ABSTRACT
In an attempt to understand the cause of a recent boat trailer accident, a study of the performance and stability characteristics of articulated towing vehicle and boat trailer combinations was performed. The performance study (braking, acceleration and climbing and steady state handling) was conducted by employing various models and criteria. The stability characteristics of the configuration were investigated by means of a three degree-of-freedom, linearized mathematical model. The propensity for trailer roll-over during evasive manoeuvres was studied by using frequency response results from the 3DOF model to obtain trailer roll angle and tyre vertical reaction forces. It was found that boat trailer configurations could experience various types of instabilities of which the divergence and the snaking (fishtailing) instability are the most important. The instabilities could be caused by a host of factors such as too heavy a trailer, its mass centre in the wrong position, too long an overhang for the tow hitch or too high a vehicle speed. The divergence or the snaking instability could appear at a certain vehicle speed and it would initially be fairly mild and probably controllable but as vehicle speed is increased, the inherent severity of the instability would increase. A driver should hence not attempt to accelerate out of an instability. It was further found that heavy boat trailers are prone to roll-over during manoeuvring at high speeds. The analysis of the UCT boat trailer accident revealed that it was caused by a snaking instability which could not be controlled and which eventually led to trailer and vehicle roll-over. The vehicle combination was inherently unstable at speeds in excess of about 115 km/h due to the trailer being too heavy for its towing vehicle. The trailer roll-over threshold was also unacceptably low at these speeds. Striving to improve the safety of boat trailer configurations, it is recommended that available mechanisms which suppress snaking, or fifth wheel connectors, be used. As a byproduct of the current investigation, a mass-spring-damper system, with vibrational characteristics similar to that of a sustained snaking motion, is proposed.
