STABILITY OF OVER-HEIGHT LOW-DENSITY FREIGHT VEHICLES AND ITS PREDICTION

Operators carrying low density freight usually operate vehicles with axle-loads below the allowable legal limit. To maximise the productivity of these vehicles, the cubic capacity is maximised. Already, tandem axle dropwell B-Doubles have been adopted by some operators and the use of these vehicles with a 4.6m rather than 4.3m height restriction has been proposed. The stability of these higher vehicles was investigated and a prediction model was also developed. Several dropwell semi-trailer vehicles were tested on the tilt deck at ARRB Transport Research to determine their static roll threshold. The vehicles were loaded with products of different densities and the load height was varied to provide comparison. A prediction formula for roll threshold (lateral acceleration producing first wheel lift) was developed from an existing method for computing the stability factor. All of the 4.6m high vehicles tested produced static roll thresholds above the target value, and the prediction model based on data from vehicles tested was found to be accurate to within 7 percent of the tested values. The vehicle tested with the lead B-Double trailer, a practice not used in operation, was the only exception. The model was used to predict the roll threshold for a "worst case" vehicle based on homogenous loading that produced maximum legal axle loads. This vehicle was borderline on the target value. Since homogenous loading is not practically possible, it was concluded that 4.6m high vehicles with low density freight would have acceptable stability. (a) For the covering entry of this conference, please see IRRD 895232.