2003-01-0954 Safety Belt Fit, Comfort, and Contact ... - Delphi

SAE TECHNICAL
PAPER SERIES 2003-01-0954
Safety Belt Fit, Comfort, and Contact Pressure
Based on Upper Anchorage Location
and Seat Back Angle
Liang Chen, Rana Balci and Alicia Vertiz.
Delphi Corporation
Reprinted From: Human Factors in Driving, Seating, & Vision
(SP-1772)
2003 SAE World Congress
Detroit, Michigan
March 3-6, 2003
400 Commonwealth Drive, Warrendale, PA 15096-0001 U.S.A. Tel: (724) 776-4841 Fax: (724) 776-5760 Web: www.sae.org

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise,
without the prior written permission of SAE.
For permission and licensing requests contact:
SAE Permissions
400 Commonwealth Drive
Warrendale, PA 15096-0001-USA
Email: permissions@sae.org
Fax: 724-772-4028
Tel: 724-772-4891
For multiple print copies contact:
SAE Customer Service
Tel: 877-606-7323 (inside USA and Canada)
Tel: 724-776-4970 (outside USA)
Fax: 724-776-1615
Email: CustomerService@sae.org
ISSN 0148-7191
Copyright © 2003 SAE International
Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE.
The author is solely responsible for the content of the paper. A process is available by which discussions
will be printed with the paper if it is published in SAE Transactions.
Persons wishing to submit papers to be considered for presentation or publication by SAE should send the
manuscript or a 300 word abstract of a proposed manuscript to: Secretary, Engineering Meetings Board, SAE.
Printed in USA

2003-01-0954
Safety Belt Fit, Comfort, and Contact Pressure Based on Upper
Anchorage Location and Seat Back Angle
Copyright © 2003 SAE International
ABSTRACT
A seat belt usability study was conducted to investigate
factors associated with seat belt comfort and convenience
related to shoulder belt contact pressure, shoulder belt fit,
and seat belt upper anchorage location.
Two major objectives were addressed in this study: (1)
Determine the shift in the contact pressure while
changing the seat back angle and seat belt attachment
points / B-pillar location by utilizing a body pressure
measurement system; (2) Identify how seat belt contact
pressure and fit affect users’ subjective feeling of
comfort.
Results from the statistical analysis shows that the seat
belt contact pressure increases when the D-ring moves
away from the driver in the fore-aft direction (X-axis)
whereas height adjustment of the D-ring (Z-axis) is not
statistically significant in terms of pressure distribution.
Seat back angle, fore-aft (X-axis) and lateral (Y-axis)
positions of the D-ring, and seat belt sub-segments are
found to be statistically significant in the analysis of
subjective comfort rating for contact pressure, which
comply with the results obtained from the statistical
analysis of mean contact pressure and distribution. In the
analysis of subjective comfort rating on seat belt
fit/routing, height adjustment of the D-ring is statistically
significant. This indicates that the height adjustment for
seat belt upper anchorage point is essential to achieve a
better seat belt fit, help reduce the risk of seat belt
rubbing the neck or slipping off the occupant’s shoulder,
given that it cannot improve the contact comfort by
reducing the contact pressure.
INTRODUCTION
The protection provided to vehicle occupants by the
three-point seat belt is no longer disputed over the world.
Vehicular seat belts were originally developed to help
prevent serious injury to a restrained occupant. Its
function is to decelerate the occupant in the vehicle by
distributing the forces of collision across the body of the
Liang Chen, Rana Balci and Alicia Vertiz
Delphi Corporation
occupant and by restraining the occupant inside the
vehicle during a collision or rollover event. Therefore,
increasing the usage rate of the seat belt is a primary
goal of all national transport safety policies in motorized
countries. Figure 1 shows the seat belt use rates in the
U.S. between 1998 and 2001. The average seat belt use
rate in the year 2001 reached its all-time high, which is
73%.
Year
2001
2000
1999
1998
71%
67%
73%
69%
0% 30% 60% 90%
Use Rate (%)
Figure 1. Seat Belt Use Rate in U.S. (Source: National
Highway Traffic Safety Administration, 2002)
To achieve high levels of usage rate, points of concern
still are effectiveness, comfort, and convenience, which
depend largely on the extent of the geometric design of
seat belts matching the occupant’s anatomical
characteristics. This study focuses on the comfort and
convenience aspects of safety belts from human factors
point of view.
A number of studies have highlighted potential issues
related to the seat belt comfort and convenience
(Woodson, et. al., 1980, and Wilcoxon, 1998). A
questionnaire administered by Delphi (Balci, et al., 2001)
concludes that a primary reason many people give for not
wearing a seat belt is that it is inconvenient to be fastened
and unfastened and uncomfortable to wear, especially in
some specific stature groups, e.g., small females and
large males. Therefore, a seat belt usability study was
conducted to investigate the factors associated with seat
belt comfort and convenience with respect to shoulder
belt contact pressure, shoulder belt fit, and seat belt
upper anchorage location.

One objective of the study is to determine the shift in the
contact pressure while changing the seat back angle and
the location of the seat belt upper attachment points (Dring)
by utilizing a body pressure measurement system.
Another is to identify how seat belt contact pressure and
fit affect users’ subjective feelings of comfort.
METHOD
Each participant was tested with 54 experimental
scenarios to investigate the seat belt pressure levels, seat
belt fit, and subjective comfort ratings resulting from
various geometric configurations of the D-ring, seat back
angles, and different anthropometric dimensions.
SET-UP
The set-up consists of a universal seating buck to
simulate a passenger car interior package. The threepoint
belt system was configured to represent an
adjustable "D"-ring anchor with three different levels of
seat back angle. Extra webbing was provided to allow
measuring the seat belt comfort and fit for extreme
geometric configurations. In addition, the seat was in a
fixed position during the entire test.
MEASUREMENTS
Tekscan ISCAN V4.23 pressure measurement system
was utilized to sample the pressure distribution on the
seat belt. For convenience in describing the pressure
distribution patterns, the pressure sensor with a resolution
of 44 x 52 is virtually divided into 8 even segments.
Segment number 1 to 8 indicate the relative seat belt
portion from top of the shoulder to the buckle location
(Figure 2). A Metrecom 3D Digitizer was also utilized to
collect the participants’ 17 standard anthropometric
measurements, e.g., standing height, seating height,
shoulder width, etc.
8: ASIS/Buckle
4: Sternum
6: Abdomen
1: Top of Shoulder
3: Upper Chest
Figure 2. Seat Belt Segments Definition
2
TEST PARTICIPANTS
A total of 15 volunteers, including 11 males and 4
females, participated the test. Their height and Body
Mass Index (BMI) data are summarized in Table 1.
Table 1. Anthropometric Data for Test Subjects (N=15)
Height (inch) BMI
MEAN 68.48 25
STDEV 3.20 6.50
MAX 72.30 43
MIN 61.70 17
Table 2 lists the height and BMI in three categories:
• 4 short stature females (66%-ile)
Table 2. Grouping information for Test Participants
BMI (Body
Mass Index)

Table 3. Three Percentile Categories of Participants
Female Male
Percentile Height (mm) Height (mm)
1761
Within-Subjects Factors:
• B-pillar Fore-aft adjustment (3 locations/levels)
0. foremost
1. middle
2. rearmost
• B-Pillar inboard-outboard adjustment (3
locations/levels)
0. Inside
1. Middle
2. Outside
• B-Pillar up-down adjustment (2 locations/levels)
0. Upper
1. Lower
• Seat back angle (3 levels)
0 degree
10 degree
20 degree
Response Variables:
1. The contact pressure and its distribution on the
seat belt
2. Subjective comfort ratings on seat belt pressure
3. Subjective comfort ratings on seat belt fit/routing
With the above experimental design, each participant took
a total of 54 tests including different scenarios and the
three response variables were recorded in each test.
RESULTS
OBJECTIVE MEASUREMENTS OF CONTACT
PRESSURE
ANOVA analysis (see Appendix ANOVA Table 4) shows
that height adjustment of the D-ring (Z-axis) is not
statistically significant in terms of pressure distribution
(p=0.325). This may indicate that the D-ring location in
up-and-down direction has a little impact on the seat belt
contact pressure. Mean pressure distribution as a
function of seat back angle, D-ring locations in fore-aft
(X-axis) and lateral (Y-axis) directions were shown in
Figures 3, 4 and 5. A very similar trend of pressure
distribution shown in these figures indicate that seat belt
pressure has peak values at chest and abdomen
(segments # 3 and # 6) while the lowest mean value
occurred at segments # 2 and # 5.
3
Figure 6 represents the mean pressure distribution as a
function of the D-ring location in both fore-aft and
inboard-outboard directions. First, it is obvious that
contact pressure is lower when D-ring moves towards the
occupant in fore-aft direction. Furthermore, in terms of
inboard-outboard adjustments, it is observed that an
optimal location exists that results in a relatively low
pressure value. Moving the D-ring either inboard or
outboard from this location would increase the seat belt
contact pressure on the occupant’s upper torso. In
addition, It was found that the lowest mean pressure
occurred when the location of the D-ring was at the most
forward position on the fore-aft axis (close to shoulder)
and at the middle position of the lateral Y-axis.
Mean Pressure (g/cm2)
.12
.10
.08
.06
.04
.02
1.00
2.00
3.00
4.00
5.00
Seat Belt Segments
6.00
7.00
8.00
Seat Back Angle
0 degree
10 degree
20 degree
Figure 3. Mean Pressure Distribution as a Function of
Seat Back Angle
Mean Pressure (g/cm2)
.10
.09
.08
.07
.06
.05
.04
.03
.02
1.00
2.00
3.00
4.00
5.00
Seat Belt Segments
6.00
7.00
8.00
D-ring in X-axis
Figure 4. Mean Pressure Distribution as a Function of
"D"-ring Location in Fore-Aft Direction
Foremost
Middle
Rearmost

Mean Pressure (g/cm2)
.09
.08
.07
.06
.05
.04
.03
1.00
2.00
3.00
4.00
5.00
Seat Belt Segments
6.00
7.00
8.00
D-ring in Y-axis
Inside
Middle
Outside
Figure 5. Mean Pressure Distribution as a Function of
"D"-ring Location in Inboard-Outboard Direction
Mean Pressure (g/cm2)
.08
.07
.06
.05
.04
.03
Inside
Middle
D-ring in Y-axis
Outside
D-ring in X-axis
Foremost
Middle
Rearmost
Figure 6. Mean Pressure Distribution as a Function of
"D"-ring Locations in Both Fore-Aft and Inboard-Outboard
Directions.
SUBJECTIVE RATING ON CONTACT PRESSURE AND
SEAT BELT FIT
ANOVA on subjective rating of contact pressure (see
Appendix ANOVA Table 4) shows that the seat back
angle, D-ring location in both fore-aft and inboardoutboard
directions are still statistically significant. This
outcome complies with the results from the ANOVA on
objective measurements of contact pressure and
distribution. Conversely, the D-ring location in height
adjustment is not a significant factor, which means that it
has a little impact on sensed seat belt pressure in terms
of occupant’s subjective feeling. However, in ANOVA on
subjective rating regarding seat belt fit/routing; all of the
4
variables, including seat back angle, D-ring in fore-aft,
lateral, and vertical directions, are statistically significant.
This finding indicates that the height adjustment for seat
belt upper attachment point is essential to achieve a better
seat belt fit/routing and to help reduce the risk of seat belt
rubbing the neck or slipping off the occupant’s shoulder.
However, it cannot improve the contact comfort by
reducing the contact pressure. Figure 7 displays the
mean value/ trend of occupants’ comfort rating on seat
belt fit with respect to the seat back angle and D-ring
location on Z-axis. There is a significant difference in
comfort rating for different levels of D-ring height, which
support the results from ANOVA table (Table 4 in
Appendix).
As a general observation from the experiment, some
participants who were in the low height and low BMI
category complained about the seat belt rubbing their
necks. Although it cannot be generalized due to the small
number of participants, a similar complaint was received
from the participants who were at the higher end of the
BMI and height scale combination.
Mean Pressure (g/cm2)
2.8
2.6
2.4
2.2
2.0
1.8
0 degree
10 degree
Seat Back Angle
20 degree
D-ring in Z-axis
Figure 7. Subjective Rating on Seat Belt Fit as a
Function of Seat Back Angle and "D"-ring Locations in
Different Height Levels
Note: Comfort Rating Scale (1-5)
1: Slipping off the shoulder
3: Just right
5: Rubbing the neck
DISCUSSION
In this study, height and BMI data of all participants were
recorded as the between-subject variables. However, the
results of ANOVA with repeated measurements for each
participant show that body height and BMI are not
statistically significant. Hypothetically, body height and
High
Low

BMI should have an impact on the seat belt pressure
because of the changes in the belt geometry, similar to
changing D-ring locations in belt configuration. One
possible reason is that seat belt pressure in normal
driving condition is in a relatively low range that results in
minor discomfort. Although the relative pressure
distribution was analyzed in this test, observation of a
difference in absolute pressure value may have been
missed due to the current setup for different stature
groups. However, the interaction of height and BMI is
found to be significant in ANOVA on comfort rating of
seat belt fit/routing. This indicates that body height and
BMI do have an influence on seat belt fit/ routing due to
the different body statures and weights. To better explain
the difference in belt contact pressure and subjective
ratings of different stature groups, a further study with
larger sample size, particularly in small and large stature
groups, is recommended.
A linear regression analysis was also conducted in each
stature group based on the current sample size and
quantitative measurements. Relatively low R-Square and
non-random residual distribution around zero level
indicates that either there are more unknown factors that
play a role in prediction of seat belt pressure, comfort
rating, and seat belt routing or the functions to all
independent variables are not linear. Theoretically, inside
each stature group, there is still a considerable difference
among participants in terms of BMI, upper body shape,
etc. Larger sample size and sub-grouping inside each
stature group can be helpful to find out the root cause of
the problem. The effect of gender was also not
considered in this study due to the limited number of
females, especially in small stature group.
CONCLUSION
1. The "D"-Ring location on fore-aft and inboardoutboard
directions and the seat back angle are
statistically significant on the seat belt pressure.
2. Height adjustment of D-Ring location has no
significant impact on contact pressure, but improve
the comfort of seat belt fit.
3. There is a similar trend of pressure distribution for all
test scenarios: higher mean pressure occurred at
upper chest (segment # 3) and abdomen (segment #
6) and lower mean pressure occurred at upper
shoulder (segment # 2) and sternum (segment # 5)
4. Height and BMI of the participants are not statistically
significant in the Repeated Measures ANOVA using
SPSS GLM. Further analysis and research work is
needed to investigate the seat belt pressure for
different stature groups.
5. A linear regression analysis on seat belt contact
pressure and seat belt fit shows poor prediction
capability possibly due to the low pressure
measurements and small sample size.
5
REFERENCES
1. Balci, R., Vertiz, A, and Shen, W., 2001, “Comfort
and Usability of the Seat Belt”, SAE Technical Paper
No. 2001-01-0051, 2001.
2. NHTSA (National Highway Traffic Safety
Administration), www.nhtsa.gov
3. Wilcoxon, K., "Improving Seat Belt Comfort on Off-
Highway Vehicles", SAE Technical Paper No.
982058, 1998.
4. Woodson, W.E., Selby, P.H., and Coburn, R., 1980,
Comfort and Convenience: Specifications for Safety
Belts: Shoulder Belt Fit, Pressure and Pullout Forces,
DOT Report HS-805 597.

APPENDIX
Table 4. ANOVA Table (Tests of Within-Subjects Effects)
Source Dependent Variable Type III Sum of Squares df Mean Square F Sig.
Corrected Model PRESSURE 4.022 a 227 1.772E-02 3.654 .000
COMP 1679.256 b 227 7.398 4.240 .000
COMR 3352.118 c 227 14.767 22.209 .000
Intercept PRESSURE 18.458 1 18.458 3806.214 .000
COMP 150145.195 1 150145.195 86049.013 .000
COMR 36712.928 1 36712.928 55214.661 .000
SEAT BACK PRESSURE .845 2 .423 87.126 .000
ANGLE COMP 837.554 2 418.777 240.003 .000
COMR 94.664 2 47.332 71.186 .000
D_RING PRESSURE .610 2 .305 62.919 .000
X-axis COMP 670.898 2 335.449 192.248 .000
COMR 1080.735 2 540.368 812.689 .000
D_RING PRESSURE 5.423E-02 2 2.712E-02 5.592 .004
Y-axis COMP 27.528 2 13.764 7.888 .000
COMR 1699.751 2 849.875 1278.176 .000
D_RING PRESSURE 4.691E-03 1 4.691E-03 .967 .325
Z-axis COMP 3.129 1 3.129 1.793 .181
COMR 209.952 1 209.952 315.759 .000
BELT PRESSURE 1.140 7 .163 33.576 .000
SEGMENT COMP .000 7 .000 .000 -
COMR .000 7 .000 .000 -
a R Squared = .117 (Adjusted R Squared = .085) on PRESSURE (pressure measurement)
b R Squared = .133 (Adjusted R Squared = .102) on COMP (subjective comfort rating on pressure)
c R Squared = .446 (Adjusted R Squared = .426) on COMR (subjective comfort rating on belt fit/routing)
6