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Energy-Absorbing C
ar
Seat
Designs
for
R
educing
Whiplash
S. HIMMETOGLU, M. ACAR, K. BOUAZZA-MAROUF, and A. J. TAYLOR
Wolfson School of Mechanical and Manufacturing Engineering, Loughborough University, Loughborough, Leicestershire, UK
Objectives: This study presents an investigation of anti-whiplash features that can be implemented in a car seat to reduce
whiplash injuries in the case of a rear impact. The main emphasis is on achieving a seat design with good energy absorption
properties.
Methods: A biofidelic 50th percentile male multi-body human model for rear impact is developed to evaluate the
performance of car seat design concepts. The model is validated using the responses of 7 volunteers from the Japanese
Automobile Research Institute (JARI) sled tests, which were performed at an impact speed of 8 kph with a rigid seat and
without head restraint and seatbelt. A generic multi-body car seat model is also developed to implement various seatback
and recliner properties, anti-whiplash devices, and head restraints. Using the same driving posture and the rigid seat in the
JARI sled tests as the basic
configuration,
several anti-whiplash seats are designed to allow different types of motion for the
seatback and seat-pan.
Results: The anti-whiplash car seat design concepts limit neck internal motion successfully until the head-to–head
restraint contact occurs and they exhibit low NICmax values (7 m2/s2 on average). They are also effective in reducing neck
compression forces and T1 forward accelerations. In principle, these car seat design concepts employ controlled recliner
rotation and seat-pan displacement to limit the formation of S-shape. This is accomplished by using anti-whiplash devices
that absorb the crash energy in such a way that an optimum protection is provided at different severities.
Conclusions: The results indicate that the energy absorbing car seat design concepts all demonstrate good whiplash-
reducing performances at the IIWPG standard pulse. Especially in higher severity rear impacts, two of the car seat design
concepts reduce the ramping of the occupant considerably.
Keywords Whiplash; Car Seat Design; Rear Impact; Human Body Model; Head-and-Neck Model
INTRODUCTION
Injury to the human neck is a frequent consequence of road traffic accidents. The term whiplash is used to describe these
injuries or disorders in which the sudden differential move- ment between the head and torso leads to damage of soft tissue
in the neck. The annual economic cost of whiplash injury has been estimated to be $8.2 billion in the United States
(Edwards et al., 2005) and $1.2 billion in the U.K. (Avery et al., 2007). The highest risk of sustaining whiplash injury has
been found to occur in rear-end collisions (Av- ery et al., 2007; Jakobsson et al., 2000; Watanabe and Ito,
2007).
Although improving head restraint geometry is the first step in reducing injury risk in case of a rear impact, research has
shown that seats with good head restraint geometry do not always offer good protection dynamically. If the seat is not
properly designed, the occupant can deflect the seatback and head restraint unfavorably. This can delay head contact time with
the head restraint and lead to higher neck loads. The ramp- ing of the occupant becomes worse at relatively higher impact severities
especially if no seatbelt is worn. In such cases the head restraint may not be able to restrict the motion of the head. Moreover,
seatbacks with strong structural cross-members do not allow the occupant to sink into the seatback; this hinders en- ergy absorption
and acts against reducing the backset between the head and the head restraint. Such structural cross-members can load the upper
torso severely and lead to S-shape deforma- tion in the neck. A strong rebound of the seatback can also exac- erbate injury, especially
in higher severity rear impacts. These problems can be overcome by designing seats that provide good energy absorption
and/or
early
head support as recommended by
International Insurance Whiplash Prevention Group
(IIWPG,
2006).
Figure 1 The human body model in its initial position.
METHODS
Design and Validation of a 50th Percentile Male
Multi-Body
Rear-Impact Human Body Model
The human body model has been developed by using MSC Vi- sualNastran 4D with Matlab-Simulink, and validated using the
responses of 7 volunteers from the JARI (Japanese Automobile Research Institute) sled tests (Himmetoglu, 2008), which were
performed at an impact speed of 8 kph with a rigid seat and without head restraint and seatbelt (Davidsson et al., 1999). The
human body model as shown in Figure 1 is composed of rigid bodies connected by rotational springs and dampers. The body shape
of the
human
body model is based on the
typical
or normal driving posture of an average 50th percentile male (Schneider et al., 1983).
The head-and-neck section of the human body model was separately validated by specifying the motion of T1 (the first thoracic
vertebra) as obtained from the JARI sled tests. The head-and-neck model, which was shown to simulate the effects of active muscle
response, is described in detail by Himmetoglu et al. (2007).
The torso model is composed of five bodies with the loca- tions of the joints chosen by analyzing the spinal vertebra
and
pelvis
rotations of the JARI sled test volunteers (Ono et al.,
1999). The vertebrae that rotated together as a unit are grouped as one separate body. The torso joints are placed approximately at the
anatomical locations
of T3 (the third
thoracic vertebra),
T5
Figure 2 T1 x
displacements
(- - - - Hybrid III,
BioRID
P3, TNO, model).
Figure 3 T1 z
displacements
(- - - - Hybrid III, BioRID P3, TNO, model).
(the
fifth
thoracic vertebra), T11/T12 (between the eleventh and twelfth thoracic vertebrae), and L3/L4 (between the third
and
fourth
lumbar vertebrae). For the neck joints, a time-varying damping coefficient function based on the recorded EMG re-
sponse of the neck muscles was found to better represent the volunteer responses. The damping functions for the torso joints also
vary in time and this is considered to reflect the equivalent increase in resistance at the joints due to muscle contraction
(Himmetoglu, 2008).
The responses of the proposed human body model are val- idated using the JARI volunteer responses, as provided by van der
Horst (2002). In Figures 2 to 5, the model responses are shown together with the responses of the JARI volunteers (grey lines) and
the responses of Hybrid III and BioRID P3 dummies and TNO model that had been
subjected
to the same impact con- ditions. BioRID
P3 and HIII (Hybrid III) responses are given by Davidsson et al. (1999). TNO responses indicate the behavior of the human body
model of TNO Automotive combined with the detailed head-and-neck model developed by van der Horst (2002).
Figures 2 and 3 show the displacement of T1 relative to the sled in the x and z directions, respectively, expressed in the
inertial coordinate system SG shown in Figure 1. Figure 4 displays the head angle with respect to T1. Figure 5 depicts the
Figure 4 Head angles wrt T1 (- - - - Hybrid III, BioRID P3, TNO, model).
Figure 5 OC x displacements wrt T1 (- - - - Hybrid III, BioRID P3, TNO, model).
displacement of OC (occipital condyles) in the x direction with respect to T1, expressed in the T1 anatomical coordinate system
attached to T1 as shown in Figure 1. Figures 2 to 5 demonstrate that the human body model shows biofidelic behavior of the head-
and-neck motion when subjected to the same rear-impact conditions as in the JARI volunteer sled tests.
Car Seat Design Methodology
An
anti-whiplash
seat should absorb as much energy as possible while reducing the occupant acceleration and minimizing the
relative movements between the adjacent cervical vertebrae. The following design criteria are considered to be essential for a car
seat equipped with anti-whiplash features:
1. Good head restraint geometry in terms of head restraint height and backset
2. Effective crash energy-absorbing characteristics
3. Minimum neck internal motion (OC relative to T1 motion), reduced S-shape (or retraction)
4. Low neck forces (compression, tensile, shear) and moments
5. Reduced ramping
6. Minimum rearward displacement of the seat
7. Limited seatback rebound
8. No activation of anti-whiplash devices during normal use
9. Improved performance at all impact severities
It is well established that a head restraint with good stiff-
ness and energy-absorbing characteristics, positioned at the right
height
and with a small backset distance, would
significantly
re-
duce whiplash risk. Therefore, this study focuses
on
the
develop- ment
of seat designs with good
energy-absorbing
characteristics that can later be combined with a good head restraint. Hence, in the
computational simulations, head restraints are not included. Moreover, without the help of a head restraint, the effectiveness of seat
designs in limiting neck internal motion can be better
identified.
In addition, as in the JARI volunteer sled tests, a seat- belt is also
not included in the models. Forward rebound is also minimized by using high damping
characteristics
in the forward direction.
Using the same driving posture as in the JARI volunteer sled tests (Davidsson et al., 1999) and the rigid seat model, several anti-
whiplash car seat design concepts have been considered. The rigid seatback without a head restraint and seatbelt can be
regarded as one of the worst systems for rear impact. A rigid seatback could imitate, to some extent, the adverse effects of a
seatback with strong structural cross-members and very stiff foam that do not allow the torso to sink into the seatback
significantly. Without the seatbelt, the occupant runs the risk of ramping up the seatback and even ejecting in high severity rear
impacts. Developing seat design concepts with a rigid seat- back can be considered as a practical approach in multi-body dynamic
modeling, since the seatback stays rigid for all con- ditions, while in the case of a typical car seat with a degree of frame
compliance, foam stiffness and suspension movement, the dynamic characteristics need to be correctly estimated for all impact
speeds.
A high severity crash pulse of b.V (delta-V)
=
35 kph with
mean and peak accelerations of 7.1 and 16 g, respectively, is used to set the limits for the rotation of the seatback and the seat
rearward displacement in order to prevent ejection and im- pact with the rear seat. This crash pulse, which is derived from
FMVSS301 flat moving barrier test results (Viano, 2002), rep- resents quite a severe case. Therefore, at this extreme condition, the
maximum seat-pan rearward displacement allowed is set to be 10 cm and the
maximum seatback angle allowed from
the ver- tical for
the retention of unbelted occupant is set as 40 degrees. This amount of rotation is based on the results of the human body model
simulations performed at this severe pulse with an initial seatback angle of 20 degrees from the vertical and with a friction
coefficient of 0.35 for all surfaces between the hu- man body and the seat. This friction coefficient is based on the experimental
data given by Verver (2004).
Test Procedure
In order to test the car seat design concepts using the human body model, the hands and arms are positioned as shown in
Figure 1 to adopt a posture practiced in whiplash dynamic tests (IIWPG, 2006). A head restraint, called WMHR, is attached to the
seatback but in the simulations the head is allowed to pen- etrate WMHR freely without resistance; hence, it has no effect on the
motion of the head. This simulates the free head motion in the JARI volunteer sled tests and also allows the evaluation of the
effectiveness of the seat in limiting neck internal motion un- til the head contacts the head restraint. The initial seatback angle is set to
20 degrees from the vertical as in the JARI sled tests.
The head restraint WMHR satisfies the minimum height re- quirement by the European standard (UN-ECE Regulation No.
17; Edwards et al., 2005). Nonetheless, an additional vertical height of 35 mm is added for this head restraint in order to com-
pensate for the spine straightening. This value corresponds to the average upward displacement of T1 as obtained in the JARI
volunteer sled tests (see Figure 3). Hence, the top of WMHR be- comes level with the top of the head. Avery and Weekes (2006)
suggested that backset values less than 45 mm could cause
discomfort. Hence, the backset for WMHR is set to 60 mm, within the range of a good head restraint geometry, to allow head
comfort. The depth of WMHR is selected as 100 mm.
IIWPG (2006) specifies head restraint contact time, maxi- mum T1 forward acceleration, upper neck (rearward) shear, and
tension forces
for the
dynamic rating
of
seats
and head restraints. For this
purpose, IIWPG
uses a
standard dynamic
test performed at
b.
V
=
16 kph with amean
=
5 g and apeak
=
10 g. In order to evaluate the anti-whiplash seat design concepts as well as the rigid
seat, and to compare the results with the IIWPG criteria, the standard dynamic crash test pulse specified by the IIWPG was
used
in
the simulations.
In the human body model, the OC loads on the head are expressed in the head coordinate system located at the
head
center
of gravity as shown in Figure 1. The positive shear and the positive normal forces on the head are defined in the di-
rections of
+
x
and
+
z
axes of the head coordinate system, respectively; therefore, tensile force is negative and compres- sion
force is positive by
definition.
As in dummies, these forces and moments are assumed to be acting at the OC. The maximum T1
forward acceleration is taken as the highest acceleration of T1 in the x direction, as expressed in the coordinate system SG (see
Figure 1). Although IIWPG criteria are specified for the BioRIDIIg dummy only, these specifications can still be used for the
human body model for comparison purposes.
The Characteristics of the
Anti-Whiplash
Car Seat Design
Concepts
A number of energy-absorbing car seat design concepts comprising anti-whiplash devices (AWDs) are proposed and the
motions that they induce on the human body model, when subjected to rear impact, are investigated. These concepts allow the
motions for the seatback and seat-pan to be independent of each other. They are controlled by passive anti-whiplash devices
consisting of spring and damper units. These devices become operational only when the corresponding breakaway forces and/or
torques are exceeded. Therefore, the crash energy is absorbed by these devices in such a way that an optimum protection is
provided at different severities. The required characteristics of the AWDs were determined using a wide range of crash
pulses (b.V between 4.5 and 35 kph) with different severities and pulse shapes (Linder et al., 2001, 2003; see also
http://www.folksam.se). Figure 6 shows six design concepts for anti-whiplash car seats. For all seats, the masses of the seat and
head restraint are representative of typical car seats (Verver, 2004).
In this article, the abbreviation RG is used to represent the basic rigid seat that simulates the seat used in the JARI volun- teer
sled tests. RO (recliner only) represents the modified rigid seat with a rotational spring-damper AWD, which enables the seatback
to rotate with respect to a
fixed
seat-pan, whereas seat- pan only (SPO) has a horizontal translational spring-damper AWD, which
permits the whole seat to translate backwards. In SPO, there is no rotational motion between the seatback and the seat-pan.
Figure 6 Anti-whiplash car seat design concepts (HR: head restraint, SB: seatback, SP: seat-pan, OF: outer seatback frame, P: translational AWD, R & R*:
rotational AWD).
The seat design concept WMS combines both the transla- tional and rotational AWDs used in SPO and RO, respectively,
whereas the DWMS concept has the same two AWDs as WMS but with the translation AWD inclined by 30 degrees from the
horizontal, allowing the seat-pan to have both backward and downward motions simultaneously. For these two designs, the
rotational and translational AWDs are activated when
b.
V(kph)
> 4.5 and 10.5, respectively.
The downward motion is introduced in order to reduce the compression forces that occur due to spine straightening in the
very early stages of the impact. A 30 degree incline from the horizontal is selected for this purpose since lower angles were not
found
to reduce the compression force appreciably, whereas higher angles could not limit neck internal motion as well as the selected angle.
Besides, higher angles would cause large nor- mal and frictional forces between the translational AWD and the supporting seat
structure.
In both RFWMS and DRFWMS, an inner seatback frame (SB) pivots about an outer seatback frame (OF) at R* as shown in
Figure 6. When the breakaway torque at the rotational AWD at R* is overcome due to the pressure applied by the torso on the
inner
seatback frame, a rotation at R* occurs that is in the opposite direction to the rotation at R of the outer seatback frame.
This action provides better occupant retention at high severity impacts by reducing the effective seatback angle; it also moves the
head restraint forward with a net effect of reducing the backset. The difference between RFWMS and DRFWMS is that the
latter
has
an inclined translational AWD by 30 degrees from the
horizontal.
In both
RFWMS
and
DRFWMS,
the AWDs at R, R*, and P are
activated when b.V(kph) > 4.5, 10.5, and
10.5, respectively.
It should be noted that for all of the design concepts, no AWD is activated for values of b.Vs less than 4.5 kph in order to
prevent activation during normal daily use. This can easily be achieved in practice by using a sacrificial shear element or through
active control.
Figure 7 Stiffness function for R.
Figure 7 shows the stiffness function of the rotational AWD at R for all systems. The breakaway torque is around 850 Nm. For
rearward rotation at R, a constant damping coefficient of
1
Nms/degree
is used, which is an
estimation
of the damping co- efficient for the recliner structure in typical car seats (Eriksson,
2002). High damping (400 Nms/degree) is applied at R when
the seatback starts rotating forward (rebound motion); thus,
seat- back
rebound is minimized.
Figure 8 shows the stiffness and damping functions of the rotational AWDs at R* for both RFWMS and DRFWMS. In order
to obtain optimum performance and to prevent undesired activation of the AWD at R* (especially at lower severities), a breakaway
torque of 1350 N was selected after having subjected RFWMS and DRFWMS to a wide range of crash pulses. The AWD at R* also
applies high damping for the reverse (rebound) motion. Finally, the stiffness and damping functions for the translational AWDs
are shown in Figure 9.
Figure 10 indicates the typical responses of the translational
AWD
at
P and the rotational AWDs
at
R and R* when the seat
de- sign
concepts with combined rotational and translational AWDs are subjected to the IIWPG standard pulse (b.V
=
16 kph, amean
=
5
g, apeak
=
10 g). Although the AWD that controls the rotational motion at the recliner (at R) is activated at a lower b.V value with
respect to the translational AWD, once both b.V thresholds are exceeded, the seat-pan moves backwards rapidly at the
initial stages
of
the
impact (between
0 to 50 ms) in compar- ison to the backwards rotation of the seatback. In other words, the seat moves backwards
initially without
considerable
recliner rotation. The response of the AWD at R* shows a delay of about
65 ms.
However,
during this period, the recliner and the seat-pan are in motion; thus, the AWDs at R and P are absorbing
energy.
When
the breakaway torque is overcome, the inner seatback
Figure 8 Stiffness and damping functions for R*.
Figure 9 Stiffness and damping functions for P.
frame rotates rapidly with respect to the outer seatback frame, providing relatively earlier head–to–head restraint contact.
RESUL
T
S
In this section, the performance of the rigid seat (RG),
recliner only motion seat (RO), seat-pan only motion seat (SPO), and the
four anti-whiplash energy-absorbing seats (WMS, DWMS, RFWMS, and DRFWMS) are evaluated by using the IIWPG standard
pulse
and the
severe crash pulse
(b.V
=
35 kph, amean
=
7.1 g, apeak
=
16 g). The results of the simulations are presented
in Table I. A friction coefficient of 0.35 is used for all contacts
between the human body and the seat (Verver, 2004). The initial value of the normal force (which is the compression force at t
=
0)
is set to zero as this is a usual practice in displaying the values for the OC normal forces. Head restraint contact times correspond
to contact with the head restraint WMHR. NICmax (Neck Injury Criterion) is also calculated. NIC is associated with the S-shape
deformation of the neck and is based on the relative acceleration and velocity between the OC and T1.
In Table I, the largest values of OC (upper neck) forces are presented. OC shear and tensile forces indicate how strongly the
head is thrown backward relative to the seat. For all seats, the largest OC tensile and shear forces occur approximately at the
same time, which corresponds approximately to the in- stant when maximum head retraction in the form of an S-shape is
developed. According to the IIWPG neck force
classification
(IIWPG, 2006), shear forces appear to be on the border of mod- erate to
high, or higher, whereas tensile forces are well within the low neck force range. The compression force occurs very early in the
impact. WMS and RFWMS reduce the maximum
Figure 10 AWD displacements in response to the IIWPG standard pulse.
Table I Seat performance in response to the IIWPG standard and severe crash pulses
IIWPG standard crash pulse
RG RO SPO WMS DWMS RFWMS DRFWMS
Fshear (N) 235 358 249 248 240 240 273
Ftensile (N)
−
333
−
343
−
329
−
316
−
295
−
306
−
297
Fcomp (N) 252 98 225 113 84 132 102
maxT1x
−
acc
(g) 10 10.2 8.9 7 7.85 6.5 7.8
NICmax
(m
2
/s
2
) 11.8 11.4 11.13 7.68 7.06 6.81 6.28
HrCt (ms) 50 101 62 108 107 95 94
maxSB-b.θ (degrees) 0 15.8 0 15.5 15.6 11.1 11.43
maxSP-b.x (cm) 0 0 5.41 5.28 4.62 5.3 4.6
maxSP-b.z (cm) 0 0 0 0 2.66 0 2.65
Severe crash pulse
WMS
DWMS
RFWMS
DRFWMS
maxSB-b.θ (degrees)
20.1
20.2
15.1
15
maxSP-b.x (cm)
6.9
5.94
6.97
6
maxSP-b.z (cm)
0
3.43
0
3.46
OC compression forces, whereas DWMS and DRFWMS cause further
reduction
in the
compression
forces by
allowing
the seat- pan
to
also move downward by an amount of 1.75 cm during the
first
50 ms of the impact. RO, the
fixed
seat-pan with a rotational AWD at
the recliner (at R), also shows low compression force as the seatback rotates quickly, but it generates the highest shear and tensile
forces.
Maximum forward T1 x accelerations (maxT1
x
−
acc
) as
shown in Table I are less than the IIWPG threshold value of 9.5 g for all the
anti-whiplash seat design concepts. The evaluation of T1 x
acceleration
and the upper neck forces become more compatible with the
IIWPG criteria when head restraint contact is enabled. The head restraint changes the dynamics of the system and the values
of
these
parameters significantly.
The head restraint contact times (HrCt) are 105 ms on av- erage, higher than 70 ms (IIWPG threshold value) for the seat
design concepts without the inner seatback frame design (RO, WMS, and DWMS). On the other hand, the head restraint con- tact
times for RG and SPO (i.e., seat designs with fixed seat- backs) are 50 and 62 ms, respectively. Backward rotation of the
seatback aids in energy absorption, but this moves the head restraint away from the head, thus extending the head restraint contact
time. This is normal for seat designs focusing on energy absorption (Avery et al., 2007). Furthermore, the rigid-body modeling
approach does not allow the human body model to sink into the seatback, consequently causing the backset dis- tances
to
remain
effectively larger, resulting in later head re- straint contact times. The seats with the inner seatback frame design
(RFWMS and DRFWMS) have slightly reduced contact times (95 ms) since the head restraint moves forward as the inner
seatback frame rotates in the opposite direction relative to the outer seatback frame under the pressure from the torso.
The anti-whiplash seat design concepts with combined rota- tional and
translational
AWDs have favorable energy absorption
characteristics and they produce much lower NICmax values compared to RG, RO, and SPO. In relation to NIC
(Bostro¨
m et al.,
1996), whiplash-mitigating seats can decrease the degree
of S-shape or retraction as shown in Figure 11. In the simu- lations, the most pronounced S-shape occurs when the lower neck
is in extension and at the same time the upper neck has the maximum flexion. The most pronounced (maximum) S-shape is
identified
by monitoring the intervertebral angles of the neck. It can be seen that, for RG, the initial neck posture as shown in
Figure 1 is transformed into an S-shape and then transition from S-shape to extension takes place, followed by hyperex- tension.
On the other hand, for WMS, head retraction relative to the upper torso is very much limited; hence, the initial neck posture is
transformed into neck extension without consider- able S-shape deformation. The simulations also indicate that the
anti-
whiplash
seat design concepts such as WMS have the potential to allow larger backset values as a result of limited head
retraction (Figure 11).
Figure 11 Comparison of head-and-neck responses of RG and WMS to the
IIWPG standard pulse.
ENERGY-ABSORBING
CAR SEAT 589
As the seats absorb energy, they can limit and delay the de- velopment of neck internal motion until the head contacts the head
restraint. While head with respect to T1 motion is being minimized with the aid of AWDs, the neck muscle activity that begins at
around 75 ms (Ono et al., 1997) becomes effective without any appreciable neck internal motion. Therefore, in the later stages of
the impact, the neck becomes more resistant to S-shape and neck extension formation as the AWDs complete
their motions.
These
results
are in
agreement with
the
findings
of
Stemper
et al.
(2006), who compared the effects
of precontracted neck muscles in aware
occupants with
reflex
muscle contraction in unaware occupants by subjecting a validated head-and-neck model to an acceleration
pulse applied horizontally at T1 with a severity of b.V
=
10.5 kph. In comparison to the reflex muscle contraction in the unaware
occupant simulation, precontracted neck musculature with maximum contraction levels before im- pact stabilized the head and neck,
eliminated S-shape curvature, and decreased spinal motions and soft tissue distortions. These findings supported the results
of
human
volunteer experiments in the literature. Considering the above discussion, the anti- whiplash seat design concepts are
expected to reduce the injury risks associated with the S-shape injury mechanism.
The inner seatback frame rotation as in RFWMS and DR- FWMS reduces the maximum seatback angular displacement
(maxSB-b.θ ) by 4 degrees in comparison to WMS and DWMS (see Table I). Simulations using the IIWPG standard pulse have not
shown much difference in the ramping effect for the seat designs considered. Besides, since the IIWPG standard pulse is a
medium severity pulse and the head restraint WMHR has good geometry, the ramping of the body has not posed any injury
risk. However, the presence of such an inner seatback frame effectively reduces the ramping of the body at higher severity
impacts. When the severe crash pulse (b.V
=
35 kph, amean
=
7.1 g, apeak
=
16 g) is simulated, the counterrotation of the inner
seatback frame decreases the maximum seatback angular displacement by 5 degrees. For each anti-whiplash seat design concept,
Figure 12 shows the instant when the torso has
Figure 12 Comparison of the ramping effect of anti-whiplash seat design concepts at the severe crash pulse.
just started to descend and this approximately corresponds to the highest position of the head. It can be seen that at this severe crash
pulse, RFWMS and DRFWMS provide better occupant retention compared to WMS and DWMS.
RFWMS and DRFWMS decrease the backset by 1.5 to 3 cm and this corresponds to 4 to 6 degrees of rotation at R*. In the
simulations, the inner seatback frame rotation at R* reduces the backset by 1.8 cm and 2.7 cm typically for the IIWPG standard and
severe crash pulses, respectively, depending on the point where the head makes the first contact with the head restraint.
As shown in Table I, the maximum rearward displacement of the seat-pan (maxSP-b.x) varies between 6 to 7 cm at
the
severe
crash pulse, whereas for the IIWPG standard pulse, it is between 4.6 to 5.4 cm. For DWMS and DRFWMS, the max-
imum
downward displacement of the seat-pan (maxSP-b.z) is
2.65 and 3.45 cm in response to the IIWPG standard and severe crash pulses, respectively.
DISCUSSION
Using passive devices only, it is a challenging task to design seats that can operate optimally for all levels of severities. The
simulation test results of several anti-whiplash seat design concepts have been investigated using mainly the IIWPG stan- dard
pulse
and the human body model specifically developed for rear impact. The design strategy used in this article has been
applied to a seat with a rigid seatback and seat-pan. No head restraint or seatbelt is used in the simulations. The results indicate
that the anti-whiplash seat design concepts, namely, WMS, DWMS, RFWMS, and DRFWMS, all demonstrate good whiplash
reducing performances (with only slight differences) at the IIWPG standard pulse. As expected, RG, RO, and SPO show poor
performance.
RG and SPO have fixed seatbacks, as a result of which a strong reaction force to the upper torso develops immediately, thus
causing severe S-shape deformation rapidly. This is ac- companied by strong spine straightening, which leads to high
compression forces. In RO, the rotational AWD at the recliner only accounts for energy absorption and therefore the seatback is
rotated rapidly. This leads to reduced compression force but, on the other hand, causes high shear and tensile forces. RG, RO,
and
SPO have high NICmax and maximum forward T1 x acceleration values, which are indications of high injury risk.
The anti-whiplash car seat design concepts (WMS, DWMS, RFWMS, and DRFWMS) show similar effectiveness in min-
imizing neck internal motion. In principle, these four designs employ controlled recliner rotation and seat-pan displacement to
limit the formation of S-shape. Their T1 forward accelera- tion is also lower than the recommended IIWPG limit (9.5
g)
specified
for energy-absorbing seats. Their NICmax values are
much lower than the
proposed
injury
threshold
value of 15
m
2
/s2
(Bostro¨
m et al., 1996). The head restraint contact time is around
100 ms on average but this would be much lower in reality with a real seat that has some compliance due to the seat foam and
suspension, which allows the torso to sink into the seatback,
hence reducing the contact time. The anti-whiplash car seat design concepts induce moderate to high shear but low tensile OC
forces according to the IIWPG ratings and much reduced
compression
forces compared to RG and SPO. However, the use of a good
head restraint in conjunction with the AWDs would provide head support in good time, which in turn would prevent neck extension
and reduce the shear force applied to the neck by the head. A good head restraint would also help to further limit S-shape
deformation and prevent the development of the most pronounced S-shape as investigated in this study.
It can be concluded that there is not much difference among the performance of the four anti-whiplash seat design concepts
regarding their responses to the IIWPG standard pulse. How- ever, the seat design concepts with the inner seatback frame have
some advantages over the ones without the inner seatback frame. With the aid of inner seatback frame rotation at R*, they provide
earlier head restraint contact and reduce the effective seatback angle. Especially in higher severity rear impacts, the inner
seatback
frame rotates further, which helps to reduce the ramping of the body considerably, preventing its ejection and interaction
with the car interior and the rear seat occupant. How- ever, the characteristics of the AWD at R* must be adjusted properly so that
the inner seatback frame rotation at R* must be accompanied by a sufficient amount of outer seatback frame (OF) rotation at R to
avoid increasing the loading on the upper torso in any case.
At the severe crash pulse, the seat design concepts having the
downward
motion produce slightly less ramping. DRFWMS
performs the best as it does not let the head rise over the head restraint and also lowers the position of the head relative to the
vehicle floor. This offers good protection for the tall and unbelted occupants in the case of a severe rear impact.
In this study, the rebound effects have not been considered as it would be immaterial due to the absence of seatbelt and head
restraint. Besides, since the forward rebound of the seat compo- nents is minimized, the rebound of the torso is insignificant for all
severities as observed from the simulations.
The proposed energy-absorbing seat design concepts have been shown to limit the neck internal motion successfully, hence
reducing injury risks associated with S-shape deforma- tion. However, early head support is also essential to limit the loading on
the head-and-neck. As indicated by Viano and Olsen (2001), early head support can enable the head and neck to bene- fit from a lower
relative velocity of impact on the head restraint. Therefore, an anti-whiplash seat can perform best if it absorbs the crash energy
effectively and at the same time provides early head support. Hence, if a good head restraint is used in con- junction with the
AWDs, the seat design concepts with the inner seatback frame are expected to produce lower head-and-neck loads than WMS and
DWMS.
It should be noted that the compliance of the seatback foam and suspension has not been taken into account in this study in
order to provide a comparison with the existing JARI test results with a rigid seatback. As in the actual commercial seats
used in the automotive industry, the seatback foam and
suspension compliance allow the occupant to sink into the seatback rapidly with little resistance at the very early stages of the
impact, reducing head restraint backset distance and contact time. Therefore, seatback foam and suspension compliance will
further improve the protection provided by the proposed whiplash-mitigating designs.
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