A new approach to roll cage design promises to make the MINI John Cooper Works WRC the safest car ever to enter the World Rally Championship. A combination of design innovation, rigorous analysis and exhaustive physical testing has allowed Prodrive to develop a new configuration of side-impact protection that could also improve the safety performance of other race and road cars.
With the introduction of new regulations in 2011, Prodrive sought a means to improve occupant safety within their demanding weight targets. A comprehensive diagnosis of previous accident damage to roll cages led Prodrive to a fundamental reappraisal of weld geometry, materials and manufacturing processes.
�The breakthrough came from completely revising the side impact protection bars, routing them farther away from each crew member and subtly changing their shape,� explains Paul Eastman, Prodrive head of rally engineering. �In an impact, this brings the structure into play much sooner, allowing softer materials to be specified to safely accelerate the driver and passenger over a longer period of time.�
The innovative new curved shape of the door beams allows them to withstand much higher impact forces than traditional straight bars, in a similar way to the improved dent-resistance of curved body panels compared to flat ones. The new shape also feeds the loads into nearby welds in a controlled direction, minimising the chance of failure by tearing. The beams are so far outboard that they pass through the car�s B-pillars, to which they are welded, contributing further strength to the structure.
The process that led to the new design is equally unusual. Before securing the MINI WRC programme, Prodrive�s engineers spent many months analysing the basic engineering ingredients that would comprise an �ideal� rally car. This required the development of a generic vehicle model and the definition of preferred characteristics such as weight distribution and body stiffness.
By making the model parametric, its values could be rapidly adjusted to simulate the behaviour of any particular car. When BMW announced the MINI Countryman, it proved to be a good match against Prodrive�s model of the ideal WRC contender and therefore the preferred choice for the company�s new rally programme.
The model of the �ideal� car included a definition of the preferred attributes of the combined body and roll cage structure. Eastman continues: �The difficulties of making an actual car, without compromising the safety gains made on the virtual car, were considerable. But because we knew just how good it could be theoretically, we kept working at it to find a way to make it feasible for manufacture without losing safety performance or compromising weight.�
For the new roll cage, going the extra mile meant performing over 50 physical tests on specially prepared tube and weld samples and using more than 100 model configurations for finite element analysis. It also meant finding new ways to integrate the cage structure into the bodyshell, for example by routing the footwell diagonal brace through the front bulkhead in order to join the bottom of the cage to the opposite strut top-mount. This was a considerable packaging challenge, especially around the pedal box, but provided improved stiffness and also helped to optimise the centre of gravity.
The new configuration also helps to improve interior space, which has been a concern of rally crews in the new smaller 2011 cars . The new roll cage is thought to make the Prodrive-engineered MINI John Cooper Works WRC not just the safest in an impact, but also the most spacious. Eastman says that now Prodrive has a clear understanding of the new structure and the factors influencing joint behaviour that allow it to be manufactured, the benefits could be brought to other race series, or potentially even road cars.
| Prodrive
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