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.
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/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).
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
Attributes that can distinguish snow tires from "all-season" and summer tires include:
- An open, deep tread, whose void ratio between rubber and spaces between the solid rubber is comparatively high
- Shoulder blocks—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 improves friction on wet surfaces
Snow tires are designed to encounter many of the same conditions that summer tires encounter, so they incorporate siping for rain. Wet-film conditions on hard-compacted snow or ice require studs or chains.
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 millimetre (0.039 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. 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. In 2015 the concept was announced of retractable studs that allow the drive to extend or retract the studs in the tire.
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.
- Tire-snow interactions
In Russia light vehicles and buses must be equipped with snow M+S or 3PMSF (Three-Peak Mountain Snow Flake) tires on all axles from December through February passenger and have a minimum tread depth of 4 millimetres (0.16 in).
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. Nationwide studded tire restrictions for passenger vehicles came in effect in 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.
The U.S. National Highway Traffic Safety Administration (NHTSA) and Transport Canada allow display of a 3PMSF (Three-Peak Mountain Snow Flake) symbol to indicate 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 period between October 1 and April 30. Stud lengths may be limited to 3.5 millimetres (0.14 in). Regulations limit the number of studs to fewer than 130 per tire on vehicles weighing less than 4,600 kg.
As of 2016, European regulations pertaining to snow tires varied by country. The principal aspects of regulations were whether use was mandatory and whether studded tires were permitted.
- Mandatory use – The following countries required snow tires between specified dates, at least for specified circumstances: Austria, Bosnia-Herzegovina, Croatia, Czech Republic, Estonia, Finland, Latvia, Lithuania, Montenegro, Norway, Serbia, Slovakia, Slovenia, Sweden, Russia, and Turkey.
- Studded tires banned – The following countries banned the use of studded tires: Albania, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Czech Republic, Hungary, Luxembourg, Macedonia, Montenegro, Netherlands, Poland, Portugal, Romania, Serbia, Slovakia, and Slovenia.
- Studded tires restricted – The following allowed the restricted use of studded tires: Austria, Denmark, Estonia, Finland, France, Great Britain, Ireland, Latvia, Lithuania, Norway, Russia, Spain, Sweden, and Switzerland.
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.
- Heißing, Bernd; Ersoy, Metin (2010). Chassis Handbook: Fundamentals, Driving Dynamics, Components, Mechatronics, Perspectives. Springer Science & Business Media. p. 591. ISBN 9783834897893.
- "Winter tyre basics". Tyremen UK. Retrieved 2012-10-24.
- "Prall Tester - Studded Tyre Wear Test". www.cooper.co.uk. Cooper Research Technology Ltd. Retrieved 1 September 2014.
- Taylor, Rich (January 1985), "How to pick the right winter tires", Popular Mechanics: 72–78
- Hays, Donald (2013). The Physics of Tire Traction: Theory and Experiment. Springer Science & Business Media. p. 428. ISBN 9781475713701. Retrieved 2016-12-25.
- Glenne, Bard (December 1989), "All about Snow Tires", Skiing: 52–55, 272
- ScienceDaily (6 January 2011). "How Studded Winter Tires May Damage Public Health, as Well as Pavement". Retrieved 26 January 2011.
- Nordström, Olle (2004). VTI Meddelande 965 - 2004. VTI - Väg- och transportforskningsinstitutet (Report).
- Gustafsson, M.; et al. (2006). VTI rapport 543 - Effekter av vinterdäck - en kunskapsöversikt. VTI - Väg- och transportforskningsinstitutet (Report).
- "Studded Tires for Winter Driving". Tirerack.com. Retrieved 24 January 2015.
- Lloyd, Alex (18 February 2014). "New tire deploys ice-gripping studs with push of a button". Yahoo Auto, Motoramic. Retrieved 24 January 2015.
- Editors (2016). "Winter Regulations—European regulations for winter equipment on trucks and buses". continental-tires.com. Continental Tires. Retrieved 2017-03-26.
- 冬の安全ドライブ事前注意報 Japan Automobile Federation
- スパイクタイヤ粉じんの発生の防止に関する法律 e-Gov Laws and Regulations Database
- Editors (February 21, 2017). "Cooper Recalls 7,067 Discoverer M+S Sport Tires". moderntiredealer.com. Modern Tire Dealer. Retrieved 2017-03-26.
- Editors. "What You Should Know—Tires for Winter Driving". tracanada.ca. Tire and Rubber Association of Canada. Retrieved 2017-03-26.
- Dudley, David (6 December 2016). "The Joy and Terror of Urban Snow Driving". CityLab. Retrieved 13 December 2016.
- "Studded Tyre Wear Test". www.cooper.co.uk. Cooper Research Technology Limited. Retrieved 3 September 2014.