Snow tire


Snow tires, also known as winter tires, are tires designed for use on snow and ice. Snow tires have a tread design with larger gaps than those on conventional tires, increasing traction on snow and ice and are often optimized to drive at temperatures below. Tires that meet specific standards and or tests are entitled to display M+S, a Three Peak Mountain Snow Flake , and or an Ice Grip Symbol on their sidewalls. Studded tires are a type of snow tires which have metal or ceramic studs that protrude from the tire to increase traction on hard-packed snow or ice and have parallel standards including the use of the 3PMSF symbol with a Ш along with "STUDDED" and the IGS. Studs abrade dry pavement, causing dust and creating wear in the wheel path. Regulations that require the use of snow tires or permit the use of studs vary by jurisdiction.
All-season tires have tread gaps that are smaller than snow tires and larger than conventional tires. They are quieter than winter tires on clear roads, but less capable on snow or ice.

Roadway conditions in winter

Snow tires operate on a variety of surfaces, including pavement, mud, ice, or snow. The tread design of snow tires is adapted primarily to allow penetration of the snow into the tread, where it compacts and provides resistance against slippage. The snow strength developed by compaction depends on the properties of the snow, which depend on its temperature and water content—wetter, warmer snow compacts better than dry, colder snow up to a point where the snow is so wet that it lubricates the tire-road interface. New and powder snow have densities of. Compacted snow may have densities of.
Snow or ice-covered roadways present lower braking and cornering friction, compared to dry conditions. The roadway friction properties of snow, in particular, are a function of temperature. At temperatures below, snow crystals are harder and generate more friction as a tire passes over them than at warmer conditions with snow or ice on the road surface. However, as temperatures rise above, the presence of free water increasingly lubricates the snow or ice and diminishes tire friction. Hydrophilic rubber compounds help create friction in the presence of water or ice.

;Dry and moist snow conditions on roadways

Tire/snow interactions

The compacted snow develops strength against slippage along a shear plane parallel to the contact area of the tire on the ground. At the same time, the bottom of the tire treads compress the snow on which they are bearing, also creating friction. The process of compacting snow within the treads requires it to be expelled in time for the tread to compact snow anew on the next rotation. The compaction/contact process works both in the direction of travel for propulsion and braking, but also laterally for cornering.
The deeper the snow that the tire rolls through, the higher the resistance encountered by the tire, as it compacts the snow it encounters and plows some of it to either side. At some point on a given angle of uphill pitch, this resistance becomes greater than the resistance to slippage achieved by the tread's contact with the snow and the tires with power begin to slip and spin. Deeper snow means that climbing a hill without spinning the powered wheels becomes more difficult. However, the plowing/compaction effect aids in braking to the extent that it creates rolling resistance.

;Tire/snow interactions

Tread

Attributes that can distinguish snow tires from "all-season" and summer tires include the following:
  • An open, deep tread, with a high void ratio between rubber and spaces between the solid rubber
  • Shoulder blocks, a specialized tread design at the outside of the tire tread to increase snow contact and friction
  • A narrower aspect ratio between the diameter of the tire and the tread width to minimize resistance from the plowing effect of the tire through deeper snow
  • Hydrophilic rubber compounds that improve friction on wet surfaces
  • Additional siping, or thin slits in the rubber, that provide more biting edges and improve traction on wet or icy surfaces.
Wet-film conditions on hard-compacted snow or ice require studs or chains.

Studs & studded tires

Many jurisdictions in Asia, Europe, and North America seasonally allow snow tires with metal or ceramic studs to improve grip on packed snow or ice.
Such tires are prohibited in other jurisdictions or during warmer months because of the damage they may cause to road surfaces. The metal studs are fabricated by encapsulating a hard pin in a softer material base, sometimes called the jacket. The pin is often made of tungsten carbide, a very hard high performance ceramic. The softer base is the part that anchors the stud in the rubber of the tire. As the tire wears with use, the softer base wears so that its surface is at about the same level as the rubber, whereas the hard pin wears so that it continues to protrude from the tire. The pin should protrude at least for the tire to function properly. Studded tires do not eliminate skidding on ice and snow, but they greatly reduce risks.
Studdable tires are manufactured with molded holes on the rubber tire tread. Usually, there are 80 to 100 molded holes per tire for stud insertion. The insertion is done by using a special tool that spreads the rubber hole so that a stud jacket can be inserted and the flange at the bottom of the jacket can be fitted nicely to the bottom of the hole. The metal studs come in specific heights to match the depths of the holes molded into the tire tread based on the tread depths. For this reason, metal studs can be inserted only when the tires have not been driven on. A proper stud insertion results in the metal jacket that is flush with the surface of the tire tread having only the pin part that protrudes.

Standards

Regardless of what conditions a tire is designed to handle, regional authorities may require minimum tread depth such as 6/32 or 3/16-inch in the U.S. or 4mm in Europe, the use of chains, or close road access all together in freezing conditions.

Snow: M+S

The M+S tire code arose in the 1970s to indicate tires whose tread design performs better in "Mud and Snow" conditions than regular "non-M+S" street tires, and is distinct from the tire code "M+T" for "Mud and Terrain".
The USTMA includes requirements that M+S tire tread has recesses that extend at least 1/2 inch from the edge toward the center, with a minimal cross-section of 1/16 inch, and be angled to the direction of travel from 35 to 90 degrees. The Tire and Rubber Association of Canada also uses USTMA standards.
The ISO defines both "snow tyre" and "mud and snow tyre" as performing better at initiating or maintaining vehicle motion under their respective road condition versus a normal tire lacking the M+S designation or a similar combination of “M” and “S”.
UN Regulation No. 117 definitions and revisions have used M+S to indicate tires whose tread pattern, tread compound or structure are designed to outperform a normal tire in "mud and fresh or melting snow". Whether a tire has a "M+S marking " is included "grip index" test report forms. Reg. No. 172 requires a retreaded tire's sidewall to display M+S if so qualified. Currently, 68 countries have signed to UN Regulations regarding tires, the United States and Canada being the two notable exceptions which use USTMA and their respective laws.

Severe Snow: Alpine Symbol, 3PMSF

The Alpine Symbol or Three-Peak Mountain Snow Flake pictogram depicts a "mountain profile" with "three peaks with the middle peak being the tallest" containing "a six-sided snowflake" half the height of the tallest peak is an international indicator of tires of various classes rated for "severe snow use", such as a passenger vehicle tire that performs 10 to 12% better in straight-line "traction test" than a Standard Reference Test Tire to meet USTMA's standard
. It is used in addition and adjacent to the M+S mark, and was standardized in 1999.
Snow tires should be installed on all wheels, never just the front which could lead to adverse handling.
ISO defines a "severe snow use tyre" as having a tread pattern, compound or structure whose snow grip is based on performance testing using ISO 18106 "Methods for measuring snow grip performance" to test braking on snow for passenger car and commercial vehicle tires, and acceleration on snow for truck and bus tires.
The USTMA requires the Snow Grip Index of tires for "severe snow conditions" equal or exceed that of a SRTT using straight line "traction test" ASTM F1805-18 on a medium packed snow surface. For example, with the SRTT having a SG of 1.00, a 14-inch tire would need a SG of 1.10, and a 16-inch tire a SG of 1.12, basically 10 or 12% better respectively.
UN R117 requires "severe snow" tires for a passenger car to equal or exceed a "brake test" SG of 1.07 and "traction test" SG of 1.10, while light commercial have a lesser "brake test" SG of 1.02, whereas heavy duty vehicles only "acceleration test" SG of 1.25. Whether a tire has a "3PMSF marking " is also included grip index test report forms. Reg. No. 172 requires a retreaded tire's sidewall to display both the 3PMSF and M+S if so qualified.

Ice Conditions: Ice Grip Symbol, IGS

The Ice Grip Symbol depicting a "mountain profile" containing "three icicles with the middle icicle being the longest" is used internationally to indicate passenger car tires rated for "ice conditions" which perform at least 18% better in straight-line "braking test" than a reference tire. It is placed in addition and adjacent to both the 3PMSF mark and M+S mark, and was standardized in 2021.
Tires for "ice conditions" should be installed on all wheels, never just the front which could lead to adverse handling.
The USTMA requires the Ice Grip Index of an "ice conditions" tire to equal or exceed 1.18 compared to a 16-inch tire meeting ASTM F2493 in a "braking test" complying with ISO 19447:2021.
UN R117 defines an "ice grip tyre" as 3PMSF-rated passenger car tire with a GI of 1.18 or greater using the same procedures used by USTMA.
ISO's definitions of tire industry terms has yet to be updated since the release of their "ice grip performance" test methods in 2021.