Capoco Design Ltd Side Impact Analysis of a Passenger Bus

Company Profile

Capoco Design Ltd (Capoco), was founded by automotive engineer Alan Ponsford, and has been at the forefront of fundamental bus design and engineering for 25 years. Capoco work has contributed to the success of market-leading products from two of the UK's most important bus producers - Mayflower (Dennis) and Optare. The Dennis Dart, probably the largest selling small bus model in the history of the UK bus market, and the Queen's Award winning Optare Solo are just two of the models with which Capoco have been involved from the original design process. Now, Alan Ponsford's expertise has become internationally recognised, with projects underway on every Continent. The most recent of these involves work on a new generation of luxury coaches destined for the burgeoning tourist market in China. Alan Ponsford is a founding partner in Newbus Technology Ltd, a design consortium focused on future bus technology, including such developments as electric drive and fuel cell hybrids. The graphic to the right is a picture of the Optare Solo low floor bus, which won the Queen's Award for Innovation in 2001. It has enjoyed tremendous sales success in the UK and is now being sold in America.

Scope of Project

The purpose of this project was to simulate the side impact of a car into a bus, in order to evaluate the amount of energy absorption by the bus, as well as the amount of side intrusion that could be anticipated. This has an effect on the level of occupant safety. The analysis required the use of ANSYS/LS-DYNA.

Simulation Details

The Finite Element model, consisting of a bus and a car bumper, was created by Capoco and meshed using 4-noded quadrilateral shell elements in ANSYS. This model was then read into ANSYS/LS-DYNA where the shell elements acquire velocity and acceleration degrees of freedom (dofs) in addition to translational and rotational dofs, giving each node 12 dofs to solve for. The model is made up of 28,700 elements & 16,300 nodes resulting in a model size of 195,600 dofs. A graphic illustrating the model used in the analyses can be seen to the left. Vertical constraints have been applied to the model at the axle suspension attachment points, in addition to the lateral restraints. The existence of lateral restraints simulates a worst-case situation, as this assumes that there will be no lateral sliding of the bus on impact. However, IDAC does have access to tyre data, which would allow the modeling of the bus slipping.

The main structure of the bus was made of steel, whereas the roof component of the bus was modelled with aluminium. Both materials have been modelled plastically. The impacting bumper was modelled as a rigid surface using 4-noded shell elements. Making use of a rigid surface to define a component in ANSYS/LS-DYNA simplifies the model, as the rigid component is reduced to 12 dofs at the centre of gravity of the component. The car bumper was modeled with linear material properties, and the density value was altered to represent a reasonable value for the total car mass.

The bumper was forced to move into the side of the bus by imposing an initial velocity of 3.14m/s to the bumper, this velocity gives rise to 9kJ of energy. The graphic to the left illustrates the velocity of the bumper decreasing with respect to time. Contact between the bus and the car bumper have been defined in ANSYS/LS-DYNA and friction has been ignored.

To evaluate the energy absorption capacity of the bus, the kinetic energy of the bus has been measured during impact. The graph above right shows how the absorption of kinetic energy varies with time. The velocity of the bumper and bus, relative displacements of points at the window pillar and longitudinal floor member were also monitored. This relative displacement measure provided means of evaluating the amount of permanent set in the model.

The graphic to the left shows the total deformation the bus experiences at a particular time point in the analysis. The full analysis was run for 160 milliseconds, which is well beyond the duration of impact, but it does include bounce back of the bumper. The analyses were run using SGI Intel based Pentium PCs.


"IDAC have helped Capoco particularly with the more complex issues of the plastic, nonlinear FE analyses. However, the benefits we have had from our co-operation with IDAC, over the years, are more than the straightforward support with ANSYS and FEA projects. Our relationship with the founders of IDAC, Dave Ellis and Steve Varnam, go back to the late 1980's when this area was covered by Strucom. It is this degree of constancy that we value in our relationship with IDAC. We have consulted them on the deeper issues of operating systems, model geometry generation and hardware."