In Sweden the current regulation for minimum tread depth for heavy vehicles in the winter period is different for trailer axles compared to drive or steer axles. While the requirement for the latter axles are 5 mm tread depth, on trailer axles 1,6 mm is required.
The purpose of this study has been to investigate whether there is reason to increase the tread depth requirement for heavy vehicle trailer tyres during the winter period in Sweden, and if so, suggest a new tread depth limit. The study was limited to ice conditions, and ice grip for a number of different trailer tyres with different tread depth was measured on smooth ice in VTI’s tyre test facility.
Before conducting the test there was no clear hypothesis for how the ice grip would be affected by a lower tread depth. An increase of the grip, as well as a decrease, were both possible outcomes for a diminished tread depth.
Four different tire models in a dimension common for heavy vehicle trailer tyres (385/55-R22.5) were chosen for the tests. Every model was tested in a new state, as well as with 5, 3, and 1.6 mm tread depth. It was decided to use naturally worn tyres which meant that in total 16 different tyres were used in the study. The difficult task of finding identical tyre models and dimensions with specified tread depth forced us to also include some tyres with the larger profile (285/65-R22.5). This is however not considered to affect the results of the test. All the tyres were winter tyres, and most of them were also marked with the 3PMSF symbol, which means that they have passed a standardized grip test on snow.
Both brake and steer test were conducted. Five different grip related attributes were investigated: peak friction at braking, friction at locked wheel braking, cornering stiffness, cornering peak friction, and cornering friction with large slip angle