Recently, new reference test tyres have been specified by the ISO to be used for tyre/road noise measurements with the close-proximity (CPX) method. Various studies have provided evidence that tyre ageing is accompanied by significant and continuous changes in the noise emission properties. It is therefore essential to consider the changing state of reference tyres when carrying out tyre/road noise measurements using the CPX method. A reliable quantification of these ageing effects and their influence on noise emission levels requires that individual reference tyre sets are monitored over time. This study aims at investigating the ageing process of the new reference tyres SRTT and Avon AV4 during one measurement season. Measurements, which test the indentation resistance of tyre rubber with the type A durometer, were repeated on a monthly basis. This revealed substantial increases in rubber hardness during the 2012 measurement season, exceeding 3 units Shore A for the SRTT tyre and 6 units Shore A for the Avon AV4 tyre. This corresponded with a considerable rise in noise levels, suggesting that tyre ageing is a primary influencing factor when carrying out tyre/road noise measurements using the CPX method. The study provides a simple tyre specific model for estimating rubber hardness changes based on the number of measurement days. The evaluation of temperature data suggested that, due to the physical and environmental strain on in-service tyres, usage influences tyre ageing to a larger extent than standardised operational storage conditions. The data implies, moreover, that individual corrections for the CPX reference tyres SRTT and Avon AV4 are needed.
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