Investigation of Snow Milling Mechanics to Optimize Winter Tire Traction
A detailed understanding of effects occurring in the contact patch between tire tread and snow surface is needed to maximize tire grip in winter conditions. The main focus of this study is quantifying the snow milling effects of individual tire tread block elements during sliding. Tests are carried out using the high-speed linear friction tester (HiLiTe), located at the Institute of Dynamics and Vibration Research at Leibniz University of Hannover, Germany. Test tracks are prepared using artificially produced snow. To solely investigate snow milling effects and exclude material properties of rubber, in a first instance the tread block samples are made of polytetrafluoroethylene (PTFE). Because PTFE is at the same time rigid and hydrophobic, known friction mechanisms such as adhesion and hysteresis can be neglected, leaving only the tread pattern milling mechanics to transmit frictional forces to the snow track. The PTFE samples are shaped in such a way that they mimic the geometry of different siped rubber tread blocks under load, varying the sipes' number, shape, and tilt angle. Results show the benefit of multiple sipes and give information on the evolution of transmittable forces with respect to sliding distance. It is found that the block element shape and tilt angle are directly linked to the frictional force, showing a distinct optimum for specific angle and shape combinations. In addition, forces are not depending on sliding speed, but on sliding distance. The snow milling results of PTFE block elements are then compared to siped rubber block samples. Corresponding high-speed videos show that PTFE sample snow milling mechanics can be directly applied to rubber samples.ABSTRACT
(a) High-speed Linear Friction Tester overview; (b) sample carriage.
(a) Conventional PTFE test sample design, tilt angles from flat to high; (b) PTFE saw tooth sample design, with angle combinations from left to right: low-low, low-medium, low-high, medium-high; (c) sample holder equipped with single lamella; (d) sample holder equipped with three lamellas.
Measurement data for conventional PTFE samples (single lamella); (a) normal force Fn; (b) frictional force Fr; (c) frictional coefficient μ; (d) penetration depth h.
Frictional force for different sliding speeds with same sample; (a) low angle samples; (b) medium angle samples.
Frictional force comparison for single lamella and three lamellas; (a) low angle samples; (b) medium angle samples; (c) high angle samples; frictional coefficient comparison for single lamella and three lamellas; (d) low angle samples; (e) medium angle samples; (f) high angle samples.
Measurement data for saw tooth PTFE samples (single lamella); (a) normal force Fn; (b) frictional force Fr; (c) frictional coefficient μ; (d) penetration depth h.
High-speed video analyses, sorted from top to bottom; (a) penetration sequence of single medium angle PTFE sample; (b) different penetration ramps for three lamella saw tooth samples (low-low, low-medium, low-high, medium-high); (c) soft compound with related average tilt angle (single block, two lamellas, four lamellas, six lamellas).
Comparison of penetration process and frictional force development for three lamellas. (left) schematic penetration process; (right) qualitative frictional force development.
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