BRAKING PERFORMANCE OF AIR SUSPENDED
In 1996 the National Road Transport Committee (NRTC) released a national heavy vehicle axle Mass Limit Review (MLR) The MLR recommended an axle mass increase for axle groups suspended by road friendly air suspension. For an air suspension to be classified as Road Friendly it is required to have a bounce frequency below 2.0Hz and have damping greater than 20% of critical. It is also a requirement that the suspension group achieves load sharing within 5% Air suspended converter dollies have become popular in Australia, particularly the triaxle type. Triaxle dollies offer a productivity benefit of between 2.5 and 4.5 tonne when compared to a tandem converter dolly. There was concern that the increased mass offered to air suspended dollies would significantly affect the performance of road trains under braking. The Roaduser Autosim Truck Engineering Dynamics (RATED) computer simulation models were used to simulate the performance of hinged and rigid drawbar tandem and triaxle dollies under braking. The results from the simulation showed that an air suspended tandem converter dolly could pitch significantly under braking when compared to mechanically suspended dollies. Triaxle air suspended dollies were found to pitch somewhat less than the tandem air suspended dolly and generated a lower longitudinal force in the coupling. This indicated that the triaxle dolly has better brake balance and should be encouraged by allowing the weight increase. Rigid drawbars on converter dollies reduce the amount of dolly pitch and hence have better brake balance. It was found that tandem axle air suspended converter dollies perform significantly better if they use a rigid drawbar. This paper presents the results of the braking performance conducted with the RATED system, briefly compares these results with test data and discusses how key results can be transferred into practice. INTRODUCTION Roadtrains in Australia commonly use tandem mechanically suspended converter dollies to connect trailers to their hauling units. However air suspended converter dollies are now becoming more popular in Australia, particularly the triaxle type. Triaxle dollies offer a productivity benefit Pages 319 335 of 2.5 4.5 tonnes when compared to tandem dollies. Triaxle dollies are also seen to have stability and tracking advantages over the tandem dolly. A converter dolly consists of a chassis supported by two or three axles with either air or mechanical suspension. The chassis has a fifth wheel placed over the centre of the axle group and a hinged drawbar is usually attached to the front of the dolly chassis. Converter dollies traditionally use a hinged drawbar to negotiate obstacles in the roadway without introducing large stresses into the chassis of the dolly. However, this also removes the means by which brake moments can be reacted. The introduction of air suspension into the dolly design further complicated braking performance. Air suspensions are typically fitted with a single ride height control value. However, the transport industry found tandem dollies fitted with a single ride height control valve tend to pitch about the axle to which the ride height control value is connected. This was of concern to operators and regulators, particularly with the prospect of axle group mass limit increases under the National Road Transport Commission's (NRTC's) Mass Limit Review (MLR) This paper looks at some aspects of the braking performance of roadtrains equipped with dollies with tandem and triaxle groups, air and mechanical suspensions and with rigid and pivoted drawbars. This work formed part of the NRTC's investigation of the effects of mass limit increases on roadtrain dollies. SIMULATED BRAKING PERFORMANCE The Roaduser Autosim Truck Engineering Dynamics (RATED) Braking Model was used to simulate the braking performance of a double roadtrain with tandem and triaxle dollies and with hinged and rigid drawbars. Simulated Vehicle The double roadtrain simulated (Fig 1) consisted of a 3 axle prime mover, two triaxle trailers and a dolly, which was simulated with the following design variations: mechanical suspension (tandem only) air suspension tandem axles triaxles reduced brake gain on the rear axle of the triaxle dolly pivoted drawbar rigid drawbar The vehicles were simulated under the following axle mass limits: current limits of 6t, 16.5t and 20t on steer, tandem and triaxle groups respectively MLR Option F limits of 6t, 17.0t and 22.5t on steer, tandem and triaxle groups respectively MLR Option F limits of 6t, 17.0t and 22.5t on steer, tandem and triaxle groups and the dolly axle loads remained at Current Limits of 16.5t and 20t for tandem and triaxle dollies respectively The vehicles were simulated in an emergency straight line braking manoeuvre similar in severity to Australian Design Rule emergency braking requirements for heavy trucks. The simulation involved stopping from a speed of 90 km/h with a deceleration in the range 0.4 0.45 g. The simulated brake line pressure was the same for all vehicles and was set at a level to produce incipient wheel lock in most of the vehicles investigated. The effects of dolly suspension type, drawbar type and axle group load on the following performance parameters were investigated: stopping distance (m) this provides a fundamental measure of braking performance peak longitudinal force applied to the pin coupling on the rear of the first trailer (kN) this provides a measure of the braking balance between the two trailers peak vertical force applied to the pin coupling on the rear of the first trailer (kN) this provides an indication of the vertical load capacity required at the pin coupling maximum dolly pitch angle (deg) this provides some guide to the stability of the dolly under braking and was included because visual impressions of dolly behaviour may be influenced by direct observation of dolly pitch peak axle load on the lead axle of the dolly (t) Fig 2 shows an animation frame from the vehicle simulation. BRAKING PERFORMANCE OF ROADTRAINS When a roadtrain brakes, the dolly is required to react the moment that is produced from the brake torque and the horizontal force that is applied at the fifth wheel from the trailer it carries. The forces and moments that are applied to the dolly during braking cause the dolly to pitch, as shown in Fig 3. When the dolly pitches, the load on the leading axle increases, while the load on the trailing axle decreases. The load on the centre axle in a triaxle dolly increases by only approximately one third of the load that is transferred from the rear trailer axle group to the dolly group (ie: it is affected little by the brake moments occurring in the dolly) Because the dolly pitches when it is braked and transfers a significant amount of load between axles, it may appear that the suspension on the dolly is brake reactive even though this is not the case for air suspension. The results in Table 1 show the minimum stable stopping distance, peak longitudinal and vertical coupling forces, the maximum dolly pitch angle and the peak axle load occurring on the front axle of the dolly. Some of these characteristics are identified in Fig 4. TABLE 1. BRAKING ANALYSIS RESULTS Vehicle Loading Condition Minimum Stable Stopping Distance (m) Peak Longitudinal Coupling Force (kN) Peak Vertical Coupling Force (kN) Maximum Dolly Pitch Angle (deg) Peak Lead Axle Load (t) Static Lead Axle Load (t) Triaxle Dolly (air suspension) Hinged Drawbar Current Weights 76.6 41 0 2.4 10.9 6.67 MLR Option F (no increase on dolly) 76.3 38 0 2.7 11.5 6.67 MLR Option F (increase on dolly) 81.0 40 0 2.9 12.6 7.5 MLR Option F (increase on dolly) (reduction of brake gain) 74.6 51 0 2.9 12.6 7.5 Rigid Drawbar Current Weights 73.9 10.2 19.7 1.3 9.2 6.67 MLR Option F (no increase on dolly) 76.1 15.0 19.5 1.3 9.3 6.67 MLR Option F (increase on dolly) 76.2 17.3 21.6 1.5 10.3 7.5 Tandem Dolly (air suspension except where indicated) Hinged Drawbar Current Weights 79.4 70 0 8.7 15.5 8.25 Current Weights (mech. suspension) 78.1 53 0 1.5 14.7 8.25 MLR Option F (no increase on dolly) 84.5 70 0 8.7 15.5 8.25 MLR Option F (increase on dolly) 85.3 72 0 8.9 16.0 8.5 Rigid Drawbar Current Weights 74.0 8.6 15.9 1.9 10.2 8.25 MLR Option F (no increase on dolly) 79.0 8.0 16.5 2.0 10.4 8.25 MLR Option F (increase on dolly) 79.4 9.6 16.8 2.1 10.6 8.5 The results in Table 1show that, with conventional hinged drawbars in double roadtrains: air suspended triaxle dollies improve the stopping distance by approximately 2% compared to current mechanical tandem dollies under MLR Option F, the stopping distance of the air suspended triaxle dolly vehicle increases by approximately 6% if the mass increase is not allowed on the dolly, this degradation in stopping distance is completely avoided adjustment of the brake gain on the rear axle of the dolly can improve stopping distance by approximately 9% and this would more than compensate for the effects of the mass limit increase on the dolly. Further insights gained into dolly braking performance with hinged drawbars include: longitudinal coupling loads are much less with triaxle dollies than with tandem dollies, indicating improved balance of braking effort between the vehicle units; the peak coupling forces are 40% lower for the triaxle dolly when compared to the air suspended tandem dolly. Peak coupling forces under braking have recently been measured for a triaxle dolly (1) The range of measured coupling loads under braking was 36 71 kN for a triple roadtrain. This indicates that the peak coupling forces as determined in the simulation are realistic. the pitch angle of the air suspended triaxle dolly is approximately 3.5 times less than that of the air suspended tandem dolly. The triaxle dolly pitch is approximately twice that of a mechanically suspended tandem dolly. the peak axle load occurring under the air suspended triaxle dolly, with