# 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. Such tires that have passed specific winter traction performance tests are entitled to display a 3PMSF (Three-Peak Mountain Snow Flake) and/or a IMP (Icy Mountain Peak) symbols on their sidewalls. Tires designed for winter conditions are optimized to drive at temperatures below 7 °C (45 °F). 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. 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 (wet or dry), 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 0.1 to 0.3 g/cm$^3$ (6 to 20 lb/cu ft). Compacted snow may have densities of 0.45 to 0.75 g/cm$^3$ (28 to 47 lb/cu ft).
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 −7 °C (20 °F), 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 −2 °C (28 °F), 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

* Vehicle in cold, dry, powder snow, which offers comparatively better traction than moist snow.
* Vehicles in warm, moist, granular snow encounter decreased roadway traction.

## Treads

Attributes that can distinguish snow tires from "all-season" and summer tires include:
* 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

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 1 millimetre (0.04 in) for the tire to function properly. Snow 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, stud metals can only be inserted 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.

## 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

* Compaction of snow under an advancing snow tire, causing rolling resistance while passing through about 10 centimetres (4 in) of snow.
* Compaction in about 5 centimetres (2 in) of snow left behind a snow tire, showing tread-snow interaction.

## Standards
### 3PMSF

The 3PMSF (Three-Peak Mountain Snow Flake) indicates 10% better acceleration on snow. ASTM International F1805 (formalized in the year 1999)

### IMP

The IMP (Icy Mountain Peak) indicates 18% better deceleration on ice. ASTM International F2493 (formalized in the year 2021)

## Regulations
### Asia

All prefectures of Japan, except for the southernmost prefecture of Okinawa, have a traffic regulation requiring motorized vehicles to be fitted with winter tires or tire chains when the road is covered by ice or snow. In addition, tire chains must be fitted for all vehicles on rural designated highways in snow country regions when regulated by traffic signs requiring tire chains.
In many prefectures, tread grooves of snow tires are worn off for more than 50% of their original depth, tires must be replaced to meet the legal requirements. Drivers will be fined for failing to comply with the snow tire or tire chains requirements, and checkpoints are in place on major highways.
Nationwide studded tire restrictions in Japan for passenger vehicles came into effect in April 1991, followed by restrictions for commercial trucks in 1993. Studded tires are still legal in Japan, but their usage is restricted by environmental law and it is a criminal offence to operate a vehicle fitted with a studded tire on dry asphalt or concrete.

### Europe

As of 2016, regulations pertaining to snow tires in Europe varied by country. The principal aspects of regulations were whether the use was mandatory and whether studded tires were permitted.
* Mandatory use – The following countries required snow tires between specified dates or when roads are snowy or icy: Austria, Bosnia-Herzegovina, Croatia, Czechia, Estonia, Finland, France, Germany, Latvia, Lithuania, Montenegro, Norway, Romania, Serbia, Slovakia, Slovenia, Sweden, and Russia.
* Studded tires banned – The following countries ban the use of studded tires: Albania, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Czechia, Germany, Hungary, Luxembourg, North Macedonia, Montenegro, Netherlands, Poland, Portugal, Romania, Serbia, Slovakia, and Slovenia.
* Studded tires restricted – The following countries allowed the seasonally restricted use of studded tires: Austria, Denmark, Estonia, Finland, France, Iceland, Ireland, Latvia, Lithuania, Norway, Russia, Spain, Sweden, and Switzerland.

### North America

The U.S. National Highway Traffic Safety Administration (NHTSA) and Transport Canada allow display of a 3PMSF symbol to indicate that the tire has exceeded the industry requirement from a reference (non-snow) tire. As of 2016, snow tires were 3.6% of the US market and 35% of the Canadian market.
US states and Canadian provinces control the use of snow tires. Of these, Quebec is the only jurisdiction that requires snow tires throughout. Some may require snow tires or chains only in specified areas during the winter.