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Chapter 4RimTyresFigure 4.4 – Tyre size parametersIt should be noted that the tread width of a tyre is less than the section width. This fact isimportant, as the impact of the tyre to the pavement is generally accepted to be determinedby the footprint width and not by the nominal section width. For radial tyres, the footprintwidth of a tyre generally equals the tread width (except for strongly over-inflated or‘under-loaded’ conditions when the footprint width may be less). Similarly, the outerdiameter of the tyre is not only determined by its rim diameter, but also by its sidewallheight, indicated by its aspect ratio and width. This too is important, since the outer tyrediameter is one of the factors influencing the footprint length, together with the wheel loadand inflation pressure.Different tyre sizes and dimensions can be found on different axles of trucks and tractorson the one hand and trailers and semitrailers on the other hand. Tyre sizes in use wereexamined by analysing the reports of tyre changes on site in Germany (n=7347) (Glaeser1997b) The following data were drawn from this research. (Note that the situation outsideGermany may be different.)• On trucks and tractors the tyre sizes 315/80R22.5 and 295/80R22.5 have nearly thesame share (23% and 27% respectively, noting that in 17% of all cases the tyre size isunknown).• On front axles of trucks and tractors one can find an increasing number of wide basesingle tyres (385/65R22.5). At present, about 4% are wide base singles on the frontaxles in Germany, whereas in Norway this figure rises to about 23%.• On trailers and semitrailers the tyre size 385/65R22.5 has a share of 45% and the tyresize 365/80R20 has a share of 20% (assuming that also 17% of the trailer tyres sizesare unknown). The tyre size 425/65R22.5 has a share of only 1%. All these tyres are ofcourse mounted on trailers and semitrailers as single tyres.• In 11% of the cases the tyre size is smaller than 22.5. The tyre size 275/70R22.5 has ashare of 5%, all other 22.5 tyre sizes together have a share of 20%.• Smaller tyre diameters, mounted as twins (e.g. on volume trucks and trailers), have thefollowing share: 17.5 tyres 9% and 19.5 tyres 4%. Other twin mounted 22.5 trailertyres have a share of about 4%.• About 50% of all tyres on heavy vehicles are retreaded tyres.version 29 November 200113