It is well known that snow and ice sticks to textiles. AutoSock is made from 100% high-technology fibers. These fibers, which become hairier with use, are arranged in a specific pattern in order to optimize grip. The specially developed textile has the advantage of handling water film found between the icy and snowy road and the tyre, (generated for example by heat from the sun, or wheel spin), thereby maximizing the grip.
Tribology/ Dry vs. wet friction
Tribology is defined as the science and technology of interacting surfaces in relative motion and of the practices related thereto.
A tribological system consists of three parts:
- Upper surface
- Lower surface
In the case of friction on icy or snowy roads the upper surface is the tyre or AutoSock, the lubricant is water film created by frictional melting and liquid layer on the ice/snow, and the lower surface is the icy or snowy road.
When the water film thickness is insignificant, we have dry friction; this is the case when braking a car at -20ºC. When the water film separates the two surfaces, we have wet friction; this is the case when a car water planes. AutoSock wants to have as large area as possible under the AutoSock with dry friction, since dry friction gives the highest friction coefficient.
Snow vs. Ice friction
- A surface pattern that makes the total contact area exposed to friction sufficiently large under both soft and hard snow or ice conditions
- A combination of surface pattern, strength and elasticity that make the contact points sufficiently sticky under hard snow or ice conditions
To some extent we have a trade off between good friction properties on snow relative to ice. It is favourable to open up the structure in order to increase the total contact area exposed to friction on snow. At the same time the contact points need to be sufficiently sticky on hard ice. The contact configuration of a tyre or AutoSock can be quite different on snow relative to ice.
The upper figures show the contact configurations of a tyre on soft snow.The figure to the left simulates compaction resistance, while the figure to the right simulates impact resistance.
The lower figure shows the contact configuration of a tyre on ice where the macroscopic impact and compaction resistances are negligible.
The AutoSock is made of a special high friction textile fabric. In a frictional AutoSock sliding situation, electrostatic pressures can be defined in:
- The air gaps between: a) the sock surface and the icy or snowy road, b) the sock surface and the water film, c) the water film and the icy or snowy road surface.
- The interfacial contacts between: a) the sock surface and the icy or snowy road surface, b) the sock surface and the water film, c) the water film and the icy or snowy road surface.
- The water film.
The dry friction process is dominated and characterised by accumulation of electrostatic charges in the slider contact points. The frictional water film initiates discharge of potential differences between the slider and the sliding surface due to the much higher electrical conductivity of water relative to snow/ice.
The topography of the slider and the sliding surface is decisive for the electrical contact configuration between the slider and the sliding surface.
The electrolytic conductivity of a melted snow/ice sample may indicate the rate of ions introduced to the interface between snow/ice and the slider by frictional melting and thereby the rate and ease of discharge between the slider and snow/ice through the frictional water film during braking. Larger frictional electrification should take place on snow/ice with low electrical conductivity compared to snow/ice with high electrical conductivity.