A STUDY OF DYNAMIC WHEEL FORCES IN AXLE GROUP SUSPENSIONS OF HEAVY VEHICLES

A wheel force transducer was used to measure dynamic wheel loads for speeds ranging between 40 km/h and 80 km/h over road surfaces ranging from as-new construction to the maximum tolerable roughness as measured using the NAASRA roughness meter. The effects of tyre inflation pressure and axle group load were taken into account and a factorial experimental design was used to determine dynamic loading, expressed as a form of coefficient of variation termed the dynamic load coefficient (dlc), as a function of speed and roughness for each suspension type. It was found that, for practical purposes, the dlc for each suspension type is a function of the product of the speed and the square root of the NAASRA roughness value. The centrally-pivoted drive axle suspensions (walking beam and single point types) show a dlc increasing with spring stiffness. Although the trailer suspensions and the torsion bar drive axle suspension showed, in general, lower dlc's than the centrally-pivoted drive suspensions, it was concluded that the level of dynamic loading generated by all suspensions is large. Three out of five drive axle suspensions were found to cause severe dynamic pavement loading. At least one of these does not comply with current Australian regulations for load sharing suspensions, and a quantitative test of dynamic loading is suggested. Most suspensions were found to share the load between the wheels to within 10 per cent of an equal share and this level is proposed as a quantitative criterion of load sharing ability. One currently accepted type did not load-share adequately and it is concluded that a test is needed and in some cases proper fitment of the suspension to the chassis needs to be ensured. (Author/TRRL)