Snow tire

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/cm3. Compacted snow may have densities of 0.45 to 0.75 g/cm3.

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 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 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 File:Mercedes-Benz S203 Yyteri.JPG|Vehicle in cold, dry, powder snow, which offers comparatively better traction than moist snow. File:Саратов пробка у Детского парка.jpg|Vehicles in warm, moist, granular snow encounter decreased roadway traction.

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 File:Snow tire compacting snow ahead of it.jpg|Compaction of snow under an advancing snow tire, causing rolling resistance while passing through about 10 cm of snow. File:Impression left by snow tire rolling through snow.jpg|Compaction in about 5 cm of snow left behind a snow tire, showing tread-snow interaction.

Tread

Main article: Tire#Components

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. (See Regulations)

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 mm 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 (~4.75mm) in the U.S. or 4mm (~5/32in) 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 (perpendicular). The Tire and Rubber Association of Canada (TRAC, formerly the 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” (e.g. M&S).

UN Regulation No. 117 (UN R117) 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 (Y/N)" 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 (3PMSF) 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 (SRTT) 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 (SG) 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 (C1) to equal or exceed a "brake test" SG of 1.07 and "traction test" SG of 1.10, while light commercial (C2) have a lesser "brake test" SG of 1.02, whereas heavy duty (C3) vehicles only "acceleration test" SG of 1.25. Whether a tire has a "3PMSF marking (Y/N)" 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 USTMA requires the Ice Grip Index (GI) 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 (2017) has yet to be updated since the release of their "ice grip performance" test methods in 2021.

Studded: 3PMSF with Ш ("Sha"), adding IGS

A new symbol coupling the Alpine "3PMSF" symbol with the Cyrillic letter "Sha" (Ш), the first letter of studs or spikes (Russian: Шипы "shipy"; Ukrainian: Шипи "shipi"), was standardized in 2022 for studded tires that meet the requirements for "severe snow use", in addition to adding the word "STUDDED" at least 4mm in height to the tire sidewall. Studded tires should be installed on all wheels, never just the front which could lead to adverse handling.

UN Regulation No. 164 (UN R164) was adopted in late 2022 to apply the "severe snow use" standards to studded tires, which are permitted in some regions yet were beyond the scope of UN R117. The following year, UN R164 was amended to allow studded tires that meet the requirements for a "ice grip tyre" to add the Ice Grip Symbol (IGS).

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 2024, 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.

• Snow tires mandatory 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 restricted seasonally: Austria, Denmark, Estonia, Finland, France, Iceland, Ireland, Latvia, Lithuania, Norway, Russia, Spain, Sweden, and Switzerland.
• Studded tires banned: Albania, Belgium, Bosnia-Herzegovina, Bulgaria, Croatia, Czechia, Germany, Hungary, Luxembourg, North Macedonia, Montenegro, Netherlands, Poland, Portugal, Romania, Serbia, Slovakia, and Slovenia.

North America

NHTSA (United States) and Transport Canada allow the 3PMSF symbol on tires that meet or exceed 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.

Canada

:{| class="wikitable mw-collapsible mw-collapsed" |+ class="nowrap" | Table of Canada laws: Snow tires, Snow chains, & Studded tires by province & territory |- ! Region !! Snow tires !! Snow chains !! Studded tires |- | Alberta || || || |- | British Columbia || MVAR 7.162 || MVAR 7.163 - 7.1634 || MVAR 7.164 |- | Manitoba || || || CCSM H60 3.21(2) |- | National parks of Canada || CRC c1126, 40 || CRC c1126, 40 || |- | New Brunswick || || MVA M17 241(3) || MVA M17 241(3) |- | Newfoundland and Labrador || CNLR 1007/96 27 || CNLR 1007/96 27 || CNLR 1007/96 28 |- | Northwest Territories || || MVER 82.(b) || MVER 82.(a) |- | Nova Scotia || || MVA 293, 198(2) || RSNS 45/79 |- | Nunavut || || MVER 82.(b) || MVER 82.(a) |- | Ontario || || RSO 1990 H.8 69(2) || RRO 625, 9.(1) |- | Prince Edward Island || "winter tires" || HTA 136.(3) || HTA WTR 1. |- | Quebec || C-24.2, r. 45 || C-24.2, r. 44 2. || C-24.2, r. 44 1. |- | Saskatchewan || || || |- | Yukon || || || |- style="text-align: center; background-color: #eaecf0;" | Region || Snow tires || Snow chains || Studded tires |}

Quebec is the only jurisdiction that requires vehicles registered therein to have 3PMSF-rated or studded tires December 1 through March 15 regardless of weather.

United States

:{| class="wikitable mw-collapsible mw-collapsed" |+ class="nowrap" | Table of United States laws: Snow tires, Snow chains, & Studded tires for 50 states & D.C. |- ! Region !! Snow tires !! Snow chains !! Studded tires |- | Alabama || || Ala. Code 32-5-210 (2)(b) || Ala. Code 32-5-210 (2)(b) |- | Alaska || || AS 28.35.155 || AS 28.35.155 |- | Arizona || || ARS 28-958 B.2. || ARS 28-958 B.3. |- | Arkansas || || ACA 27-37-401. (c)(2) || ACA 27-37-401. (c)(3) |- | California || 13 CCR 1071, CVC 27459, CVC 558 || CVC 27454 (a) CVC 27459 CVC 27460 CVC 27460 || CVC 27454 (a) CVC 27459 CVC 27460 CVC 27460 |- | Colorado || CRS 42-4-106 (5)(a)(I)(B) || CRS 42-4-228 (3) || CRS 42-4-228 (3) |- | Connecticut || || || CGS 14-98 |- | Delaware || || || Del. Code 21-4302 (b) |- | Florida || || FS 316.299 || FS 316.299 |- | Georgia || || OCGA 32-6-5 (a) || OCGA 40-8-74 (c)(2) |- | Hawaii || || HRS 291-33 (1) || HRS 291-33 (2) |- | Idaho || "snow tires"1, 2, 3 || IS 49-948 (3) || IS 49-948 (3) |- | Illinois || || 625 ILCS 5/12-401 3¶ || 625 ILCS 5/12-401 1¶ |- | Indiana || || IC 9-19-18-3 (c) || IC 9-19-18-3 (d) |- | Iowa || || IC 321.442 (2) || IC 321.442 (3) |- | Kansas || || KSA 8-1742 (c)(2) || KSA 8-1742 (c)(3) KAR 36-7 |- | Kentucky || || KYRS 189.190 || |- style="text-align: center; background-color: #eaecf0;" | Region || Snow tires || Snow chains || Studded tires |- | Louisiana || || LRS 32:362 (B)(2) || |- | Maine || || MRS 2381 (2)(C) || MRS 1919 |- | Maryland || MTS 21-1119 || MTS 22-405.2 (b)(2) || MTS 22-405.2 (c) |- | Massachusetts || || idem MGL 90-16 || MGL 90-16 |- | Michigan || "winter tires" || MCL 257-710 (b) || MI Admin. Code R 247.174 |- | Minnesota || || MS 169.72 Subd. 1. (d)(2) || MS 169.72 Subd. 4. |- | Mississippi || || MC 63-7-67 || |- | Missouri || || by cities 1, 2, 3, 4, 5 || RSMo 307.171 |- | Montana || MCA 61-9-406(3) || MCA 61-9-406(3) || MCA 61-9-406(4) |- | Nebraska || || NRS 60-6,250 (2)(c) || NRS 60-6,250 (2)(a) |- | Nevada || NAC 484D.230, NRS 484D.515 (2) || NAC 484D.215 (2)(a), NRS 484D.510 (2)(a) || NAC 484D.215 (1)(e) , NRS 484D.510 (2)(c) |- | New Hampshire || || NHRSA 266.48, NHAC Saf-C 3244.16 || |- | New Jersey || || NJRS 39:3-73 || NJAC 13:20-32.19 (g) |- | New Mexico || || NMSA 66-3-847 (C) || NMSA 66-3-847 (C) |- | New York || CLNY VAT 145-D || CLNY VAT 145-C || CLNY VAT 375, 35. 35-a |- | North Carolina || || NCGS 20-122 (b) || NCGS 20-122 (b) |- | North Dakota || || 39-21-40 (3) || 39-21-40 (3) |- style="text-align: center; background-color: #eaecf0;" | Region || Snow tires || Snow chains || Studded tires |- | Ohio || || ORC 5589.081 (C) || ORC 5589.081 (B)(1) |- | Oklahoma || || OSC 47-12-405 (C)(1)(b) || OSC 47-12-405 (C)(2)(b) |- | Oregon || OAR 734-017-0005 (1)(a) || ORS 815.140, OAR 734-017-0005 || ORS 815.165 (7), OAR 734-017-0005 |- | Pennsylvania || 67 PaC 213.2 || 75 PaCSA 4525 (d) || 75 PaCSA 4525 (c) |- | Rhode Island || || RIGL 31-23-20 (3) || RIGL 31-23-20 (1) |- | South Carolina || || SCCLU 56-5-5040 || SCCLU 56-5-5040 |- | South Dakota || || SDCL 32-19-2 || SDCL 32-19-3 |- | Tennessee || || TC 55-9-106 (c) || TC 55-9-106 (b) |- | Texas || || TTC 547.612 (c)(2) || |- | Utah || UAC R920-6 || UCA 41-6a-1636 (5)(c) || UCA 41-6a-1636 (5)(a) |- | Vermont || "snow tires" || || 23 VSA 1092 |- | Virginia || || CoV 46.2-1044 || CoV 46.2-1044 |- | Washington || WAC 204-24-040 (3)(d) || WAC 204-24-050, RCW 46.37.420 (2) || RCW 46.37.420 (2) |- | Wash., D.C. || || DCMR 18-732.5(b) || DCMR 18-732.5(c) |- | West Virginia || || WVC 17C-15-37 (c)(2) || WVC 17C-15-37 (c)(3) |- | Wisconsin || || WS 347.45 (2)(b) || WS 347.45 (2)(c) |- | Wyoming || WS 31-5-956 (e) || WS 31-5-956 (c)(ii) || WS 31-5-956 (c)(iii) |- style="text-align: center; background-color: #eaecf0;" | Region || Snow tires || Snow chains || Studded tires |} Some U.S. states, especially those in snowy or mountainous regions, require certain vehicles to use approved winter tires or carry traction devices such as tire chains. when traveling on designated roads during the winter season. Where allowed, studded tires and similar traction devices are usually restricted to specific months to prevent road damage. Requirements can vary widely by jurisdiction and may also depend on vehicle class, axle configuration, weight, and other factors.

References

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