Influence of snow microstructure and properties on the grip of winter tyres

The friction of tyres on roads has been of practical importance for many years with&#13;\nnearly 80% of terrestrial traffic making use of rubber tyres. Tyres provide the grip&#13;\nrequired for vehicle acceleration, braking and cornering. In order for a tyre to grip on a&#13;\nsnow covered surface friction mechanisms such as “ploughing”, (where sharp tread block&#13;\nedges dig into and break bonds between the snow grains) and fluid film lubrication must&#13;\nbe considered. These are not present when a tyre interacts with tarmac. In addition&#13;\nmetamorphism of the snow over time can result in variations of the structure and&#13;\nmechanical properties, this can occur rapidly especially when dealing with temperatures&#13;\nclose to snows melting point.&#13;\n&#13;\nWhen full car-scale outdoor testing is carried out the snow conditions cannot be&#13;\ncontrolled and vary daily. This means the snow properties must be measured every day&#13;\nso that any observed variations in friction can be attributed to the tyres rather than&#13;\nthe snow. At present the simple measurements being carried out on the snow tracks&#13;\nhave not proved sufficient to pick up on the variations in the snow. This leads to&#13;\ninconsistent results: one tyre behaves differently on two different days, even though the&#13;\nsnow was measured to be the same. This has resulted in the need for further study of&#13;\nthe way snow variations influence the grip of winter tyres.&#13;\nThe primary aim of this study is to identify which snow properties contribute to the&#13;\nfriction of tyres on snow and be able to estimate the friction from measurements of snow&#13;\nproperties. This work is the first comprehensive study to combine: multiple snow&#13;\nproperties, microstructure characterisation, measurement of friction behaviour and&#13;\ndifferent snow (both artificial and natural).&#13;\n&#13;\nIn order to study the way snow affects the grip of winter tyres, methods of&#13;\nmanufacturing artificial snow with consistent mechanical properties and microstructure&#13;\nare used. A method of blending ice chips (a solid state fracturing process) and&#13;\ncompressing the resulting snow to form a test track was developed during a previous&#13;\nPhD carried out in our group. An alternate snow microstructure was created by using&#13;\nan established process of creating snow by vapour deposition. The process was simplified&#13;\nand downscaled, the resulting snow consisted of large dendritic grains, very different to&#13;\nthe blended ice chips. Both snows were pressed in identical manners to create snow&#13;\ntesting tracks. In addition, natural snow collected from the field was tested to compare&#13;\nwith the artificial snow.&#13;\n&#13;\nIn order to investigate how the variations in the snow affected the friction of tyres&#13;\nextensive testing was carried out in a cold room using a linear tribometer, using&#13;\nprocedures established in previous studies. Two analytical rubber samples were used to&#13;\ninvestigate the friction, a rounded edge sample and a siped sample. Testing was carried&#13;\nout at -10°C at speeds of 0.01m/s, 0.1m/s and 1m/s.&#13;\n&#13;\nA significant part of this PhD involved the development of new methods and equipment&#13;\nwhich have not been used to study snow in this way before. In order to characterise&#13;\nmechanical properties, shear testing, compression testing and cohesion testing were&#13;\ncarried out. To investigate snow microstructure, surface profilometry, microscopy and&#13;\nX-ray microtomography were used.&#13;\n&#13;\nCorrelating the changes observed in snow characteristics with the changes recorded in&#13;\nthe coefficient of friction has allowed the development of an empirical equation. This&#13;\ncan be used to predict the coefficient of friction of a given snow based on three relatively&#13;\nsimple snow measurements: a compression test to calculate the effective modulus, a&#13;\nroughness measurement to calculate the peak count density and a snow penetration&#13;\ntest.&#13;\n&#13;\nFor the first time this study allows us to use the empirical equation to estimate the&#13;\nrelative contributions of the ploughing and surface friction mechanisms to the total&#13;\nfriction. This allows the comparison of full car-scale test data as it is now possible to&#13;\naccount for variations in the snow test tracks.

Topic: Winter Sports Injuries and Performance
Subfield: Pulmonary and Respiratory Medicine
Field: Medicine
Domain: Health Sciences