forward motion of the body is slowed
down may be low minimizing the impact between the body and the
car acting through the safety belt and between parts of the body
as well as proceeding loads affecting the body during the "whiplash-mechanism".
The slow down of forward motion of the body is
a mechanism you will find in frontal collisions too. But there
are differences between the mechanism in an incident where the
car is going through a reduction of it's velocity and if it's
increasing it's velocity like in a rear-end collision, differences
which results in high loads on parts of the accelerated body even
in collisions at low velocities. I will explain this difference
in a later chapter. Of course I would prefer to explain this mechanism
in a simple way. But such a mechanism must be rather complex necessarily
making an explanation of how it works rather complex.
A commonly accepted statement:
"The head is thrown towards the seat" ?
But: "Is the
body and so the head really thrown backwards towards the back
of the seat or the "neck support" during a rear end
collision?"
This question may seem unimportant. The acting
forces between the body and the seat will not be changed if it
is the seat which accelerates the body as the body is picked up
by the seat or the body is thrown towards the seat as long as
the difference in the velocities between the body and the car
is not changed. But to understand what happens to a body during
such an event it's of interest to know if the body is thrown towards
the back of the seat or if the body is picked up by the seat in
a series of impacts.
To those who will claim that the body is thrown
towards the seat and even colleagues do, I will ask: "What
force is throwing the body backwards?" The answer
is well known: "It's the inertial force."
As I have already stressed: " The inertial
force is a reaction to the acting force." It's
only the body's resistance against changes of it's velocity, not
a force able to do any work. The only acting forces are the forces
acting in the same direction like the acceleration of the car.
3. The head is thrown forward in a curved path.
Depending on the elastic abilities of the seat
structures and the properties of the "head- support"
the forward velocity of head will be higher than the velocity
of any other part of the body. The forward motion of the head
motion is along a curved path with a centre of rotation shifting
vertically up and down along the spine as the inclination of the
neck region change.
As a result of such a curved path the neck region
will be affected by stretching forces during this forward movement,
varying with the relative velocity of the head and the inclination
of the neck, additional to the accelerating forces acting through
the neck.
In a mechanism like the "whiplash-mechanism"
where the head is swinging freely in a curved path, the neck region
is affected by changing stretching loads both in the first acceleration
of the head in Phase A, in the retardation of Phase B, the proceeding
pick up in Phase C as well as in the relative forward motion the
head.
4. The influence of how the person
is seated.
The reason for the appearance of the "whiplash"
mechanism is the delay between the acceleration of the car and
the body inside it. This delay makes a difference in the velocities
of the car and the body which will cause impacts between the car
and the body. If there was no delay as it will be only if the
body is fixed to the car, no difference of velocities would occur.
The difference in velocities will, in all collisions except at
very low velocities, increase with an increasing delay.
To reduce the impacts between the car and the
body, acting through the seat, the delay of the acceleration has
to be minimized. Because of the elasticity of the seat structure
it is impossible to avoid every delay. But seated with an upraised
position of the back of the seat, and in a seat giving a firm
support to all parts of the body, this delay will be as small
as possible.
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