Snow tires—often also called winter tires—are tires designed for use in colder weather, snow and ice. Snow chains can be a slower-speed, temporary alternative in snowy conditions. Snow tires have a tread design with bigger gaps than those on summer tires, increasing traction on snow and mud. Some have metal studs to increase traction on ice. Tires designed for winter conditions are optimized to drive at temperatures below 7 °C (45 °F).
Studded tires with metal pins that protrude from the tire can greatly reduce skidding and accidents on snow or ice-covered roads. However, the metal studs make contact with the road pavement and eventually cut into the pavement, allowing water to get in. The water can cause road damage and create hydroplaning hazard.
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/cm2 (0.20 to 0.61 lb/sq ft). Compacted snow may have densities of 0.45 to 0.75 g/cm2 (0.92 to 1.54 lb/sq ft).
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 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.
In much of Scandinavia, Canada, and the US, snow tires may have metal studs to improve grip on packed snow or ice, but such tires are prohibited in certain other jurisdictions because of the damage they cause to the road surface. 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 mm 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. Typically, 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 out.
There is a new development of tires with retractable studs. The concept is to allow the pin part of the stud to be able retractable based on the activation and deactivation with a press of a button by the driver.
Regional symbols and rules
Light vehicles and buses must be equipped with snow M+S or 3PMSF tires on all axles from December through February passenger and have a minimum tread depth of 4mm.
All prefectures of Japan, except for the southernmost prefecture of Okinawa, require the motorized vehicles to be fitted with winter tires or tire chains if the road is icy or slushy. If tread grooves of snow tires are worn off for more than 50% of its 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.
Expressways and other controlled-access highway have mandatory snow tire restrictions in advance when snowfall is expected. Snow tire checkpoints are often set up in the early winter or on the roads leading to snow country to ensure all vehicles are fitted with appropriate snow tires with good tread depth. In addition, regions prone to severe snow and ice accumulation may require a tire chain to be fitted on top of the snow tires.
Studded tires were popular in Japan until mid 1980s, but some cities have enacted by-laws preventing the use of studded tires in an effort to reduce dust pollution caused by studded tires. In 1987, group of lawyers in Nagano Prefecture have brought up the issue to Environmental Dispute Coordination Commission to ban the sales of studded tires in Nagano Prefecture. In 1988, Environmental Dispute Coordination Commission and major tire manufactures in Japan have agreed to stop the manufacturing and sales of studded tires by March 1991.
Nationwide studded tire restrictions for passenger vehicles came in effect on April 1991, followed by restrictions for commercial trucks in 1993. Studded tires are technically still legal in Japan, but the usage is restricted by environmental law and it is a criminal offence to operate a vehicle fitted with studded tire on dry asphalt or concrete.
In the United States and Canada, a "3PMSF ("Three Peak Mountain Snow Flake")" symbol means that the tire has exceeded the industry requirement from a reference (non-snow) tire. Snow tires are 3.6% of the US market and 35% of the Canadian market.
Many US states permit the use of studded tires in colder months.
Canadian provinces control the use of snow tires. They require snow tires or chains only in certain areas during the winter:
- British Columbia – Snow tires are only required by law in certain mountainous regions. In these areas, motorists must use winter tires or carry tire chains.
- Alberta – Banff National Park or Jasper National Park require cars to have snow tires or tire chains between November 1 to March 31, or any other period during which the road is covered with snow or ice.
- Quebec – Winter tires or studded tires must be used from December 15 to March 15.
Certain Canadian provinces ban the use of studded snow tires outside the the period between October 1 and April 30. Stud lengths may be limited to 3.5 mm. Regulations limit the number of studs to fewer than 130 per tire on vehicles weighing less than 4,600 kg.
In Europe, requirements for snow tires vary by country: in Andorra, Austria, Czech Republic, Estonia, Finland, Germany, Latvia, Lithuania, Luxembourg, Montenegro, Norway, Romania, Serbia, Slovenia and Sweden, the use of snow tires is a legal requirement during winter months (usually November to mid-April) or if snow or slush is present on road surfaces; failure to comply can result in on-the-spot fines from the police. In Norway, drivers of light vehicles (less than 3,500 kg) are required to use winter tires according to conditions, studded tires are not allowed before November 1 (October 15 in northern parts) unless required by road conditions. Andorra, north and central Italy and Switzerland all recommend snow tires and they are a requirement in many areas. In Bulgaria, Croatia, Czech Republic, Germany, Netherlands, Poland, Romania, Serbia, Slovakia and Slovenia, the use of spiked tires is forbidden. In Switzerland, spiked tires are forbidden on motorways.
In Germany tires with a mud and snow (M+S) marking must be used during winter conditions.  M+S tires can be snow tires but all-season tires may also qualify. Studded tires are forbidden for motor vehicles, but allowed for bicycles.
Snow chains fitted to tires are required in Andorra, Austria, France, Germany, Italy, Spain, and Switzerland in certain winter conditions. In Norway, heavy vehicles (3,500 kg or more) are required to bring chains in winter conditions and to use winter tires from November 15 to March 31.
Since July 2008, the Czech Republic uses the Europe-wide road sign requiring the use of snow tires in marked mountainous areas during winter. The duration of obligatory snow tire use was originally November–April (Ordinance 208/2008 Sb.). This was later changed to November–March (Ord. 91/2009 Sb.).
Winter/Snow tyre requirements vary significantly across Europe, with different countries having different statutory requirements
Economic and environmental impact
The studs present in some snow tires come into contact with the road surface, and in certain cases they wear off the roads and cut ruts. Use of these types of snow tires can be costly because of abrasion of road surfaces. In wet weather this can further cause problems when the ruts are filled with water and cause hydroplaning hazard. Furthermore, it can result in creating polluting dust.
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