Tests of radial and cross ply tyres for a HGV have measured the vertical deflections and stiffnesses of rotating tyres and established the effect on the deflection and stiffness of the tyre speed, temperature and age. Tyre speed was found to have little effect on deflection or stiffness but an increase of 30 degrees C in the mean temperature of a cross ply tyre increased deflection by 15 per cent and decreased tyre stiffness by 19 per cent, and in a radial ply tyre, about a half of this. The stiffness of radial and cross ply tyres increased by about 5 per cent over a 21 month period, probably due to age hardening. The deflection of a radial ply tyre was around 37 per cent greater than that of a cross ply tyre at the same conditions; if tyre deflection is used to indicate applied dynamic force, an increase in the temperature of the tyre above the calibration temperature will result in a small over-estimate of load. (A)

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