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1. Bus Rollover Simulation & AnalysisShare
Bus Rollover- Introduction
Bus rollover event is one of the most crucial hazards for the safety of passengers and the crew riding in a
bus. In the past few years it has been observed that after rollover accidents the deforming body structure
seriously threatens the lives of the passengers. Hence, rollover strength of the bus bodies has become an
important issue for bus and coach manufacturers.
In India, the general trend noticed in the market is that OEMs sell their drive away bus chassis. The bus
body design and building is done by third party bus body builders. Many bus body builders unknowingly
employ poor designs, resulting in large amounts of fatal accidents, mostly rollover. The existing bus body
structures are inadequately designed, sub-optimal and unsafe. Wood is used in the construction of the bus
bodies to a large extent.
The cause for death or injury in bus roll over is primarily due to intrusion of structure inside the
passenger’s survival space. Hence, the bus body (superstructure) should be strong enough to absorb the
energy of impact caused by bus rollover and the resulting deformation of the superstructure should not
result in intruding the minimum survival space for the passengers.
Fig.1 Bus Rollover
The government bodies have regulated the bus rollover strength requirements of the bus bodies by
releasing Automotive Standards like AIS-052 & AIS-031. These regulations ensure that the bus body
superstructure is strong and safe in an event of a rollover accident.
Types of Bus Rollover Accidents

2. Mainly there are 2 possible cases of bus rollover accidents.
Fig.2 Bus Rollover Cases
The case that needs to be considered for compliance mainly depends on the usability of the vehicle. A city
bus has to run on good quality roads and the possibility of roll over is only during high speed cornering
(E.g. BRTS Corridor), so Case-2 (bus rolls over on ground) will be selected. On the other hand if it is an
intercity/tourist bus, the probability of roll over into a ditch is high, Hence Case-1 (bus rolls over into a
ditch of 800 mm depth) is selected.
However, the structural strength requirement is mostly driven by AIS-031. AIS 031 standard ensures that
the bus body superstructure has sufficient strength such that no part of the structure intrudes into the
residual space (explained below) of passenger in the event of rollover.
Physics behind Bus Rollover
Bus rollover is a phenomenon in which the fully laden bus rolls about an axis parallel to the longitudinal
direction of the bus passing through the outermost edge of the outermost tyre (Right/Left/Front/Rear) on
ground.
Bus rollover happens due to collision or high speed during cornering. As a vehicle rounds a corner, three
forces act on it: tire forces (the centripetal force), inertial effects (the centrifugal force), and gravity. The
cornering forces from the tire push the vehicle towards the center of the curve. This force acts at ground
level, below the center of mass. The force of inertia acts horizontally through the vehicle's center of mass
away from the center of the turn. These two forces make the vehicle roll towards the outside of the curve.
The force of the vehicle's weight acts downward through the center of mass in the opposite direction.
When the tire and inertial forces are enough to overcome the force of gravity, the vehicle starts to turn
over.
Methods for Bus Rollover Analysis
Physical Testing
During physical testing, the bus placed on platform is tilted about the rotation point (specified as per AIS-
031) with the angular velocity of not more than 0.087 rads/sec. At some specific angle, the bus C.G.
imbalances and the bus rolls into the ditch of depth 800 mm due to its own gravity. The intrusion of the
side structure into the residual space is checked manually and regulation pass/fail is decided based on the
AIS-031 criteria.
Computer Aided Engineering - Bus Rollover Simulation

3. In this method the actual case is simulated to an extent that is accurate enough to confirm the protection
from rollover. It is a cost effective & less time consuming process. Moreover, bus rollover simulation in
comparison to physical testing provides enough scope for optimizing the bus body structure which further
helps in reducing the overall weight of the bus.
Conventionally many bus body builders go for actual vehicle’s physical testing which incurs huge cost
and time. It is advisable to achieve the same result with bus rollover simulation. The method is explained
in brief below.
For bus rollover simulation we require final angular velocity of the bus during impact on ground & the
residual space as per AIS-031 definition.
Residual Space Creation
Fig.3 Residual space
1. Residual Space (Survival Space) is to be created in the bus as per AIS-031 shown in Fig.3.
2. The position of the "R" point shown in Fig.3 shall be assumed to be 500 mm above the floor under the
passenger’s feet, 300 mm from the inner surface of the side of the vehicle and 100 mm in front of the
seat back in the Centre-Line of the outboard seats.
3. Height of the Residual space should be minimum 750 mm.
Bus Rollover Kinematics
It explains the position of rotation points about which bus rolls and calculation of angular velocity with
which bus rolls into the ditch.

4. Fig.4 Bus Rollover Kinematics
1. First Rotation (θ1, Position A to B): The bus along with stoppers should be rotated about the bottom
point of rear wheel stopper (O1) till the CoG of bus reaches maximum height (Unstable position).
2. Second Rotation (θ2, Position B to C): The bus excluding stoppers should be rotated about the top most
point (O2) of the rear stopper (Point height=80 mm) till it touches impact floor. (Note: Keep 1-1.5 mm
gap between Impact floor and part which comes first in contact)
 Angular velocity is calculated by using formula: (Refer Fig.5)
1/2 IRA ω2
= M * g * ∆H
Where,
IRA = ICOG_XX + Mass * D2
(Mass moment of inertia about rotational axis)
∆H = Height between Max. CoG & Final CoG
D = Distance between CoG and Second point of rotation O2
ω= Angular Velocity
M = Total mass of the bus
g = Acceleration due to gravity
CAE- Bus Rollover Simulation
Bus rollover simulation can be performed in several ways (1D/2D/3D mesh) & using various softwares,
but the most appropriate method & softwares are considered in terms of accuracy and speed of solution.
The CAE simulation includes four major steps, viz. CAD Preparation, Rollover Kinematics, Residual
Space, Deck Pre-processing, Deck Solving and Post-processing as described below.
Deck Pre-processing
1. Accurate finite element 2D mesh model is prepared from the CAD model of the bus super structure &
chassis.
2. Connections i.e. welding and bolts are modelled using 1-D elements.
3. All components mounted on the structure are either meshed or used as lumped mass (0D Mesh).
4. The overall weight & CoG of the bus is matched with the actual bus.
5. Assignment of Material properties.
6. Application of Gravity load at global CoG of bus and Angular velocity to all components of bus.
7. Assignment of Termination time, Time step required for simulation and output files needed for post
processing.
Solution

5.  The Deck prepared should be imported in solver and number of CPU’s required for run should be
mentioned.
 Check for any errors; make corrections as per error and Rerun the analysis.
 Check for mass addition and percentage increase mass.
 Run the Solution, this generally takes 24 Hours to solve the mathematical model.
Post-processing
 Check energy balance i.e. Total Energy, Internal Energy, Kinetic Energy, Rigid Wall energy and
Sliding Interface Energy of each component of bus.
Fig.5 Energy balance
 Gap Measurement:
Fig.6 Gap measurement between residual space and pillars
 If gap is more than 5 mm, then the structure is not penetrating into the residual spaces and it passes
AIS-031.
 If gap is less than 5 mm, then the structure fails and suitable countermeasures need be taken.
Factors Affecting Bus Rollover Performance
Position of Centre of Gravity (CoG)

6. Stability of bus while turning is dependent on the position of CoG of bus. If height of bus CoG is more,
the bus will be more prone to rollover than one having low CoG. Also as height of CoG increases, the
radius of rotation will increase, which increases the kinetic energy in structure before impact resulting
more damage to superstructure.
Fig.7 Position of CoG
Hence, during design proper caution should be taken to ensure minimum CoG of the Bus.
Number and Position of Pillars
As the structural pillars absorb energy during roll over impact, it is necessary to optimise the no of pillars.
More no of pillars will add unnecessary weight and less no of pillars will fail in rollover simulation.
Tubes Cross Section
The strength of the structure is mostly depending on the cross section of the tubes used in the structure. If
the bus is failing in rollover analysis, the strength of the structure can be increased by increasing cross
section (Breadth, width or thickness) of crucial members in rollover analysis.
Material Properties
The stronger the material, the stronger the bus is against rollover. Material should absorb more energy of
impact by deforming rather than failing. Aluminium can be used for stretch panels which weighs less and
absorbs more energy.
Reinforcement Optimization

7. Fig.8 Failure Bus Rollover Simulation &
Analysis
Share
Bus Rollover- Introduction
Bus rollover event is one of the most crucial hazards for the safety of passengers and the crew riding in a
bus. In the past few years it has been observed that after rollover accidents the deforming body structure
seriously threatens the lives of the passengers. Hence, rollover strength of the bus bodies has become an
important issue for bus and coach manufacturers.
In India, the general trend noticed in the market is that OEMs sell their drive away bus chassis. The bus
body design and building is done by third party bus body builders. Many bus body builders unknowingly
employ poor designs, resulting in large amounts of fatal accidents, mostly rollover. The existing bus body
structures are inadequately designed, sub-optimal and unsafe. Wood is used in the construction of the bus
bodies to a large extent.
The cause for death or injury in bus roll over is primarily due to intrusion of structure inside the
passenger’s survival space. Hence, the bus body (superstructure) should be strong enough to absorb the
energy of impact caused by bus rollover and the resulting deformation of the superstructure should not
result in intruding the minimum survival space for the passengers.

8. Fig.1 Bus Rollover
The government bodies have regulated the bus rollover strength requirements of the bus bodies by
releasing Automotive Standards like AIS-052 & AIS-031. These regulations ensure that the bus body
superstructure is strong and safe in an event of a rollover accident.
Types of Bus Rollover Accidents
Mainly there are 2 possible cases of bus rollover accidents.
Fig.2 Bus Rollover Cases
The case that needs to be considered for compliance mainly depends on the usability of the vehicle. A city
bus has to run on good quality roads and the possibility of roll over is only during high speed cornering
(E.g. BRTS Corridor), so Case-2 (bus rolls over on ground) will be selected. On the other hand if it is an
intercity/tourist bus, the probability of roll over into a ditch is high, Hence Case-1 (bus rolls over into a
ditch of 800 mm depth) is selected.
However, the structural strength requirement is mostly driven by AIS-031. AIS 031 standard ensures that
the bus body superstructure has sufficient strength such that no part of the structure intrudes into the
residual space (explained below) of passenger in the event of rollover.
Physics behind Bus Rollover
Bus rollover is a phenomenon in which the fully laden bus rolls about an axis parallel to the longitudinal
direction of the bus passing through the outermost edge of the outermost tyre (Right/Left/Front/Rear) on
ground.
Bus rollover happens due to collision or high speed during cornering. As a vehicle rounds a corner, three
forces act on it: tire forces (the centripetal force), inertial effects (the centrifugal force), and gravity. The
cornering forces from the tire push the vehicle towards the center of the curve. This force acts at ground
level, below the center of mass. The force of inertia acts horizontally through the vehicle's center of mass
away from the center of the turn. These two forces make the vehicle roll towards the outside of the curve.
The force of the vehicle's weight acts downward through the center of mass in the opposite direction.
When the tire and inertial forces are enough to overcome the force of gravity, the vehicle starts to turn
over.
Methods for Bus Rollover Analysis

9. Physical Testing
During physical testing, the bus placed on platform is tilted about the rotation point (specified as per AIS-
031) with the angular velocity of not more than 0.087 rads/sec. At some specific angle, the bus C.G.
imbalances and the bus rolls into the ditch of depth 800 mm due to its own gravity. The intrusion of the
side structure into the residual space is checked manually and regulation pass/fail is decided based on the
AIS-031 criteria.
Computer Aided Engineering - Bus Rollover Simulation
In this method the actual case is simulated to an extent that is accurate enough to confirm the protection
from rollover. It is a cost effective & less time consuming process. Moreover, bus rollover simulation in
comparison to physical testing provides enough scope for optimizing the bus body structure which further
helps in reducing the overall weight of the bus.
Conventionally many bus body builders go for actual vehicle’s physical testing which incurs huge cost
and time. It is advisable to achieve the same result with bus rollover simulation. The method is explained
in brief below.
For bus rollover simulation we require final angular velocity of the bus during impact on ground & the
residual space as per AIS-031 definition.
Residual Space Creation
Fig.3 Residual space

10. 1. Residual Space (Survival Space) is to be created in the bus as per AIS-031 shown in Fig.3.
2. The position of the "R" point shown in Fig.3 shall be assumed to be 500 mm above the floor under
the passenger’s feet, 300 mm from the inner surface of the side of the vehicle and 100 mm in front
of the seat back in the Centre-Line of the outboard seats.
3. Height of the Residual space should be minimum 750 mm.
Bus Rollover Kinematics
It explains the position of rotation points about which bus rolls and calculation of angular velocity with
which bus rolls into the ditch.
Fig.4 Bus Rollover Kinematics
1. First Rotation (θ1, Position A to B): The bus along with stoppers should be rotated about the
bottom point of rear wheel stopper (O1) till the CoG of bus reaches maximum height (Unstable
position).
2. Second Rotation (θ2, Position B to C): The bus excluding stoppers should be rotated about the top
most point (O2) of the rear stopper (Point height=80 mm) till it touches impact floor. (Note: Keep
1-1.5 mm gap between Impact floor and part which comes first in contact)
 Angular velocity is calculated by using formula: (Refer Fig.5)
1/2 IRA ω2
= M * g * ∆H
Where,
IRA = ICOG_XX + Mass * D2
(Mass moment of inertia about rotational axis)
∆H = Height between Max. CoG & Final CoG
D = Distance between CoG and Second point of rotation O2
ω= Angular Velocity

11. M = Total mass of the bus
g = Acceleration due to gravity
CAE- Bus Rollover Simulation
Bus rollover simulation can be performed in several ways (1D/2D/3D mesh) & using various softwares,
but the most appropriate method & softwares are considered in terms of accuracy and speed of solution.
The CAE simulation includes four major steps, viz. CAD Preparation, Rollover Kinematics, Residual
Space, Deck Pre-processing, Deck Solving and Post-processing as described below.
Deck Pre-processing
1. Accurate finite element 2D mesh model is prepared from the CAD model of the bus super
structure & chassis.
2. Connections i.e. welding and bolts are modelled using 1-D elements.
3. All components mounted on the structure are either meshed or used as lumped mass (0D Mesh).
4. The overall weight & CoG of the bus is matched with the actual bus.
5. Assignment of Material properties.
6. Application of Gravity load at global CoG of bus and Angular velocity to all components of bus.
7. Assignment of Termination time, Time step required for simulation and output files needed for
post processing.
Solution
 The Deck prepared should be imported in solver and number of CPU’s required for run should be
mentioned.
 Check for any errors; make corrections as per error and Rerun the analysis.
 Check for mass addition and percentage increase mass.
 Run the Solution, this generally takes 24 Hours to solve the mathematical model.
Post-processing
 Check energy balance i.e. Total Energy, Internal Energy, Kinetic Energy, Rigid Wall energy and
Sliding Interface Energy of each component of bus.

12. Fig.5 Energy balance
 Gap Measurement:
Fig.6 Gap measurement between residual space and pillars
 If gap is more than 5 mm, then the structure is not penetrating into the residual spaces and it
passes AIS-031.
 If gap is less than 5 mm, then the structure fails and suitable countermeasures need be taken.
Factors Affecting Bus Rollover Performance
Position of Centre of Gravity (CoG)
Stability of bus while turning is dependent on the position of CoG of bus. If height of bus CoG is more,
the bus will be more prone to rollover than one having low CoG. Also as height of CoG increases, the

13. radius of rotation will increase, which increases the kinetic energy in structure before impact resulting
more damage to superstructure.
Fig.7 Position of CoG
Hence, during design proper caution should be taken to ensure minimum CoG of the Bus.
Number and Position of Pillars
As the structural pillars absorb energy during roll over impact, it is necessary to optimise the no of pillars.
More no of pillars will add unnecessary weight and less no of pillars will fail in rollover simulation.
Tubes Cross Section
The strength of the structure is mostly depending on the cross section of the tubes used in the structure. If
the bus is failing in rollover analysis, the strength of the structure can be increased by increasing cross
section (Breadth, width or thickness) of crucial members in rollover analysis.
Material Properties
The stronger the material, the stronger the bus is against rollover. Material should absorb more energy of
impact by deforming rather than failing. Aluminium can be used for stretch panels which weighs less and
absorbs more energy.
Reinforcement Optimization

14. Fig.8 Failure locations
As shown in the fig.9, the superstructure fails at the shown junction points. To avoid that we can increase
the tube's cross-section, but that will increase the overall weight of the bus.
Hence, it is advised to provide local stiffeners, it adds less weight and increases the strength of the bus
against rollover.
-
Rushikesh Savant
Advanced Structures India Pvt Ltd is an independent automotive product development company based out
of Bangalore, India with operations in India, China and US. Above is a blog entry from our engineers
about Bus Rollover. We can be contacted on business@advancedstructures.in for business enquiries and
careers@advancedstructures.in for open positions
locations
As shown in the fig.9, the superstructure fails at the shown junction points. To avoid that we can increase
the tube's cross-section, but that will increase the overall weight of the bus.
Hence, it is advised to provide local stiffeners, it adds less weight and increases the strength of the bus
against rollover.
-
Rushikesh Savant

15. Advanced Structures India Pvt Ltd is an independent automotive product development company based out
of Bangalore, India with operations in India, China and US. Above is a blog entry from our engineers
about Bus Rollover. We can be contacted on business@advancedstructures.in for business enquiries and
careers@advancedstructures.in for open positions