Life, limb and money continues to be lost in agriculture world-wide as a result of vehicles overturning on sloping land. It appears that vehicle operators lack the information required to prevent this. Studies of overturning accidents on hillsides have suggested that overturning is often the result of dynamic vehicle behaviour and in particular the result of striking discrete terrain obstacles. Currently, the only standard measure of the stability of vehicles for hillside use is the static stability limit. Several researchers have studied the response of vehicles striking discrete obstacles and have computed reductions in safe slope with increases in speed or obstacle height. However, there are many variables influencing the vehicle response; and no 'dynamic stability' test has been proposed and adequately backed up with theoretical analysis. This thesis describes work to provide more insight into the complex problem of vehicle dynamic stability, leading to a proposal for a dynamic stability test. A DADS computer model has been developed and validated by comparison with a farm trailer constrained on a test rig, with test variables speed, slope, tyre pressure, moment of inertia, static wheel load, static stability limit and obstacle length. It was found in both empirical and simulated cases that the trailer response reached a maximum at a certain speed, dependent on the natural frequency of the system, and that the overturning slope was at a minimum at that speed. The completed model predicted the slope on which the trailer overturned to within 27% of the slope in 95% of cases. Attaining this level of accuracy involved a detailed study of tyre properties, especially radial stiffness. It was found, unexpectedly, that this decreased by around 50% during large amplitude dynamic compression such as occurs when striking an obstacle. The relationship between the proposed index and the vehicle parameters was studied and it was found that static wheel load and tyre stiffness affected it most significantly. The index appeared to increase with the square root of the tyre stiffness and decrease with the square root of the static wheel load. The index could be used in conjunction with the static stability limit to compute a slope below which it would be theoretically safer to use the more dynamically stable vehicle.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:657363 |
| Date | January 1995 |
| Creators | Martin, Colin Patrick |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/12584 |
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