The Dynamics of Tank-Vehicle Rollover
and the
Implications for Rollover-Protection Devices

Final Report
UMTRI-98-53
November 1998

Prepared by
The University of Michigan
Transportation Research Institute
2901 Baxter Road, Ann Arbor, Michigan 48109
under Contract No. DTFH61-96-C-00038, Task Order #1
for
Federal Highway Administration
U.S. Department of Transportation
400 Seventh Street S.W., Washington, D.C. 20590

EXECUTIVE SUMMARY
Federal regulations DOT 406, 407, and 412 require that cargo tank motor vehicles
used on U.S. highways be equipped with rollover-protection devices to protect manhole
covers, valves, vents, and other top-mounted hardware from damage and to prevent
leakage of product during rollover accidents. In its 1992 Special Investigation Report
entitled “Cargo Tank Rollover Protection,” the National Transportation Safety Board was
critical of existing rollover-protection devices and found that “insufficient
guidance… [exists] about the factors and assumptions that a cargo tank manufacturer must
consider when calculating loads on the rollover-protection devices… ” and that “there is
inadequate information about the forces that can be encountered in a rollover accident and
the extent to which rollover-protection devices for cargo tanks can reasonably be designed
to withstand these forces… ”
This study has attempted to expand the knowledge base on the dynamics of tank
vehicle rollover events in order that such guidance and information can be improved.

DYNAMICS OF TANK-VEHICLE ROLLOVER
The primary contribution of this study is a broad-ranging examination of the dynamics
of tank-vehicle rollover accomplished through computer simulation. Seven cargo tank
motor vehicles, two tank trucks, and five tractor semitrailer combinations were simulated.
Each was subjected to 126 simulated maneuvers intended to result in rollover. Test
maneuvers included mild, low-speed turns that just barely produced rollover, more
dynamic maneuvers on smooth surfaces, and high-speed impacts with curbs and guardrails
that result in rapid rollover combined with substantial pitch and yaw.
Simulation runs were allowed to continue until the moment that the vehicle tank
contacted the ground. Measures of the dynamic motions of the vehicles at that instant of
impact were compiled and analyzed. The results describe the range of initial conditions of
the common impact events that can occur subsequent to a rollover and that engage the
rollover-protection devices. Three such impact scenarios were defined and analyzed:

• In a mild rollover, the vehicle may fall onto its side but continue to roll on the flat ground surface to engage the rolloverprotection devices. The primary dynamic parameter of interest in this mechanism is the value of roll rate at touchdown. Vehicles in the least severe of such rollovers achieve roll rates on the order of 100 deg/sec. Vehicles Simulations show that tankers land on their sides in mild rollovers ii landing on their sides in more energetic events have roll rates up to and beyond 150 deg/sec.

• In more dramatic
rollover events, the
vehicle may become
airborne and roll rapidly
enough to bring the
rollover-protection
devices into direct impact
with the ground. This
result can occur with unit
trucks on level ground
but appears to require a sloping or depressed roadside surface for it to happen to a
semitrailer tank. Trucks landing on their tops on the road achieve downward speeds of at
least 6 ft/sec. but rarely more than 18 ft/sec. The semitrailers allowed to fall sufficiently to
reach 180 degrees of roll achieve downward speeds as high as 30 ft/sec.

• In moderate and severe rollovers, the vehicle may land on its side and slide sideways into
any of the many objects with vertical surfaces that are typically oriented parallel to the
roadway such as guardrails, retaining walls, or embankments. Simulation showed that the
impact speed normal to such a surface often exceeds 20 or 30 percent of the initial
forward speed and occasionally can reach well beyond 40 percent.

IMPLICATIONS FOR ROLLOVER-PROTECTION DEVICES
A supplemental element of this study was an effort to provide some insight into the
design and engineering requirements for rollover-protection devices and the tanks to
which they are mounted, given the type and severity of impact conditions that developed
in the process of rollover.
Simple impact simulation models using idealized force/deflection characteristics were
developed and applied in a large matrix of conditions to evaluate the design implications
for the impacts defined in the vehicle dynamics study. The effort was intended to provide
broad guidance for the design of protective systems rather than precise determination of
forces in any given device.
Results indicate that impact due to rolling is of little concern for low-profile, rail-style
rollover-protection devices but may be a major challenge for discrete devices which
constitute a significant discontinuity in the profile of the tank. This is true even for roll
velocities associated with the mildest of rollovers.
Vertical and lateral impacts, even into simple planar surfaces, appear to pose a
significant challenge for all impact protection devices. The dynamic simulation study
showed that virtually every rollover event involved impact speeds of at least 6 ft/sec and
that 24 ft/sec was a reasonable upper bound to cover the majority of impacts.

Initial velocities of 6, 12, 18, and 24 ft/sec were used in the impact study. Of these velocities, 12 ft/sec is the lowest which could be judged as covering a significant fraction of realistic events. In many impacts at this velocity, if the combined structure of tank and protection devices provided a foot of crush distance, then it was often the case that the protective structure had to be of such strength that it could support ten times the weight of the vehicle. Since impact energy is proportional to velocity squared, the situation is four times more severe for impacts at 24 ft/sec.
Angular orientation of the tank relative to the impact surface was found to be an important factor. Compared to impact flat against the top surface of the tank, angular misalignments of just 10 and 15 degrees can increase the effective severity of impact by about ten-fold.

These observations suggest that effective protective systems that could survive such
impacts would represent a substantial increase in performance relative to that required by
today’s regulations. The magnitude of the forces involved would appear to demand that
effective designs spread the loading over a large portion of the tank structure, a fact which
probably eliminates the so-called staple-type devices and other discrete styles of protective
devices. Further, it appears that effective protective systems must involve controlled crush
of the tank to provide adequate crush distances without resorting to excessively large,
high-profile devices.

RECOMMENDATIONS
Three recommendations are presented below. By way of preamble, however, we take
care to note the nature of this study. The study was theoretical in that all results were
obtained from computer simulations. Although the vehicle dynamics simulation that was
used is well established and rather comprehensive, the impact simulations were newly
developed for this purpose. Further, the study endeavored to examine the dynamic and
physical properties of a large range of rollover events. However, as noted in the previous
section on background and philosophy, there is no adequate accident data base to establish
how these properties are distributed among accidents occurring in the real world. Thus,
while the second recommendation below is quantitative and suggests some consideration
of costs versus benefits, we frankly acknowledge that it is in part based on the authors’ experience and judgement rather than wholly on the content of the study. The third
recommendation recognizes the need for substantive cost/benefit considerations to be
accomplished in the future.

• Performance goals for rollover-protection devices should be expressed in terms of
impact events to be survived, not in terms of the strength of the device.

Rollover-protection devices must effectively manage impact energy. To do this, good
designs may benefit by allowing greater crush deformations, thereby maintaining low
forces. Such engineering trade-offs are best left to the design process, but designers
need to know the basic parameters describing the design task. A description of the
impact event is what is needed and such descriptions constitute the primary product of
this study. Expressing requirements through a description of the impact event is the
approach already employed by the USDOT in bumper standards and in standards for the
impact protection of passenger car occupants.

• A minimum design goal for rollover-protection systems should be effective
performance in impacts onto flat surfaces at speeds normal to the surface of at
least 12 ft/sec and with representative angular orientations of the tank with respect
to the impact surface. Designing for impact at speeds up to 24 ft/sec is desirable.

The vehicle dynamics study showed that virtually all rollover events yield impact
velocities of at least 6 ft/sec and that impact speeds of 24 ft/sec can be achieved in many
situations. It seems reasonable to recommend that any rollover-protection device, if it is
to deserve such a name, must be able to protect tank fittings during impacts covering at
least a significant portion of this range.

At the very least, protection should be ensured when the impact occurs squarely in
relation to the top surface of the tank, but consideration should also be given to
covering a representative range of angular misalignments between the tank and the
impacted surface.

• Evaluation of the cost effectiveness of performance standards for rollover
protection should be undertaken.

This study has sought to identify the pertinent physical properties of cargo tank rollover
events as a basis for specifying performance requirements for rollover-protection
devices. The question of the cost-effectiveness of such devices remains to be addressed.
By way of example, impact velocities of 12 and 24 ft/sec could be attained by dropping
a tank from rest onto a flat surface through distances of 2.2 and 8.9 feet, respectively.
The latter number certainly suggests a significant design challenge even if the impact
were square against the top surface of the tank. Reasonably representative angular
misalignments could increase the effective severity of impact by about ten-fold.
Evaluation of the incremental cost to the transportation versus the potential societal
savings which might result from various levels of such performance requirements is
appropriate. Such analysis will require, among other things, knowledge of the statistical
distribution of rollover accidents in terms of their physical severity and the occurrence of
cargo spillage.

Table of Contents
EXECUTIVE SUMMARY ....................................................................................I
DYNAMICS OF TANK-VEHICLE ROLLOVER ........................................................ I
IMPLICATIONS FOR ROLLOVER-PROTECTION DEVICES................................. II
RECOMMENDATIONS............................................................................................III
INTRODUCTION................................................................................................. 1
BACKGROUND AND PHILOSOPHY................................................................. 3
DYNAMICS OF TANK-VEHICLE ROLLOVER................................................. 7
VEHICLE SIMULATIONS..........................................................................................7
SIMULATED VEHICLES ...........................................................................................8
SIMULATED MANEUVERS ....................................................................................12
Intersection turn (I-turn)..........................................................................................12
Highway or exit-ramp turn (H-turn) ........................................................................13
Curb-strike and rail-strike maneuvers (trip and rail).................................................13
Spiral turn (spiral) ..................................................................................................13
High-speed avoidance maneuver (swerve)................................................................13
Step-turns (step) .....................................................................................................14
RESULTS FROM THE VEHICLE-DYNAMICS SIMULATION RUNS ...................14
ANALYSIS OF SIMULATION RESULTS................................................................18
Description of sample rollover events ......................................................................18
Quantitative examination of the simulation results ...................................................25
IMPLICATIONS OF THE RESULTS FOR MINIMUM PERFORMANCE
REQUIREMENTS FOR ROLLOVER-PROTECTION DEVICES.........................32
IMPLICATIONS FOR THE STRUCTURE OF ROLLOVER-PROTECTION
DEVICES AND TANKS............................................................................. 33
IMPACT SIMULATION MODELS...........................................................................33
IMPACT SIMULATION MATRIX............................................................................39
RESULTS OF THE IMPACT SIMULATION STUDY..............................................44
Observations on the results of the rolling-impact model............................................45
Observations on the results of the normal-impact model...........................................48
OBSERVATIONS REGARDING THE CURRENT REGULATIONS .......................56
CONCLUSIONS AND RECOMMENDATIONS................................................ 57
DYNAMICS OF TANK-VEHICLE ROLLOVER ......................................................57
IMPLICATIONS FOR ROLLOVER-PROTECTION DEVICES................................57
RECOMMENDATIONS............................................................................................59
REFERENCES.................................................................................................... 61
APPENDIX A. VEHICLE ROLLOVER SIMULATION INPUT PARAMETERS
......................................................................................................................... A-1
APPENDIX B. VEHICLE ROLLOVER SIMULATION RESULTS ................ B-1
APPENDIX C. IMPACT SIMULATION RESULTS........................................ C-1

List of Figures
Figure 1. Photo of a truck similar to that of the Bronx accident.................................................... 9
Figure 2. Line drawing of the Hamilton accident vehicle with photo of the tank............................ 9
Figure 3. Photo of a semitrailer similar to that of the Albuquerque accident.................................10
Figure 4. Photo of the vehicle from the Columbus accident .........................................................10
Figure 5. Photo of the vehicle from the Lantana accident ............................................................10
Figure 6. Estimating the roll moment of inertia of the liquid load.................................................11
Figure 7. Closed-loop simulated maneuvers................................................................................12
Figure 8. The open-loop high-speed avoidance maneuver............................................................14
Figure 9. Plan-view of a vehicle rolled onto its side at the completion of a simulation run ............17
Figure 10. The Bronx vehicle in a minimal rollover (run #116, Dt = 0.25 sec) .............................19
Figure 11. Plan-view sketch of the Bronx vehicle at the end of run #116 .....................................20
Figure 12. The Hamilton vehicle in a rapid, flat-surface rollover (run #63, Dt=0.1 sec) ...............20
Figure 13. Plan-view sketch of the Hamilton vehicle at the end of run #63...................................21
Figure 14. The Bronx vehicle rolling over after striking a curb (run #28, Dt=0.1 sec) ..................21
Figure 15. Plan-view sketch of the Bronx vehicle at the end of run #28 .......................................22
Figure 16. The Hamilton vehicle rolling over after striking a rail(run #52, Dt=0.1 sec) ................22
Figure 17. Plan-view sketch of the Hamilton vehicle at the end of run #52...................................23
Figure 18. The Albuquerque vehicle rolling over and falling after striking a curb at 20 degrees and
45 mph (run #124) ..................................................................................................24
Figure 19. The Albuquerque vehicle rolling over and falling after striking a curb at 30 degrees and
45 mph (run #126) ..................................................................................................25
Figure 20. The final roll angle for all successful simulated test runs............................................26
Figure 21. Final vertical speeds of the centers of gravity of the tank units....................................27
Figure 22. Comparison of final vertical speeds from the simulation and from the simple calculation
based on the center of gravity falling from its apex height ........................................28
Figure 23. Final pitch angles of the tank units.............................................................................28
Figure 24. Final roll rates of the tank units .................................................................................29
Figure 25. Final speed of the tank units across the roadway ........................................................30
Figure 26. Final speed of the tank units across the roadway grouped by maneuver ......................30
Figure 27. Final yaw angle of the tank units relative to a vertical surface parallel to the road......31
Figure 28. Impact deformation may take place in the tank or in the rollover-protection devices ....34
Figure 29. Comparison of the energy dissipation performance of idealized and realistic structures
...............................................................................................................................35
Figure 30. Animation frames from a simple run of the normal-impact model ...............................37
Figure 31. Animation frames from a simple run of the rolling-impact model................................38
Figure 32. The design variations of the rollover-protection devices..............................................40
Figure 33. Dimensional properties of the rollover-protection devices represented in the impactsimulation
study......................................................................................................41
Figure 34. Example results from the impact simulation study......................................................44
Figure 35. Rolling-impact results for the trucks in their as-designed configurations .....................46
Figure 36. Rolling-impact results for the semitrailers in their as-designed configurations .............46
Figure 37. Forces on the protectors during rolling impact............................................................47
Figure 38. The influence of variations of discrete-element design on the rolling-impact results for
the Albuquerque vehicle ..........................................................................................48
Figure 39. The relationship between percent of initial forward speed in miles per hour and impact
speed in ft/sec .........................................................................................................49
Figure 40. Comparison of results from similar vertical and lateral impacts of the Lantana
semitrailer (all at 12 ft/sec)......................................................................................49
Figure 41. The influence of velocity on the results from simulations of the Lantana semitrailer in
flat-on-the-top impacts ............................................................................................50
Figure 42. Comparison of results from the as-design vehicles in 12-ft/sec impacts, flat against the
top..........................................................................................................................51
Figure 43. The performance of discrete elements approaches that of rails as their numbers increase
...............................................................................................................................51
Figure 44. The impact data of figure 43 presented in normalized form ........................................52
Figure 45. Results from impacts at 12 ft/sec and –5 degrees of pitch presented in normalized form
...............................................................................................................................53
Figure 46. Results from lateral impacts of the Lantana semitrailer (R1) at 18 ft/sec and at various
roll and yaw angles .................................................................................................54
Figure 47. Comparison of results for various protector configurations ........................................55

Figure 33. Dimensional properties of the rollover-protection devices represented in the impactsimulation
study......................................................................................................41
Figure 34. Example results from the impact simulation study......................................................44
Figure 35. Rolling-impact results for the trucks in their as-designed configurations .....................46
Figure 36. Rolling-impact results for the semitrailers in their as-designed configurations .............46
Figure 37. Forces on the protectors during rolling impact............................................................47
Figure 38. The influence of variations of discrete-element design on the rolling-impact results for
the Albuquerque vehicle ..........................................................................................48
Figure 39. The relationship between percent of initial forward speed in miles per hour and impact
speed in ft/sec .........................................................................................................49
Figure 40. Comparison of results from similar vertical and lateral impacts of the Lantana
semitrailer (all at 12 ft/sec)......................................................................................49
Figure 41. The influence of velocity on the results from simulations of the Lantana semitrailer in
flat-on-the-top impacts ............................................................................................50
Figure 42. Comparison of results from the as-design vehicles in 12-ft/sec impacts, flat against the
top..........................................................................................................................51
Figure 43. The performance of discrete elements approaches that of rails as their numbers increase
...............................................................................................................................51
Figure 44. The impact data of figure 43 presented in normalized form ........................................52
Figure 45. Results from impacts at 12 ft/sec and –5 degrees of pitch presented in normalized form
...............................................................................................................................53
Figure 46. Results from lateral impacts of the Lantana semitrailer (R1) at 18 ft/sec and at various
roll and yaw angles .................................................................................................54
Figure 47. Comparison of results for various protector configurations ........................................55

INTRODUCTION
This document is the final report of the University of Michigan Transportation
Research Institute (UMTRI) on the research project entitled “Determination of Forces in
Cargo Tank Rollover-Protection Devices.” Funding for the project was provided by the
Federal Highway Administration (FHWA) of the U.S. Department of Transportation
(USDOT) under Task Order No. 1 of Contract No. DTFH61-96-C-00038.
The purpose of the study was to outline requirements for cargo tank rolloverprotection
devices, typically affixed to the top of tank vehicles, which are meant to protect
manhole covers, valves and other tank openings during rollover events. The project was
analytical in nature. Conventional vehicle simulations were used to examine the dynamics
of the rollover of tank vehicles up to the point of crash impact. Additional computer-based
analyses were then used to broadly characterize the force-deflection qualities required of
rollover-protection devices to be effective in such events.
This report begins with a discussion of the background for and philosophy of the
project. Two technical sections follow which address the dynamics of tank-vehicle rollover
and the implied requirements for protection devices, respectively. The final section of the
main text presents conclusions and recommendations. Other technical materials are
appended.

BACKGROUND AND PHILOSOPHY
U.S. Federal regulations DOT 406, 407, and 412 require that cargo tank motor
vehicles used on U.S. highways be equipped with rollover-protection devices[1].1 Such
devices are typically mounted on the top of the tank body and are intended to protect
manhole covers, valves, piping, vents, et cetera. from damage and potential leakage of
product during rollover accidents.
In 1992, the National Transportation Safety Board (NTSB) issued a Special
Investigation Report entitled “Cargo Tank Rollover ” [2]. This report was critical of both
the rollover-protection devices which are commonly deployed on the U.S. cargo-tank fleet
and the adequacy of the related regulations and their application. The report examined
seven cargo tank rollover accidents. Given that a rollover had occurred in each case, these
accidents, with one exception, can be broadly characterized as ranging from mild to
moderate. Nonetheless, substantial cargo leakage occurred in each of these accidents due
to failure, in one form or another, of manhole covers and fittings on the top of the cargo
tank. The report concluded that some rollover-protection devices involved in these
accidents simply did not meet federal requirements. However, it also found that
“insufficient guidance… [exists] about the factors and assumptions that a cargo tank
manufacturer must consider when calculating loads on the rollover-protection devices… ”
and that “there is inadequate information about the forces that can be encountered in a
rollover accident and the extent to which rollover-protection devices for cargo tanks can
reasonably be designed to withstand these forces… ”2
It should be specifically noted, however, that the NTSB investigated this particular set
of accidents partly because leakage was reported. There is no effort in [2] to establish the
probability of cargo leakage in all accidents of comparable severity. In fact, there is little
basis in the various national accident data bases by which to establish statistical
relationships between rollover events, their physical severity, and the occurrence of
leakage by the failure of rollover-protection devices.
The most detailed information on truck accidents collected annually is in the Trucks
Involved in Fatal Accidents (TIFA) files produced by the UMTRI Center for National
Truck Statistics. As the name implies, only fatal accidents are covered by this file. The

1 Numbers in brackets designate bibliographic references given at the end of this text.
2 Note that the applicable federal regulations in 1992 where MC 306, 307, and 312. These have since
been replaced with DOT 406, 407 and 412 which call for some increase in the strength of rolloverprotection
devices. In later sections of this report, results will be presented suggesting that the new
specification are not likely to be effective.

this type of accident becomes more severe, it is likely to be more “individualized” and less
readily classifiable. Indeed, looking at cause and effect, the greater the severity, the more
likely the cause is rare or unique.
In this context, the NTSB findings imply that even the large class of mild accidents can
be too much for many rollover-protection devices to manage. It follows that to improve
rollover-protection devices to a level at which they could be effective in these relatively
mild events would be to make substantial progress with a large fraction of tanker rollover
accidents. However, to proceed beyond that level is to advance into a morass of highly
individual events which (1) would require extraordinary effort to characterize, and (2) are
probably not amenable to “standard” solutions— and even if they were, the solutions
would be excessively expensive.
If the unique qualities of a crash event can be expected to influence the forces in
rollover-protection devices, so too, of course, can the very design of the device itself (as
well as the design of the tank on which it is mounted). Rollover-protection devices suffer
the forces they do as a result of impact events. As in all impact events, the magnitude of
the forces involved is related to the dissipation of kinetic energy. The moving vehicle
possesses kinetic energy in proportion to its mass (M) times the square of its velocity (V).
This energy must be dissipated through the action of forces (F) exerted on it during the
event times distance traveled during that exertion (D). That is, in the simplest form: FD =
1/2MV2. The vehicle’s mass and velocity are given for a particular event, but force and
distance are influenced by design characteristics. If the vehicle and the object it strikes are
nearly rigid, then the distance traveled during impact is very small and the force required
to stop the vehicle in this very short distance is very large. On the other hand, if the vehicle
or the object it strikes is designed to crush, then distance is increased and the required
force is proportionately smaller. Thus, on modern passenger cars there are such things as
energy absorbing bumpers and collapsing steering columns, and by the roadside we find
various types of barriers intended to deform during a crash. Likewise, if rolloverprotection
devices and/or the tanks to which they are attached give way during impact,
forces in the structures are reduced. Of course, the design challenge is to insure that crush
takes place in a way that leaves the tank openings protected. One approach is to provide
crush space in the design of the protection devices themselves, in which case forces
experienced by these devices would depend mostly on their own design. But, in concept, it
is also possible to mount rather rigid protection devices on top of a tank structure which,
itself, provides the needed deformation. In this case, forces experienced in the devices
depend more heavily on the tank design. Realistic solutions probably encompass both of
these design elements.
In light of all this, this project has sought to:

• generally describe the dynamic vehicle motion conditions which prevail in tank-vehicle
rollover events and to distill from that broad description simplified crash conditions
that could be used to describe performance demands for rollover-protection devices,
and
• examine in a broadly applicable manner, the force-deflection characteristics required of
rollover-protection devices and the tank structure if they are to meet these demands.
The first bulleted item— a broad description of rollover dynamics and related
“performance requirements” for rollover-protection devices— is viewed by the authors as
the primary result of the project. Regardless of whether or not these results are ever used
in a regulatory scenario, they provide tank designers with a heretofore unavailable
resource to aid in the development of effective rollover-protection devices.
The second element of the study is intended to provide a fundamental basis for the
design of protective systems and to serve as an aid to the reader in interpreting the
engineering demands implied by specified crash parameters.

DYNAMICS OF TANK-VEHICLE ROLLOVER
A broad examination of the dynamic conditions which prevail during the rollover of
tank vehicles was undertaken via computer simulation. UMTRI’s TruckSim computer
programs, with modifications, were used as the basic simulation tool. The set of test
vehicles simulated were essentially defined by the vehicles of the NTSB special report.
Rollovers were elicited with turns on flat surfaces, turns involving contact with curbs and
guard-rails, and evasive lane-change-like maneuvers. Severity of the maneuvers ranged
from the minimum required to produce rollover to very severe maneuvering. The
simulation runs were allowed to proceed until the point in time when either the vehicle
tank profile contacted the ground or, in some cases, when the roll angle reached 180
degrees. The motion variables of the tank at these final impact conditions were assembled
and analyzed. In the following subsections, we will briefly describe the technical elements
of the simulation study and then examine the results.

VEHICLE SIMULATIONS
UMTRI’s TruckSim computer simulation system formed the basis of the vehicle
dynamics models used in this study [3]. TruckSim is a software package for predicting
braking, steering, and roll behavior of heavy trucks and combination vehicles. It combines
advanced simulation models with an easy to use point-and-click interface. The vehicle
models are built on over twenty years of research at UMTRI where the emphasis has been
on understanding the most important factors contributing to the vehicle dynamics.
TruckSim includes a “fleet” of simulation models covering single-unit and combination
heavy trucks and busses. The models range in complexity from a 26-degree-of-freedom
(DOF) 2-axle truck model to a 67-DOF tractor-semitrailer model. The TruckSim models
have been used extensively by UMTRI in previous research for the USDOT (e.g., [4]) and
are currently used by many others interested in truck dynamics. FHWA has recently
acquired a version of the TruckSim models for aiding in highway design [5].
For this study, the TruckSim models were modified (1) to allow the simulations to
proceed to high vehicle roll angles, (2) to include forces applied to the vehicle due to
contact with curbs and guardrails, and (3) to include a description of the geometry of the
tank shell, including protection devices, and identify the time of impact of the tank with
the ground plane.
In traditional vehicle dynamics analyses, the investigator’s interest in vehicle behavior
typically ends when it is clear that the vehicle is rolling over. Consequently, simulation
programs often include provision to automatically stop when a specified roll angle is
passed. Such programs often take advantage of the certain knowledge that roll angles are
small to make appropriate simplifications in the calculations. The TruckSim programs used in this project were modified to allow valid calculation of very large roll angles (180
degrees of roll angle and beyond).
Rollover of heavy trucks may take place due to relatively severe maneuvering on flat
road surfaces, but rollover events in the real world often involve “tripping” over curbs or
“tumbling” over guard-rails. In its original form, TruckSim does not include provisions for
forces imposed by curbs or guardrails. These forces were included through the addition of
rather simple functions. Forces normal to the road-side object were introduced using a
spring-like function of contact interference, and forces tangential to the object were
included as a friction force. Interference was determined by tracking the position of
specified points on the vehicle relative to a defined arc on the ground plane. Location of
the interference points on the vehicle (i.e., low on the tires, or higher up on unsprung and
sprung masses) determined whether the arc represented a curb or a guard-rail.
Finally a matrix of points fixed in the vehicle’s sprung mass was added to the
simulation. The position of these points in the vehicle coordinate system represented the
outline of the tank shell. The programs tracked the position of the points in the earth
coordinate system during the simulated runs. Decent of any point on the tank to the
ground plane indicates the occurrence of a “strike” of the tank and the end of the
simulation run. (In some cases, simulations were allowed to proceed further on the
assumption of a downward-sloped road-side surface.)

SIMULATED VEHICLES
The descriptions of the simulated vehicles were derived from the seven vehicles
reported on in the NTSB special report [2]. In simplified outline, the simulated vehicles
were
• a three-axle unit truck like the gasoline delivery truck of the Bronx, New York accident
(figure 1),
• a two-axle unit truck like the fuel-oil delivery truck of the Hamilton, Ontario accident
(figure 2),
• a five-axle tractor semitrailer combination like the vehicle of the Albuquerque, New
Mexico accident, that is, with relatively low-volume, round-profile tank intended for
high-density liquids (hydrochloric acid in the accident) (figure 3),
• a five-axle tractor semitrailer combination like the vehicle of the Columbus, Ohio
accident, that is, having oval-profile tanks nominally intended for petroleum products
(figure 4),
• a five-axle tractor semitrailer combination like the vehicle of the Lantana, Florida
accident, that is, having oval-profile tanks nominally intended for petroleum products
(figure 5).











The latter two vehicles differ primarily in cross-sectional geometry of the tank, the
trailer of the Columbus vehicle being a longer lower design than that of Lantana. The
vehicles of the other two accidents (Edenton, North Carolina and Ethelsville, Alabama)
were very similar tankers hauling petroleum products.
The truck from the Bronx accident was simulated as carrying a full load of gasoline
(6.1 lb/gal), and the Hamilton truck as having a full load of fuel oil (8.0 lb/gal). The
Albuquerque semi was simulated as filled with hydrochloric acid (9.8 lb/gal). Each of the
other two semitrailers were simulated with two full loads, one of gasoline and one of fuel
oil. With payload differences, there were then seven test vehicles.
The geometric properties of the vehicles were taken from the design drawings
available from the NTSB investigation file. Tire and suspension properties were “typical”
as derived from UMTRI’s library data files.

In developing the parameter sets describing these vehicles, mass, center of gravity
location, and inertial properties were derived from a combination of the cargo type, tank
volume, vehicle weight data available in the NTSB file, and UMTRI’s understanding of
the tare properties of truck, tractor, and trailer chassis. Distributions of cargo as implied
by the tank geometry were used to determine payload moments of inertia. Also in regard
to mass properties, note that the TruckSim programs do not have special capabilities to
simulate liquid loads, but in large part liquid motion is not at issue for full loads. The
exception is in regards to the roll moment of inertia of the payload. The general cylindrical

shape of the tanks and the absence of longitudinal baffles implies that much of the cargo
mass does not rotate in roll along with the tank shell. To account for this, roll moments of
inertia were represented as a fraction of the inertia which would result from a solid load of
the same geometry and density. Two values believed to span the range of reasonable
representations were used. For the oval tanks, values equal to 25 percent and 50 percent
of the rigid-material values were used. (Twenty-five percent was identified as a minimum
estimate for the case of an elliptical tank as shown in figure 6.) For a tank of circular cross
section, 10 percent and 50 percent were the values used.

More detailed parametric descriptions of the simulated vehicles appear in appendix A.

SIMULATED MANEUVERS
Each test vehicle was run through some 126 simulated maneuvers. Rollover occurred
in the majority of these runs but not in all since some maneuvers were designed to search
out a minimum condition for rollover. Many maneuvers were conducted on a flat surface.
Other maneuvers had the vehicle “tripping” over a raised curb or a guardrail. Some
maneuvers were executed in a closed-loop manner with the vehicle attempting to follow a
predefined, constant-radius turn. Other maneuvers were open-loop, that is, using a
predefined steering time-history. A brief description of each type of maneuver follows.
Sketches showing the geometry of some of the maneuvers appear in figures 7 and 8.
Intersection turn (I-turn)
The intersection turn is a closed-loop maneuver in which the vehicle attempts to
follow a 100-foot radius curve. The maneuver was conducted at speeds of 20, 23, 25, 27,
40, and 55 miles per hour. The approach and the accident landing area were level and flat.

Runs at lower speeds were intended to search out a minimum rollover condition. The
maneuvers at higher speeds were meant to represent a surprise, avoidance maneuver.

Highway or exit-ramp turn (H-turn)

The highway turn is a closed-loop maneuver in which the vehicle attempts to follow a
500-foot radius curve. The maneuver was conducted at speeds of 50, 55, 60 and 70 miles
per hour. The approach and the accident landing area were level and flat. The maneuver
was intended to represent rollover due to excessive speed in highway or exit-ramp turns.

Curb-strike and rail-strike maneuvers (trip and rail)

These maneuvers are adaptations of the highway turn in which the vehicle strikes
either a six-inch curb or a guardrail— a vertical surface from 16 to 36 inches above the
ground. The object struck is also arranged on a 500-foot radius arc but the path of the
vehicle has been offset to result in a specific angle of impact with the object. These
maneuvers were conducted at 35, 40, 45, 50, and 55 miles per hour and with impact
angles of 5, 10, 20, and 30 degrees. In most cases the surface landing area was flat and
level. Two additional maneuvers were conducted, however, in which the landing area was
assumed to lie outside the curb and to fall down and away from the road surface. In these
maneuvers, the rollover event was precipitated just as in the trip maneuvers using 20 and
30-degree curb-strike angles and a forward speed of 45 miles per hour. However, the
vehicle was allowed to roll up to 180 degrees regardless of vertical position of the tank.
These maneuvers are designated as trip-fall maneuvers.

Spiral turn (spiral)

The spiral turn is an open-loop maneuver in which the steering-wheel angle is slowly
increased (at a rate of 2 deg/sec) in order to elicit a quasi-steady-state rollover of the
vehicle. The maneuver was conducted at 40 miles per hour and was intended to produce a
minimum-severity rollover for this speed range. The approach and the landing area were
level and flat.

High-speed avoidance maneuver (swerve)

The high-speed avoidance maneuver is an open-loop maneuver simulating a severe
lane change. The maneuver begins with a turn to the right which is not sufficient to
rollover the vehicle but does initiate rolling motion. This is followed by a strong correction
to the left which results in rollover. The maneuver was conducted at 50 miles per hour.
The approach and the landing area were level and flat. The maneuver is intended to elicit
higher levels of roll rate than do the other flat-surface maneuvers.

Step-turns (step)
The step turn is an open-loop maneuver in which the steering wheel is displaced very
rapidly (i.e., within an interval of 0.2 seconds) from the straight-ahead position to a
predefined angle and then is held fixed. The maneuver was conducted with steering-wheel
angles and speeds as follows: 80 degrees at 45, 60 and 70 miles per hour; 100 degrees at
35, 50, and 65 miles per hour, 120 degrees at 30, 45, and 60 miles per hour. The
approaches and the landing areas were level and flat. These maneuvers were conducted
essentially to insure that the simulation matrix included maneuvers with steering inputs of
the highest possible frequency content.

RESULTS FROM THE VEHICLE-DYNAMICS SIMULATION RUNS

A compilation of results from all the vehicle dynamics simulation runs is presented in
appendix B. This section will explain the data presentation of appendix B using table 1 as
an example. A discussion of the analysis of these data follows in the next section.
The first six columns of table 1 identify the simulation run. Column 1 presents the run
number for the vehicle. Column 2 identifies the test vehicle, including the roll inertia
factor. Column 3 gives the maneuver type per the previous discussion. Column 4 gives the
vehicle’s forward speed. For runs involving impact with curbs or guardrails, column 5
presents the angle at which the vehicle strikes the object. For runs in which the vehicle
follows a prescribed path, the radius of the curve is given in column 6.
The remainder of the table presents results of the run in terms of the conditions which
prevail at its completion. Completion is nominally defined as the first moment at which the
tank strikes the ground. In some cases the vehicle does not rollover, in which case the run
simply times out. In others, the ground is assumed to slope down and away beside the road, in which case the run is allowed to proceed to high roll angle even though some
points on the tank may have penetrated the nominal ground plane.
Before describing the content of the individual columns, we will define some related
terminology. The axis systems and angles referred to are in accordance to ISO definitions
and nomenclature [6]. The axes systems are right-hand orthogonal. The earth axes (XE, YE, ZE) and the vehicle axes (XV, YV, ZV ) do not appear in the table directly but are the
reference axes for the roll, pitch, and yaw angles.

Table 1.
Example of results from the simulation study: semitrailer of the Albuquerque vehicle