Advanced modelling of ovoid balls

Sports played with an ovoid ball may be considered as minority sports in comparison to the numerous games played with spherical balls, however the ovoid ball market is considerable, with $84million spent on the purchasing of American footballs in the US alone (SGMA 2007b). In comparison to spherical balls, it is apparent that little research has been performed on ovoid types, which presented an opportunity for a detailed study into their dynamic properties in game related situations. With the development of this knowledge new ball design concepts have been investigated to improve consistency and performance, allowing manufacturers to create balls with improved physical characteristics. Experimental procedures have been created which allow the dynamic behaviour of an ovoid ball to be characterised. It was found that the measured parameters varied depending upon the position of impact, orientation angle of the ball and the position of the valve at impact. The inclusion of the valve within a rugby ball creates a non uniform mass distribution resulting in unstable rotation about the axis with the intermediate moment of inertia. This unstable rotation results in the ball performing a series of half twists, thus increasing the drag force during the flight and wobble. Prototype rugby balls have been manufactured with various mass distributions, allowing the effect on the unstable rotation to be analysed. Results showed that the inclusion of multiple valves, within the dynamically balanced bladder, resulted in stable rotation about all ptinciple axes. FE models have been created with isotropic and anisotropic material properties, with all models validated using experimental procedures. Results suggest that the anisotropic FE simulation accurately predicts the coefficient of restitution, contact time and deformation during experimental testing, and as a result it can be used to predict the dynamic behaviour of a rugby ball during various impact scenarios. Using a thermo-bonded construction, it was shown that a novel rugby ball could be developed with a unique carcass configuration and outer panels with increased design flexibility. A number of carcass designs, based upon mathematical shapes, have been created which increase the consistency of the dynamic behaviour of the ball when impacted at different locations.