rollover impact in buses
DESCRIPTION
This is a literature review on recent articles dealing with rollover analysis in buses and coaches. It includes experimental and FEM modeling samplesTRANSCRIPT
Department of Mechanical and Manufacturing Engineering
Rollover Impact in Buses and Coaches
Impact Mechanics- MECG-7780
Khashayar Pejhan
7718719
2 Khashayar Pejhan - 7718719
Abstract
There exists a generalized interest in the improvement of safety issues in
buses due to the social alarm brought on by accidents in which buses are involved.
Accident statistics show that bus rollover accidents occur with relative low
frequency, taking into account all kinds of bus accidents. Nevertheless the risk of
mortality in the case of rollover is five times greater compared with any other
possible bus accident typology [1]. As a result the coach and bus producers have a
great responsibility in matter of safety issues regarding rollover. Although road
condition and driving skills are important influencing parameters that may lead to
or prevent rollover but producers must perform various test on their coaches to
ensure that the structure is as safe as possible in case of rollover. There are various
standards and regulations in this area that would be pointed out in this article, in
addition substitute computer simulation for these tests would be introduced and
some main research in this area would be explained to show their weak points and
strength. Although much have been done in area of rollover impact, but due to the
dynamic sense of regulation (becoming stricter each year) and incompleteness of
current tests and computer simulations, there are still necessary steps to be taken to
improve the bus structure for facing rollover impact. Some of these points are
mentioned for future studies in conclusion section.
Key words: Rollover-Impact- Bus Safety
3 Rollover Impact in Buses and Coaches
Content
Title Page number
Introduction 4
Definitions, Regulations and Standards 7
Literature review and discussion 12
Jordan Rollover System 27
Conclusion 30
References 33
4 Khashayar Pejhan - 7718719
1. Introduction
One of the main concerns of automotive manufacturers is safety. Due to the
significant amount of money and time that has been spent in this area in last decade
the fatality rate dropped to 1.44 fatalities per 100 million of vehicles traveling in
2004 [1].
Figure 1 shows the percentage of different types of crash in vehicles for a
time span of 1997 till 2001 [2] while Figure 2 shows the statistics of injuries and
fatalities in the same time limit. As it could be seen although rollover accidents
doesn’t seem to be an important type of accident to study due to their insignificant
part in whole crashes (only 8%) but according to Figure 2, more than 30% of
fatalities in accidents have happened in rollover crashes. As a result many
manufacturers and independent researchers have dedicated their time to analyse
this type of accidents.
Figure 1) Percentage of different types of crash in a 5 year time span [2]
8% 13%
16%
49%
14%
Percentage of different types of crash
Rollover
Rear
Slide
Front
Other
5 Rollover Impact in Buses and Coaches
Figure 2) Total number of injuries (blue) and fatalities (red) in car crashes in 5 year time span due
to different type of impacts [2].
Prior to study about different researches that have focused on roll over, it is
necessary to present a definition for this type of impact. A rollover is a type
of vehicle accident in which a vehicle tips over onto its side or roof. The most
common cause of a rollover is traveling too fast while turning. Vehicles with a
high center of gravity are easily upset or "rolled." Figure 3 shows a SUV rolled
over in an accident.
Based on the 2001 Traffic Safety Facts published by NHTSA1, rollovers
account for 10.5% of the first harmful events in fatal crashes; but, 19.5% of
vehicles in fatal crashes had a rollover in the impact sequence. Based on an
analysis of the 1993-2001 NASS2 for non-ejected occupants, 10.5% of occupants
1 National Highway Traffic Safety Administration
2 National Automotive Sampling System
0
10000
20000
30000
40000
50000
60000
70000
Roll over Frontal Rear Slide Other
Injuries 30000 67143 2858 25714 17142
Fatalities 10138 12754 981 8176 655
Nu
mb
er
of
pe
op
le
Total number of injured and dead occupants in different crash types
6 Khashayar Pejhan - 7718719
are exposed to rollovers, but these occupants experience a high proportion of AIS1
3-6 (3:serious, 6:Maximum) injury (16.1% for belted and 23.9% for unbelted
occupants). The head and thorax are the most seriously injured body regions in
rollovers. As a result, it is very important to consider certain standards for safety
regarding roll over in vehicles and many researchers have focused on topics related
to roll over of vehicles. Governing dynamic equations, protective structures, safety
test, injury analysis are some of the concepts that have got engineers attention in
recent years.
Considering this definition, buses are generally believed to be very safe due
to their low centers of gravity and their tendency to be driven slowly. However,
since a number of passengers are being carried on these large vehicles, any
accident involving a bus is often devastating. This is especially true for a rollover
accident.
In each rollover accident of buses, an average of 25 people loses their lives.
[3]. Also, a rollover can cause up to five rotations to occur on the road. A “serious
rollover” occurs when the vehicle flips more than two times. Lastly, a combined
rollover involves two different dangers; for example, a head-on collision that leads
to a rollover, or a rollover that ends with the bus in a lake. The most typical
collision configurations involving buses and coaches are side, rear, frontal and
rollover. In most parts of the world, especially in Europe, safety requirements are
continuously inspected to improve passenger safety in buses or coaches.
In this study a review over different researches about roll over impact,
standard roll over test and related subjects would be presented with a focus on
studies on buses or coaches.
1
The Abbreviated Injury Scale (AIS) is an anatomical-based coding system created by the Association for the
Advancement of Automotive Medicine to classify and describe the severity of specific individual injuries.
7 Rollover Impact in Buses and Coaches
Figure 3) a rolled over SUV (safety test) [4]
2. Definitions, Regulations and Standards
Before going through details of different test, standards and studies about
rollover various types of rollover should be defined. Rollover test are divided in to
two categories; static and dynamic tests. Although there are different examples in
literature for both types of tests but there has been a tendency to static tests due to
difficulty in simulating a dynamic test that can predict any type of rollover;
because there are many reasons for a rollover to happen.
Considering the reason behind the occurrence of rollover two general
categories could be defined for rollovers:
a) Tripped rollover
b) Untripped rollover
A tripped rollover happens when a tire hits an obstacle on the road, or when
the tire rolls over soft soil, in these conditions the lateral motion of tire would stop.
This would lead to rolling of vehicle around the object. Unlike this type of rollover
8 Khashayar Pejhan - 7718719
that usually happen when vehicle leaves road’s surface there is another type of
rollover that happens on the road surface, untripped rollover. In this type of
rollover severe steering maneuvers (like J-turns, fast lane changes and etc.) lead to
losing control of vehicle. Improving structure and control systems of cars could
only prevent untripped rollover, because it is independent of external parameters;
therefore, the main safety analysis is concentrated on this kind of rollover rather
than tripped one [5].
There are many rules and regulations in different countries trying to set a
minimum standard for safety issues of motor vehicles. In case of rollover and
impact, NHTSA has considered three major rules, namely FMVSS1 208, 216 and
220 [6]:
FMVSS 208 (Occupant Crash Protection): This standard originally
specified the required type of occupant restraints (i.e., seat belts). It was amended
to specify performance requirements for anthropomorphic test dummies seated in
the front outboard seats of passenger cars and of certain multipurpose passenger
vehicles, trucks, and buses, including the active and passive restraint systems. The
purpose of the standard is to reduce the number of fatalities and the number and
severity of injuries to occupants involved in frontal crashes.
FMVSS 216:Roof Crush Resistance - Passenger Cars (except convertibles)
(Effective 9-1-75) and Multipurpose Passenger Vehicles, Trucks and Buses (except
school buses) with a Gross Vehicle Weight Rating of 2722 kg (6,000 lbs.) or less
(Effective 9-1-94). This standard specifies requirements for roof crush resistance
over the passenger compartment.
1 Federal Motor Vehicle Safety Standards
9 Rollover Impact in Buses and Coaches
FMVSS 220 (School Bus Rollover Protection): This standard establishes
performance requirements for school bus rollover protection. The purpose of this
standard is to reduce the number of deaths and the severity of injuries that result
from failure of the school bus body structure to withstand forces encountered in
rollover crashes. This standard requires a quasi-static application of a loading equal
to 1.5 times of the unloaded vehicle weight (UVW) to assess the static response of
the bus roof structure. The resistance of the roof structure is judged as satisfactory
when: the downward vertical movement at any point on the application plate does
not exceed 130 mm and each emergency exit of the vehicle can be opened during
full application of the force and after the release of the force [7].
These regulations are not always perfect and new concerns or new
technological improvements lead to modification of regulations. For example,
FMVSS 208 is a dynamic roof crush test standard and almost all vehicle industries
execute this test on their products. But in 2006, Mao et al. [8] showed that this test
lacks repeatability. Through their simulations they showed that 2 models with
completely similar roofs could react totally different to such test. This difference is
such significance, that even the number of rolls each model has in a test differs
from another similar model.
In the case of FMVSS 216, the vehicle’s roof should be loaded quasi-
statically up to a specific level, and its roof crush resistance then should be
checked. The first weak point that could be observed in this test is the fact that in
real rollovers, there are rollover forces and different velocities that are not
considered as mandatory conditions for performing this test. In addition, the force
application in this test is in a way that makes windscreen an important load
carrying part of body which limits roof deformation. The force applied to roof in
this test is not also in a same angle and magnitude as in actual accident.
10 Khashayar Pejhan - 7718719
These problems led to some action in NHTSA and a new version of FMVSS
216 was proposed in 2005, in the new version the standard includes more vehicles
because of the increase in the magnitude of limit weight for vehicles (from 6000 lb.
to 10000 lb.). In addition the applied force was increased 2.5 times to simulate the
real rollover more precisely. Finally NHTSA revised the amount limit for intrusion
of roof, and made it mandatory for manufacturers to maintain enough head room
for a mid-sized adult male occupant.
ECE R66 - Another important regulation in this criterion is the ECE R66
regulation. If a vehicle passes ECER66 test, it is guaranteed that parts like pillars,
luggage racks and etc. would not intrude the residual space during a rollover. [9].
As shown in Figure 4, the envelope of the vehicle’s residual space is defined by
creating a vertical transverse plane within the vehicle which has the periphery
moving this plane through the length of the vehicle.
Figure 4) Residual space in ECE R66 test [7]
The ECE R66 test is actually a kind of lateral tilting test. As in Figure 5, in
the rollover test procedure, a vehicle resting on a tilting platform is quasi-statically
rotated onto its weaker side. Depending on the attachments of the staircase and the
11 Rollover Impact in Buses and Coaches
door frame to the bus frame, it is usually the road side of the bus. When the center
of gravity reaches the highest (critical) point, the rotation of the table is stopped.
Further the gravity causes the bus to free-fall into a concrete ditch. The flooring in
the ditch is located 800 mm beneath the tilt table horizontal position [10].
Figure 5) ECE R66 test specifications [10]
Usually, the rollover test is carried out on that side of the vehicle considered
more dangerous with respect to the residual space. The decision is taken by a
competent Technical Service on the basis of the manufacturer’s proposal,
considering the following:
1) Lateral eccentricity of the center of gravity and its effect on potential
energy in the unstable starting position of the vehicle,
2) Asymmetry of the residual space,
3) The different, asymmetrical constructional features of the two sides of the
vehicle,
12 Khashayar Pejhan - 7718719
4) Which side is stronger and better supported by partitions or inner boxes
(e.g., wardrobe, toilet and kitchenette.)
3. Literature review and discussion
One of the most important studies that have shown the importance of
research on rollover of coaches is the study by Martinez et al. [11] in 2003.
According to the data collected by them probability of different kinds of injury for
bus passengers is higher in roll overs than other types of accident (See Figure 6).
Figure 6) Injury distribution in coach accidents, Spain 1995-1999 [11]
Therefore, it could be concluded that rollover is a menace to the safety of
bus passengers too. Rollover is a complex, chaotic and unpredictable event. This
complexity is due the fact that rollover would be influenced by different factors
like driver, road, vehicle and environmental factors. Whenever a car revolves at
least 90 degrees a rollover has happened (even if car rolls till it comes back on tires
at end). Some of major studies in the area of rollover of buses would be discussed
0.00%
10.00%
20.00%
30.00%
40.00%
50.00%
60.00%
FatalitiesSeriouslyinjured
Minorinjured Not injured
9.60%
32.10%
55.26%
2.60%
2.50% 7.70%
43.30% 46.50%
Rollover Others
13 Rollover Impact in Buses and Coaches
in details in this section and a quick over view on some other important articles in
literature is presented in Table 3.1 in a timely sequence.
Table 3.1) Some major researches about rollover
Author Research Description Year
A. Sances Jr. et
al. [12]
Biomechanical Analysis of Head and Neck Injury
with Rollover Glass Impact
(Glass consideration and injury)
2001
C. Parenteau et
al. [13]
Near and Far-Side Adult Front Passenger Kinematics
in a Vehicle Rollover
(Focus on passenger dynamics)
2001
D. Friedman et
al. [14]
Rollover Roof Test Results for a Production Vehicle
(Experimental value) 2004
D. Valladares,
R. Miralbes, L.
Castejon [15]
Development of a Numerical Technique for Bus
Rollover Test Simulation by the F.E.M. 2010
L. Yan et al.
[16]
Comparison of vehicle kinematics and occupant
responses between Jordan rollover system and an
over-the-road rollover 2012
As mentioned in previous section, standards and regulations are always an
interesting topic for researchers to work on. A good example is the study made by
Bojanowski et al. in 2011[7]. They considered a special case of Paratransit buses in
United States.
14 Khashayar Pejhan - 7718719
Paratransit buses are a special type of buses in US. They are smaller than
ordinary buses and are being used for handicapped people. As a result they are
equipped with wheelchair lifts and some other special structures. Since in these
coaches, the passenger compartment is assembled on an adopted chassis of
ordinary buses, there is no specific regulation or crashworthiness standard for these
vehicles. Bojanowski et al. found that Paratransit bus manufacturers in US try to
comply with FMVSS 220, which is originally for school bus protection. So they
proposed a FEM model to check if they can predict whether paratransit buses can
pass other test too, or not.
According to their research [7] FMVSS 220 is a useful test but since in this
test a quasi-static loading is applied symmetrically to roof, it cannot be considered
as a trustworthy representative of dynamic loading in actual rollover in which
loads have varying value, direction and intensity [7]. This weakness influences the
repeatability of FMVSS 220 test in a bad way. On the other hand, since the number
of paratransit buses that are produced is not significant, performing expensive test
like ECE-R66 is not economically wise for small manufacturers.
As a result Bojanowski et al. proposed a FEM model, as shown in Figure 7,
to predict the paratransit bus behavior in both tests (FMVSS 220 and ECE-R66)
and compare the results. In order to validate their model, Bojanowski et al.
compared the results of their model with those of a full rollover test, performed at
Florida Department of Transportation (DOT) testing facility in 2010 [17].
The bus passes the rollover test if the residual space is not compromised
during the tests [18, 19]. The so called residual space was shown in Figure 4. As it
could be seen there are 6 hypothetical hinges in that figure pointed out as to .
In order to quantify a limit for passing the rollover test the rotation of these hinges
play an important role.
15 Rollover Impact in Buses and Coaches
Figure 7) Paratransit bus in Rollover test and FE model [7]
The assumption is that during the rollover impact the deformation occurs
only at these points which are the weakest connections in bus. Then a Deformation
Index (DI) is defined as:
( )
( )
( ) (3-1)
The test would be evaluated on basis of DI as follows [20]:
Table 3-2) Evaluation of Rollover safety, regarding the Deformation Index
Acceptable Design
Deforming walls start to touch the residual space
Vehicle Failed the test
16 Khashayar Pejhan - 7718719
Using a same concept Bojanowski et al. used their FE model to check the
condition of Paratransit buses under an ECE-R66 test. Figure 8 shows the results
they gained form FEM modeling using LS-Dyna software package. Obviously the
paratransit bus has not passed this test, and the residual space is intruded by
deformed structure.
Figure 8) FE model of Paratransit bus after ECE-R66 test simulation (a: with skin, b: without skin)
[7]
Then, Bojanowski et al. calculated DI to check if it can also predict a same
result as the FE model. Figure 9 shows the variation of DI by time. As it can be
seen at 0.15 seconds after impact, paratransit bus fails the minimum requirements
for safety as DI passes 1.
Bojanowski reported the result in this way: “The bus is deformed in the
torsional mode with rear part being considerably less deformed. As an outcome of
the impact, the plastic deformations were developed at the front cap structure and
at the waistrail beam. The cantrail beam was also deformed locally at the
17 Rollover Impact in Buses and Coaches
connections of the roof bows to the walls.” [7]. By this simulation Bojanowski et
al. showed that the connections between the body and the driver’s cabin are one of
the weakest points of the bus body structure.
Figure 9) variation of Deformation Index (DI) with time in ECE-R66 simulation of
Paratransit bus [7]
As shown in Figure 10 on the road side the only connection between the
body and driver’s cabin are two flat pieces of steel while on the curb side only two
welded spots connect the two parts.
In the next step they tried to use their model for simulate a FMVSS 220 test.
After considering proper loading condition the result they gained was to their
surprise in contrary with previous test. The paratransit bus passed FMVSS 220
conditions. Figure 11 shows the diagram they gained for force applied on roof
versus displacement of roof panel.
18 Khashayar Pejhan - 7718719
Figure 10) Weak connections found by simulations on right and they shape in reality in left
picture [7].
According to FMVSS 220 a force equal to 1.5 times of unloaded vehicle
weight (UVW) should be applied to the roof, and the intrusion of roof structure to
residual space should not be more than 130.2 mm. As it could be seen in Figure 11,
at the time that applied force reaches 1.5 times of UVW the displacement is only
119 mm. So the paratransit buses pass FMVSS 220.
Figure 11) Force vs. displacement results of FE simulation of FMVSS 220 test [7].
19 Rollover Impact in Buses and Coaches
This research proved that FMVSS 220 could not be trusted for paratransit
buses since it does not consider a fair simulation of actual loading condition in
rollovers.
In the final stage of their study, Bojanowski et al. tried to identify and rank
the most relevant components of the structure in the two tests. Not surprisingly
roof bows were the first choice of variables to be modified to bear more quasi-
static load. By improving them the roof resistance force was changed about 75%
but the important thing is that their influence on Deformation index was only 14%.
As Bojanowski reported in [7], these results lead to a conclusion that testing
paratransit buses according to the FMVSS 220 standard may lead in some cases to
erroneous conclusions regarding the bus strength and integrity of its structure.
One of the most recent researches in area of rollover of buses is the study on
rollover analysis of bus body structure by Jeyakumar and Devaradjane [9] in 2012.
They analyzed rollover of bus structure considering the conditions of ECE-R66
standard. A 36 seat ultra-deluxe bus was selected for this research and necessary
dimensions was gained by reverse engineering method. This study is one of the
most recent Finite Element Analysis (FEA) of bus body structure, which has used
LS-DYNA explicit solver. Their main goal as they mentioned in their article [9]
was: “to investigate the impact on passenger residual space along the entire vehicle
during a rollover by simulation method”.
Using their computer model (see Figure 12), Jeyakumar and Devaradjane [9]
simulated an ECE-R66 for bus and found that residual space is secure.
But there are some important factors that seem to be neglected in this
research. First of all, they used lump masses instead of real parts in bus without
checking the effect of this assumption on accuracy of results. The second important
20 Khashayar Pejhan - 7718719
factor that was shown earlier is the importance of considering the special effect of
contacts between roof panel and structure. In this study they have considered same
contact elements for all contacts with same friction which is not precise enough,
regarding the importance of its influence. In addition, adaptive meshing seemed to
be necessary (more elements in contact areas and fewer in other parts which are
not important in rollover), but as their model shows, such consideration was not
made. So their model could become more precise and less time consuming by a
few modifications.
Figure 12) LS-DYNA model of bus in rollover test [9]
Another important factor that was not considered in the research, discussed
above, is the mass of passengers. First researchers to consider the effect of mass of
passenger weight on rollover crashworthiness were that of Guler et al. [21].
Likewise, Belingardi et al. [22] considered the passengers’ weight in their research
in which they used a FEM approach to study the structural behavior of a M3 bus in
rollover accident. Although this study was limited to a bay section of the bus, but
21 Rollover Impact in Buses and Coaches
their analysis on passenger injury risk showed that even if a bus comply with ECE-
R66 regulations it may be inadequate, concerning passive safety.
Next case to be studied here is a comprehensive FEM analysis by Volvo in
2012 [23]. In their research, engineers of Volvo presented a more complete model
of the bus than the one in the research discussed above. The ultimate aim of this
study was to investigate the impact on passenger residual space. One of the most
interesting aspects of this study is the method of validation of results. In order to
have a reliable computer model, Volvo engineers executed some actual tests on
critical joints of bus’ body. Then they used their computer model to predict the
behavior of same joints. After getting acceptable results from the model they
modified model in way to be more conservative. So they were sure that if the
computer simulation shows that this bus passes the test, the actual case would be
the same.
As mentioned earlier, the advantage of this study comparing to [roll-bus] is
its precise modeling. In this case FE model of the full vehicle was comprised of
first order shell elements, spring elements, mass elements and rigid elements. So
the whole structure was not considered to be made of a same rigid element. Mass
balancing was also done in this study using balancing tool in HyperCrash V11
4021.
The test regulation that was set as an aim in Volvo is AIS 031, titled
“Resistance of the Superstructure of Oversized Vehicles for Passenger
Transportation” [18]. This regulation states that vehicle structure should be strong
enough that during a rollover it does not intrude inside the residual space.
Although it seems that AIS 031 and ECE-R66 are same, but a more detailed view
will show that there are some important differences between them.
1 HyperCrash is a CAE pre-processor tool developed to support the non-linear finite element solver, Alter RADIOSS
22 Khashayar Pejhan - 7718719
Figure 13 and 14 shows the CAD model and FEA model of the coach
studied by Volvo in 2012.
Figure 13) CAD model of bus studied in Volvo’s research [23]
After preparing the CAD model, it is necessary to consider masses in the
system before performing meshing for FEM analysis. Engine, Power transmission
and suspensions were modeled by 1-D element. Mass elements were used for
modeling fixed masses. The whole vehicle had 1646623 shell elements, 7308
spring elements and 44 mass elements. In order to increase the accuracy the
elements on the roof were selected to be 10 mm long while in other parts where
there necessity for precision was not same as roof some elements were 30 mm
long. Considering the axle, it was set to be rigid and its mass was added at
appropriate locations, according to Volvo engineers [23]. The final FEM model of
bus is illustrated in Figure 14.
23 Rollover Impact in Buses and Coaches
Figure 14) FEM model of bus studied in Volvo’s research [23]
The next step is the loading process. Figure 15 shows the model after rolling
over. Both sides of the bus were tested in a rollover situation. The colliding
surface was set as a rigid wall. A torque equal to 0.75 Mgh (Nm) was applied to
the structure by a rotational velocity to all parts of the vehicle. Table 3.3 shows the
formulas used to calculate proper loading condition.
Figure 15) Loading and rollover of model [23]
24 Khashayar Pejhan - 7718719
Table 3.3) Loading condition and necessary formula
Loading condition Necessary formulas
Angular velocity and
gravity load applied on
whole body
( )
⁄ ( ( ) )
( )
(
)
Before presenting the results found by Volvo research team and analysing
them it is necessary to show how they have checked the accuracy of their model. In
order to check the accuracy of simulation results, Kumar et al. [roll-bus 4] checked
the total energy during simulation time period. Figure 16 shows the energy
distribution in buss rollover simulation. It could be noticed that while kinetic
energy transform to sliding energy and strain energy through time, total energy
remains same in whole simulation. As a result the results of simulation are accurate
and could be trusted.
Knowing the fact that simulation is accurate, Kumar [23] tried to check if
the residual space would be intruded after rollover or not. Figure 17 shows the
summary of the simulation results. As it could be seen, the driver side seems to
face less danger. The minimum distance between superstructure & residual space
was found to be 38.0 mm on the driver side.
25 Rollover Impact in Buses and Coaches
Figure 16) Energy balance graph- LHS rollover [23]
Volvo engineers have also ignored some important issues. First and probably
the most important fact is that AIS 031does not include the driver and co-driver. So
although a bus may look safe to passengers by passing AIS 031, but drivers cannot
be sure if they are also safe in case of a rollover. In addition according to AIS
031[18] structural parts, members and panels and all projecting rigid parts such as
luggage racks, on-the roof ventilation equipment, should not intrude the residual
space.
26 Khashayar Pejhan - 7718719
Figure 17) Summary of the results for both sides of bus [23]
But regarding the safety of passengers there many other hazards in a rollover
that could be considered as a menace to safety, Including:
Bulkheads
Partitions
Rings or other members reinforcing the structure of vehicle
Bars, kitchenettes or toilets (fixed appliances)
Unfortunately, none of the above is included in ECE R66/ AIS 031 and it would be
wise to consider them as possible research subjects, because there might be
mandatory regulations in future that consider them, and more importantly, they can
be dangerous for passengers in rollovers.
27 Rollover Impact in Buses and Coaches
4. Jordan Rollover System- an ignored method for
testing buses:
Since the earliest dynamic rollover test was introduced in 1934 [16], many
different rollover test devices have been designed to obtain the realistic dynamic
rollover test results. One of these devices is the Jordan rollover system (JRS). JRS
is a versatile and repeatable rollover test system developed to evaluate the
performance of roof structure and occupant restraint system during rollover. It can
also be used to study the kinematics of the occupant.
Many physical tests [24] and computational simulations [25] have been performed
and showed the effectiveness of this method. But this method is not popular in
coach producers. The reason would be obvious by a glance at the methodology of
Jordan Rollover System (JRS).
Methodology of JRS
As shown in Figure 18, a vehicle is mounted on an axis through its centre of
gravity (CG), which permits it to be rotated about its longitudinal axis. A roadbed
is released to move under the suspended vehicle when the vehicle starts to rotate
and drop. The vehicle contacts the moving roadbed in a controlled impact
configuration. This configuration includes roadbed speed, vehicle roll rate, drop
height, impact yaw angle, impact pitch angle and impact roll angle.
The standard values of these parameters are listed in Table 3.4 [26]. Only the
lateral translation and yaw rotation are continuously restrained once the vehicle is
released.
28 Khashayar Pejhan - 7718719
Figure 18) Jordan Rollover System [27]
After both the near side and the far side of the roof have contacted the
roadbed, the vehicle is halted in vertical direction to avoid further contact with the
tracks along which the roadbed moves [26].
Table 3-4) Initial impact configuration of JRS [26]
Initial condition Value
Roll rate
Drop height
Roadbed speed ⁄
Roll angle
Pitch angle
Yaw angle
29 Rollover Impact in Buses and Coaches
A dummy can be seated in the vehicle to investigate the occupant responses
and to evaluate the protection performance of the occupant restraint system during
rollover. Obviously executing such a test for heavy vehicles like buses or coaches
is very difficult, and expensive. This means that a versatile and repeatable rollover
test for buses is either impossible or very difficult and restricted to rich companies.
Computer simulation is a possible useful approach to fulfill the need of such
test for heavy vehicles. In 2008, Friedman and Hutchinson [27] performed a
computer simulation of JRS experiment on a vehicle to compare the results
between JRS, as a repeatable experiment, with CRIS (Controlled Rollover Impact
System). Figure 19 shows some sample results gained from computer simulation in
[27].
Figure 19) Jordan Rollover system initial impact configuration [27]
30 Khashayar Pejhan - 7718719
CRIS is another dynamic test for simulating rollover impact which involves
a semi-tractor and trailer at the back of which is mounted a fixture holding the
vehicle. The fixture can be adjusted to cause the vehicle to drop from a given
height giving it a defined vertical velocity. However, there is no direct
measurement of the horizontal or vertical speed of the vehicle at impact. The
traveling speed of the semi-tractor/trailer controls the horizontal speed of the
vehicle at impact1 . This computer modeling and similar researches has shown the
capabilities of FEM analysis in simulating the actual JRS experiment. As a result it
seems very useful to model a JRS test for a bus structure and check the reliability
of structure in such impact.
5. Conclusion:
During a bus or coach rollover, the occupant will have a larger distance from
the center of rotation as compared to that of a car occupant. So it is necessary to
investigate more about this type of accident in buses to ensure that passengers are
safe enough. NHTSA has considered this issue and introduced some obligatory
standards that all bus manufacturers in North America should pass. One of the
most important and famous standards to mention is FMVSS 220. But as shown in
this research there are some weak points in this regulation.
Bus manufacturers frequently strengthen up the roof structure to pass the
FMVSS 220 testing procedures. The symmetric loading applied to the roof in the
FMVSS 220, actually examines primarily the strength of the roof bows, without
applying excessive loading on the rest of the structure. This approach need to be
1 The results of [27] showed that JRS is more conservative, more versatile and more repeatable than CRIS
and as a result in this review CRIS is not covered in details.
31 Rollover Impact in Buses and Coaches
revised. As Bojanowski et al. [7] showed passing FMVSS 220 does not guaranty
the safety of residual space. So it would be vise to alter the focus from
strengthening the roof structure to considering more secure and strong connections
between the wall and the roof and the front cap.
In addition another important conclusion that could be made from this
review is the necessity of more focus on computer simulations. As shown on the
case of Paratransit buses, many manufacturers try to evade high costs of more
conservative test like ECE-R66 by some simpler test like FMVSS 220. As a result
they may produce vehicles that are not safe enough. In addition even if tests like
ECE-R66 motivate NHTSA to update its regulations, it will be a very expensive
test for many local manufacturers in North America. So by expanding the
computer simulations and trying to make them as similar as possible to actual case
engineers might find the weak points of the products without high costs of
producing expensive prototypes that might fail in very preliminary tests.
Another conclusion that could be made here is the importance of passenger
weight on accuracy of results. As was shown in section 3, by comparing two
studies ([9 and 22]) it was proved that considering passenger weight may put a
previously believed safe bus, in category of unsafe ones. The results gained [roll-
bus] in 2012 shows the necessity of more studies in future on FEM simulation of
bus rollover test with more accurate and detailed models that consider all neglected
influencing parameters like: more precise contact element selection, adaptive
meshing, full scale modeling, considering detailed geometry in order to simulate a
more accurate residual space and considering passenger weight.
It seems that industries have done comprehensive research on rollover, in
order to produce buses and coaches that comply with regulations, (as the example
presented from Volvo in part 3 [23]), but there are 2 reasons for continuing studies
32 Khashayar Pejhan - 7718719
in this area. First, the current standards and regulation would be updated in a few
years, and they will be for sure stricter. Second, currently many parts of buses are
capable of intruding the residual space (like: bulkheads, partitions, rings or other
members reinforcing the structure of vehicle, bars, kitchenettes or toilets (fixed
appliances)), that have not been considered in current studies due to their
complication. So it would be appropriate to improve current progresses in safety
criterion and research on more complete computer models of buses and coaches.
The last conclusion to be made is about repeatable vehicle rollover dynamic
physical testing methods. Because of their heavy weight and large structures,
motor coaches and buses have not been tested in experiments like Jordan Rollover
System (JRS) or Controlled Rollover Impact System (CRIS). But since there are
many validated FE models for such test for light vehicles, it is possible to model a
JRS test for a bus in computer and check the reliability of its structure by checking
different parameters that this versatile test provides. Performing such test may lead
to major revisions in design of structures; because these test represents the actual
behavior of vehicle much more realistic than current standard, obligatory tests (as
FMVSS 220).
33 Rollover Impact in Buses and Coaches
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35 Rollover Impact in Buses and Coaches
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