Paper presented at the 26th Annual Southern African Transport Conference 9 - 12 July 2007 "The challenges of implementing policy?", CSIR International Convention Centre, Pretoria, South Africa. ABSTRACT:This paper summarises the results of a study in which the maximum vertical contact stresses of the 1/3rd scale test tyres of the Model Mobile Load Simulator (MMLS3) were compared with those measured for three types of full-scale test tyres of the Heavy Vehicle Simulator (HVS). The comparative tests were done using the Stress-In-Motion (SIM) device designed to capture three-dimensional (3D) tyre-pavement contact stresses on a relatively rough-textured test surface. As it is generally accepted that these stress conditions within the moving tyre contact patch are quite complex, a single parameter of the vertical stresses measured for the different test tyres was selected for the comparison. This parameter, referred to as the Maximum Vertical Contact Stress (MVCS) indicated, on average, that it increased linearly with tyre inflation pressure for both types of the 1/3rd scale MMLS3 test tyres and full-scale test tyres. However, it was found that the MVCSs for the Diamond patterned square profile 1/3rd scale tyres of the MMLS3 were, on average, much lower than those of the full-scale test tyres as they represented, at most, only 52.5 per cent of those measured for the 11R22.5 (full-scale HVS) test tyre, 37.5 per cent for the single 315/80 R22.5 (full-scale HVS) test tyre, and 12.5 per cent and 20 per cent respectively respectively for the smooth and rough-texture tests on the 12R22.5 (full-scale HVS) test tyre. Earlier studies of vertical contact stresses of the test tyres of the MMLS3 by Sime M & Ashmore SC, (1999), Epps Martin et al (2000) and Doupal et al (2002) that were conducted with different stress measuring devices reported higher vertical contact stresses at the upper level of tyre inflation pressures. It is therefore recommended that the influence of test surface characteristics and the impact on SIM measurements of HVS and MMLS3 tyres be investigated in greater detail in future similar studies. It is nevertheless strongly recommended that the foregoing be incorporated into MMLS3 and HVS comparative and/or individual testing programs (and protocols) to permit a more rational interpretation of test data from both these devices relative to structural road pavement performance issues, especially on the surface of flexible road pavements.
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