Engineers and researchers working with tyre/road noise often need to have available one or a few tyres which are stable over time with regard to noise emission. This is not easy since tyres may change with time even when not used, and in some climates it may be enough with a few months storage to make the tyre unsuitable. The most commonly practiced example when reference tyres are essential is when comparing or classifying noise properties of road surfaces in the Close Proximity (CPX) method (or its US counterpart which is called OBSI). Any tyre differences or changes will directly give uncertainties in the result. This paper describes how time-depending parameters of a tyre, such as storage, wear, and ageing may change tyre/road noise emission. Especially the reference tyres used in the CPX method; i.e. ASTM SRTT and Avon AV4 tyres, are considered. One of the most important suggestions is to inflate the tyres with nitrogen gas instead of air. Examples of how tyres should be stored are given. Correction procedures for temperatures and for increases in rubber hardness are proposed. By correcting for a reasonable hardness increase from new condition, one may use a reference a tyre longer time than otherwise. The authors also suggest means of comparing or calibrating reference tyres to compensate for differences in new condition due to production tolerances. If advices given in the paper are followed tyres may be kept at stable conditions for years.
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