Introduction to Bus Roll Over
Bus roll-over 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 roll-over 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 roll-over. 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 vehicle 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 roll-over 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.
Type of Bus Roll Over Accident
Mainly there are 2 possible cases of bus roll-over accidents.
Fig.2 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 Roll Over
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.
Evaluation Methods for Bus Roll Over Performance
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 Roll Over Simulation
In this method the actual case is simulated to an extent that is accurate enough to confirm the protection from roll over. It is a cost effective & less time consuming process. Moreover, CAE 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 CAE simulation. The method is explained in brief below.
For CAE 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
- Residual Space (Survival Space) is to be created in the bus as per AIS-031 shown in Fig.3.
- 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.
- Height of the Residual space should be minimum 750 mm.
Bus Roll Over 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 Rollover Kinematics
- 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).
- 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
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
The CAE 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, Roll Over Kinematics, Residual Space, Deck Pre-processing, Deck Solving and Post-processing as described below.
- Accurate finite element 2D mesh model is prepared from the CAD model of the bus super structure & chassis.
- Connections i.e. welding and bolts are modelled using 1-D elements.
- All components mounted on the structure are either meshed or used as lumped mass (0D Mesh).
- The overall weight & CoG of the bus is matched with the actual bus.
- Assignment of Material properties.
- Application of Gravity load at global CoG of bus and Angular velocity to all components of bus.
- Assignment of Termination time, Time step required for simulation and output files needed for post processing.
- 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.
- 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 Roll Over Performance of a Bus
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 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 roll over 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.
The stronger the material, the stronger the bus is against roll over. 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.
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 roll over.
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 firstname.lastname@example.org for business enquiries and email@example.com for open positions.