A tire (North American English) or tyre (Commonwealth English) is a ring-shaped part that covers a wheel’s rim. It helps move a vehicle’s weight from the axle through the wheel to the ground and provides traction for the wheel to roll on surfaces. Most tires, like those on cars and bicycles, are filled with air. This creates a soft cushion that helps absorb shocks when the tire rolls over rough surfaces. Tires have an area called a contact patch, which is the part that touches the ground. This area is designed to support the vehicle’s weight and the pressure it puts on the surface without damaging it.

Modern tires are made from materials such as synthetic rubber, natural rubber, fabric, wire, carbon black, and other chemicals. They have two main parts: the tread and the body. The tread helps the tire grip the road, while the body holds the compressed air inside. Before rubber was used, tires were made of metal bands around wooden wheels to hold them together and reduce wear. Early rubber tires were solid, not filled with air. Pneumatic tires, which are filled with air, are used on many vehicles, including cars, bicycles, motorcycles, buses, trucks, heavy machinery, and airplanes. Metal tires are used on trains and railcars, while solid rubber tires are used in non-automotive items like casters, carts, lawnmowers, and wheelbarrows.

Poorly maintained tires can cause serious problems, such as flat tires that stop a vehicle from working or blowouts, where tires burst during use and may damage vehicles or hurt people. Because of these dangers, tire production is often closely controlled by laws. Tires are widely used in motor vehicles, so tire waste makes up a large part of global waste. Recycling tires is important and can be done through methods like mechanical recycling, which turns tires into materials like crumb rubber, or pyrolysis, which breaks them down for chemical reuse, such as tire-derived fuel. If tires are not recycled or burned properly, they can release harmful chemicals into the environment. Additionally, regular use of tires creates tiny plastic particles that carry these chemicals. These particles enter the environment and increase the risk of harm to human health.

Etymology and spelling

The word "tire" is a shortened form of "attire," based on the idea that a wheel with a tire is a "dressed" wheel. The spelling "tyre" was the first used, and both "tyre" and "tire" were common in writing during the 15th and 16th centuries. In the 17th and 18th centuries, "tire" became more frequently used in printed materials. The spelling "tyre" was not used again until the 1840s, when the English started fitting railway car wheels tightly with malleable iron. However, many publishers still used "tire" for a long time. For example, The Times newspaper in London used "tire" as late as 1905. The spelling "tyre" became more common in the 19th century in the UK for describing pneumatic tires. The 1911 edition of the Encyclopædia Britannica noted that "the spelling 'tyre' is not now accepted by the best English authorities, and is unrecognized in the US," while Fowler's Modern English Usage from 1926 stated that "there is nothing to be said for 'tyre,' which is not correct in terms of its origin, as well as needlessly different from British and American usage." Over the 20th century, "tyre" became the standard spelling in the UK.

History

The earliest tires were made of leather bands in Sumer, then iron (later steel) placed on wooden wheels used for carts and wagons. A skilled worker, called a wheelwright, would heat the tire in a forge fire to make it expand. The heated tire was then placed over the wheel and cooled quickly, causing the metal to shrink and fit tightly around the wheel.

The first patent for a standard pneumatic tire was filed in 1847 by Scottish inventor Robert William Thomson. However, this idea was never made into real tires. The first practical pneumatic tire was created in 1888 on May Street, Belfast, by John Boyd Dunlop, a Scottish man who owned a successful veterinary practice in Ireland. He made the tire to help his 10-year-old son, Johnnie, ride his tricycle more comfortably on rough roads. His doctor, Sir John Fagan, recommended cycling for the boy’s health and helped design the first pneumatic tires. Cyclist Willie Hume showed how much better Dunlop’s tires were in 1889 by winning races in Ireland and England. In Dunlop’s patent from October 31, 1888, he only mentioned using the tire on bicycles and light vehicles. In September 1890, Dunlop learned about an earlier invention but kept the information private. In 1892, Dunlop’s patent was declared invalid because of earlier work by Robert William Thomson, a fellow Scot. However, Dunlop is credited with proving that rubber could last long while staying strong enough for use as a tire. John Boyd Dunlop and Harvey du Cros worked through challenges, hiring inventor Charles Kingston Welch and acquiring other patents to protect their business. Their company eventually became Dunlop Rubber and Dunlop Tyres. The development of this technology relied on many engineering advances, such as vulcanizing natural rubber with sulfur and creating the "clincher" rim to hold tires securely on wheels.

In 1895, Michelin made the first pneumatic tires for cars, using them in a car called L’Eclair during the 1895 Paris-Bordeaux-Paris race.

Synthetic rubbers were invented in the 1920s by scientists at Bayer. During World War II, rubber shortages in the United Kingdom led to research into alternatives, such as leather, compressed asbestos, rayon, felt, bristles, and paper.

In 1946, Michelin developed the radial tire construction method. Michelin had purchased the bankrupt Citroën car company in 1934 to use this new technology. Because radial tires improved handling and saved fuel, they became popular in Europe and Asia. In the United States, older bias-ply tires remained in use until Ford Motor Company started using radial tires in the early 1970s after an article in the American magazine Consumer Reports in 1968 highlighted their advantages. The U.S. tire industry then lost business to Japanese and European companies, which eventually bought American tire companies.

Applications

Tires can be grouped based on the vehicles they are used for. They can be separated by the weight they carry and the type of vehicle, such as cars, planes, or bicycles.

Light-duty tires used in passenger vehicles can carry between 250 to 500 kilograms (550 to 1,100 pounds) on the drive wheel. Light-to-medium duty trucks and vans can carry between 500 to 1,500 kilograms (1,100 to 3,300 pounds) on the drive wheel. These tires are separated by speed ratings for different vehicles, including (from lowest to highest): winter tires, light truck tires, entry-level car tires, sedans and vans, sport sedans, and high-performance cars. Other types of tires include:

• Snow tires are made for use on snow and ice. They have larger gaps in their tread patterns, which help them grip snow and ice better. Tires that pass a specific winter traction test can show a "Three-Peak Mountain Snow Flake" symbol on their sides. These tires work best when temperatures are below 7°C (45°F). Some snow tires have metal or ceramic studs that stick out to increase traction on hard-packed snow or ice. These studs can wear down dry pavement, causing dust and tire wear. Rules about using snow tires or studs vary by country in Asia and Europe, and by state or province in North America.
• All-season tires are usually rated for mud and snow (M+S). They have smaller gaps than snow tires and larger gaps than regular tires. They are quieter on clear roads but less effective on snow or ice.
• All-terrain tires are made to have good traction off-road but are still safe and quiet for driving on highways. These tires perform better on snow and rain than regular street tires and are rated as "good" on ice, rock, and sand.
• Mud-terrain tires have deeper, more open treads than all-terrain tires for better grip in mud. However, they work less well on paved roads.
• High-performance tires can handle speeds up to 270 kilometers per hour (168 mph), and ultra-high-performance tires can reach 299 kilometers per hour (186 mph). However, these tires may provide a rougher ride and less durability.
• Electric vehicle tires have special needs because of the vehicle’s weight, higher torque, and the need for less rolling resistance.

Other types of light-duty car tires include:
– Run-flat tires allow a vehicle to continue driving at a reduced speed if a tire is punctured. These tires use a stiff sidewall to protect the wheel rim. Vehicles without run-flat tires rely on a spare tire, which may be a compact size.
– Race car tires are divided into three types: DOT (legal for street use), slick (no tread for maximum contact), and rain (with channels to remove water and avoid hydroplaning). These tires are made for high friction during turns and acceleration but wear out faster.

Heavy-duty tires used in large trucks and buses carry between 1,800 to 2,500 kilograms (4,000 to 5,500 pounds) on the drive wheel. These tires are often used in pairs on the drive axle.

• Truck tires come in different designs, such as "low profile" (70 to 45% of tread width), "wide-base" for heavy vehicles, and "super-single" tires that match the pressure of two tires combined.
• Off-road tires are used on construction, farming, and forestry equipment. They are also used on machinery that moves over hardened surfaces at industrial sites, ports, and airports. Tires for soft terrain have deep, wide treads to grip loose dirt, mud, sand, or gravel.

Tires for other vehicles have special needs:
– Aircraft tires are made for landing on paved surfaces and rely on landing gear to absorb impact. They are usually small compared to the aircraft they support. Most are radial-ply construction. They are designed to handle the heaviest load when the plane is stopped, though side forces during landing are important. Aircraft tires typically have radial grooves but no lateral grooves or sipes. Some small planes use large, low-pressure tundra tires for landing on unpaved surfaces.
– Bicycle tires can be used on roads or rough terrain and may be on vehicles with more than two wheels. The main types are clincher (most common, with a bead that fits the wheel rim), wired, and tubular. Most bicycle tires are clinchers. An inner tube provides air pressure and holds the tire to the rim. The ISO 5775 standard manages tire sizing.
– Industrial tires support vehicles like forklifts, tractors, and excavators. Tires on smooth surfaces have smooth treads, while those on soft surfaces have large treads. Some industrial tires are solid or filled with foam.
– Motorcycle tires provide traction, resist wear, and help the motorcycle turn. The contact of both tires with the ground affects safety, braking, fuel use, noise, and rider comfort.

Construction types

Tire construction includes pneumatic tires used on cars, trucks, and aircraft, as well as non-automotive uses such as slow-moving, light-duty, or railroad applications, which may use non-pneumatic tires.

After the 1968 Consumer Reports report highlighted the benefits of radial tire design, radial tires began to increase in popularity. By the 1980s, radial tires had taken over 100% of the North American market. Today, radial tire technology is the standard for most automotive tires, though other designs are still used.

Radial (or radial-ply) tires use body ply cords that run straight across the tread from one side to the other, positioned nearly perpendicular to the center of the tread and parallel to each other. Stabilizer belts are placed directly under the tread. The plies are usually made of nylon, polyester, or steel, and the belts are made of steel, fiberglass, or Kevlar. Radial tires have a wider footprint and flexible sides, which improve grip during turns. Circumferential belts help keep the tire stable. Compared to bias tires, radial tires offer longer tread life, better steering control, lower rolling resistance, improved fuel efficiency, more even wear, higher heat resistance, fewer blowouts, and a smoother ride at high speeds. However, they are more expensive and may provide a harsher ride at low speeds. Radial tires are rarely used in sizes larger than 42 inches, as such large tires are hard to produce.

Bias (bias-ply or cross-ply) tires use body ply cords that run diagonally from one side to the other, typically at angles between 30 and 40 degrees from the direction of travel. Layers of plies are placed in opposite directions, forming a crisscross pattern to which the tread is attached. This design makes the tire more resistant to damage and better suited for rough use. Because the tread and sides share the same plies, the tire moves as a single unit, which helps it grip uneven surfaces and move smoothly over rough ground. However, this design increases rolling resistance and reduces control, traction, and comfort at high speeds. The overlapping plies also create friction, which generates heat. Bias tires are often cheaper to make and are still used on heavy machinery and off-road vehicles, though many industries now use radial tires.

Belted bias tires start with two or more bias plies, with stabilizer belts added directly under the tread. This design improves ride comfort compared to regular bias tires and reduces rolling resistance because the belts make the tread stiffer. The concept was first introduced by Armstrong, and Goodyear popularized it with the "Polyglas" tire, which used a polyester structure and fiberglass belts. Belted bias tires begin with two main plies made of polyester, rayon, or nylon, treated similarly to regular tires. Circumferential belts at different angles are then added on top, improving performance over non-belted bias tires. These belts may be made of fiberglass or steel.

Tubeless tires are pneumatic tires that do not need a separate inner tube.

Semi-pneumatic tires have a hollow center but are not pressurized. They are lightweight, low-cost, puncture-proof, and provide cushioning. These tires often come as a complete unit with the wheel and may include built-in ball bearings. They are used on lawn mowers, wheelchairs, and wheelbarrows. They are also durable and used in industrial settings, designed to stay attached to the wheel during use.

Airless tires are non-pneumatic tires that do not rely on air pressure. They are often used on small vehicles like golf carts and utility vehicles in environments where punctures are a risk, such as construction sites. Many industrial and commercial tires are non-pneumatic and made from solid rubber or plastic through molding. Solid tires are used on lawnmowers, skateboards, golf carts, scooters, and light industrial vehicles, carts, and trailers. A common use for solid tires is on material handling equipment like forklifts, which are installed using a hydraulic tire press.

Wooden wheels on horse-drawn vehicles often have a wrought iron tire. This design was later used on wagons on horse-drawn tramways, which ran on granite or cast iron rails.

Some railway engines and older train models use railway tires to avoid replacing entire wheels. These steel tires surround the wheel and are held in place by an interference fit.

Aircraft tires can operate at pressures higher than 1,400 kilopascals (14 bar; 200 psi). Some aircraft tires are filled with nitrogen to prevent chemical reactions between oxygen in the air and gases inside the tire, which could cause a tire explosion.

Manufacturing

Pneumatic tires are made in about 450 factories worldwide. Tire production begins with raw materials like rubber, carbon black, and chemicals, which are used to create parts that are later assembled and cured. Many types of rubber are used, with the most common being a type called styrene-butadiene copolymer.

Forecasts show the global tire market will continue to grow through 2027. In 2022, worldwide tire sales were valued at about $126 billion. By 2027, sales are expected to reach over $176 billion. Tire production is also increasing. In 2015, the United States made nearly 170 million tires. About 2.5 billion tires are made each year, making the tire industry a major user of natural rubber. In 2019, it was estimated that at least 3 billion tires would be sold globally each year. However, other estimates say 2,268 million tires were made in 2021 and are expected to reach 2,665 million by 2027.

As of 2011, the top three tire companies by revenue were Bridgestone (making 190 million tires), Michelin (184 million), and Goodyear (181 million). These were followed by Continental and Pirelli. The Lego Group produced over 318 million toy tires in 2011 and was recognized by Guinness World Records for the highest annual tire production by any manufacturer.

A tire has several parts: the tread, bead, sidewall, shoulder, and ply.

The tread is the part of the tire that touches the road. The area of the tread that touches the road at any one time is called the contact patch. The tread is made of thick rubber or a rubber mixture designed to provide good traction without wearing too quickly.

The tread pattern includes grooves, sipes, and slots for road tires or lugs and voids for tires used on soft terrain or snow. Grooves run around the tire and help remove water. Lugs are the parts of the tread that touch the road. Grooves, sipes, and slots help tires drain water away.

Tread design and tire type affect road noise, which is a type of pollution from moving vehicles. Noise increases with higher speeds. Tread patterns may use different distances between slots (called pitch lengths) to reduce noise at certain frequencies. Sipes are small slits cut across the tire, usually perpendicular to grooves, which help water escape sideways and reduce hydroplaning.

Different tread designs work for different driving conditions. As the ratio of tread area to groove area increases, tire friction on dry pavement also increases, as seen on Formula One tires, which sometimes have no grooves. High-performance tires often have smaller void areas to increase rubber contact with the road for better traction, but they may use softer rubber that wears faster. Mud and snow tires have larger and deeper slots to grip mud and snow. Snow tires have even larger and deeper slots to compact snow and improve braking and cornering.

Wear bars are raised features in the bottom of tread grooves that show when a tire has reached its wear limit. When the tread lugs wear down to the point where wear bars connect across them, the tire is fully worn and should be replaced, usually when the tread depth is about 1.6 millimeters (0.063 inches).

The bead is the part of the tire that connects to the wheel’s rim. This part is made with strong steel cables covered in durable rubber to resist stretching. The bead’s fit is important because it seals the tire to the wheel, keeping air pressure intact and preventing air loss. The bead’s design ensures the tire stays securely attached to the wheel during motion. The bead’s size and the wheel’s width also affect how well the vehicle turns and stays stable.

The sidewall is the part of the tire between the tread and bead. It is mostly rubber but reinforced with fabric or steel cords to provide strength and flexibility. The sidewall holds air pressure and transfers the force from the drive axle to the tread for traction, but it supports little of the vehicle’s weight. If a tire is punctured, the sidewall collapses, showing how little weight it carries.

Sidewalls are molded with details like manufacturer information, required warnings, and other consumer data. They may also have decorative elements like white or red inserts and tire lettering.

The shoulder is the part of the tire where the tread meets the sidewall.

Plies are layers of stiff cords embedded in rubber to keep the tire’s shape by preventing it from stretching under internal pressure. The direction of the plies affects tire performance and is a key way tires are classified.

A "blem" is a term for a tire that failed inspection during manufacturing due to minor issues like smudged lettering. These tires are still functional and usually have the same warranty as perfect tires but are sold at a discount.

Modern tire materials are divided into two groups: cords that make up the plies and the elastomer that covers them.

Cords, which form the plies and bead and help contain air pressure, can be made of steel, natural fibers like cotton or silk, or synthetic fibers like nylon or Kevlar. Good adhesion between cords and rubber is important. Steel cords are coated with a thin layer of brass, and additives like resorcinol are added to rubber to improve bonding.

The elastomer, which forms the tread and protects the cords from wear, is a key part of tire design. It is often made of a rubber mixture called styrene-butadiene copolymer, combined with chemicals like silica and carbon black.

Reducing rolling resistance in the elastomer is important for lowering fuel use in vehicles. Passenger cars use about 5%–15% of their fuel to overcome rolling resistance, while trucks use even more. However, there is a balance between rolling resistance and wet traction. A low dissipation factor (measured as tan(δ)) reduces rolling resistance, while a high tan(δ) improves wet traction. Rolling resistance is affected by tan(δ) at low frequencies, while wet traction benefits from tan(δ) at higher frequencies. Measuring tan(δ) at high frequencies was historically difficult, so low-frequency measurements were often used instead.

On the wheel

Associated components of tires include the wheel on which it is mounted, the valve stem through which air is introduced, and, for some tires, an inner tube that provides the airtight means for maintaining tire pressure.

• Wheel: Pneumatic tires are attached to wheels that usually have rims built into their outer edges to hold the tire. Car wheels are often made from pressed and welded steel or lightweight metal alloys, such as aluminum or magnesium. There are two ways pneumatic tires support the rim of the wheel. First, the tension in the cords pulls the bead of the tire evenly around the wheel, except above the contact patch. Second, the bead transfers this force to the rim. Tires are placed on wheels by pressing their beads into the channel between the inner and outer rims.

• Valve stem: Pneumatic tires receive air through a valve stem—a tube made of metal or rubber with a check valve. Most cars and bicycles use a Schrader valve, while high-performance bicycles use a Presta valve. The valve stem is attached directly to the rim in tubeless tires or is part of the inner tube. Many modern cars now require a tire pressure monitoring system, which typically includes a valve stem connected to an electronic module.

• Inner tube: Many bicycle tires, motorcycle tires, and large vehicle tires, such as those on buses, trucks, and tractors, are designed to work with inner tubes. Inner tubes are ring-shaped balloons made from airtight materials, like soft, elastic synthetic rubber, to prevent air from escaping. Inner tubes are placed inside the tire and inflated to keep air pressure. Large inner tubes can be reused for activities like swimming, rafting, sledding, and skitching. Special inflatable rings are also made for these uses, offering different colors, fabric covers, handles, decks, and other features, and they do not have a valve stem sticking out.

Performance characteristics

The way a tire interacts with the road is complicated. One widely used model for tire behavior is called Pacejka's "Magic Formula." Below are explanations of key tire-related terms, listed alphabetically.

Balance: Wheel and tire combinations must have an even weight distribution around their edges to stay balanced when turning at high speeds. Tires are tested for balance during manufacturing and again after being mounted on wheels. If a tire is unbalanced, weights are added to the wheel to fix the issue. Another method uses internal balancing agents that use centrifugal force to counteract imbalance. High-performance tires often have red and yellow marks on their sides to help align tires and wheels correctly. These marks are used in two ways to ensure proper balance.

Centrifugal growth: When a tire spins at high speeds, its diameter may increase slightly due to centrifugal force pushing the tread away from the center. This can cause speedometer errors and may lead to tire rubbing against the vehicle. Motorcycle tires are often reinforced to reduce this effect.

Pneumatic trail: Pneumatic trail is the distance behind the tire’s contact point where a force acts when the tire is turned. This happens because the tire is flexible and experiences side forces during a turn.

Slip angle: Slip angle is the angle between the direction a tire is actually moving and the direction it is pointing. This occurs when the tire is turning and the motion of the tire is not perfectly aligned with its orientation.

Relaxation length: Relaxation length is the time it takes for a tire to adjust to a turn after a change in direction. It measures how quickly the tire responds to changes in movement.

Spring rate: Spring rate is how stiff a tire is when pressed down. It affects how the tire interacts with the vehicle’s suspension system. Higher inflation pressure generally increases the spring rate.

Stopping distance: Tires designed for performance often have patterns and materials that improve grip, leading to shorter stopping distances. However, specific tests are needed to determine exact stopping distances.

Camber thrust: Camber thrust is the force created by a tire’s tilt angle and the shape of its contact area with the road. This force helps the tire turn when it is not perfectly aligned.

Circle of forces: The circle of forces is a concept that helps explain how tires interact with the road during movement. It shows the limits of a tire’s ability to handle forces like turning and braking.

Contact patch: The contact patch is the area of the tire’s tread that touches the road. This area transfers forces between the tire and the road through friction. The shape of the contact patch influences how well the tire steers and turns.

Cornering force: Cornering force is the sideways force a tire produces when a vehicle turns. This force helps the vehicle stay on the road during a turn.

Dry traction: Dry traction measures how well a tire grips the road when it is dry. This depends on the stickiness of the tire’s rubber material.

Force variation: As a tire rolls, its tread and sides change shape. These changes create forces that are measured as tire uniformity. If these forces are too high, tires are rejected during manufacturing.

Rolling resistance: Rolling resistance is the effort needed to keep a tire moving. It depends on factors like inflation pressure and the tire’s structure. Lower rolling resistance improves fuel efficiency, especially in vehicles like trucks.

Self-aligning torque: Self-aligning torque is the force that helps a tire turn by rotating it around its vertical axis.

Wet traction: Wet traction is how well a tire grips the road when it is wet. Tires with designs that channel water away reduce hydroplaning. Some tires, like those on racing bikes, are less likely to hydroplane due to their small contact area.

Load sensitivity: Load sensitivity describes how tires behave under weight. Unlike simple friction models, real tires become less grippy as the weight on them increases.

Work load: Work load measures the stress a tire experiences. It is calculated in Ton Kilometer Per Hour (TKPH) and is important for heavy vehicles. Tires used in mining and construction are rated for TKPH based on their size and materials.

Tire wear is a major source of rubber pollution. Unlike exhaust emissions, tire wear pollution is not regulated.

Sizes, codes, standards, and regulatory agencies

Automotive tires have numbers and letters printed on the side, called a tire code. These markings show the tire's size, performance ratings, and other important details.

The National Highway and Traffic Safety Administration (NHTSA) is a U.S. government group under the Department of Transportation (DOT). It works to improve car safety in the United States. NHTSA created the Uniform Tire Quality Grading System (UTQG), which helps compare tire performance based on rules in the Code of Federal Regulations 49 CFR 575.104. This system requires tires to be labeled with information about tread wear, traction, and temperature. The DOT Code is a mix of numbers and letters printed on the tire's side. It helps identify the tire and its age. The U.S. Department of Transportation requires this code, but it is used worldwide. The code is also useful for finding tires that are part of a recall or have reached the end of their useful life due to age. The Tire and Rim Association (T&RA) is a U.S. group that sets standards for tires, rims, and related parts. It publishes important details like tire sizes, rim shapes, valve standards, and rules for how much weight tires can hold and how much air they need.

The National Institute of Metrology Standardization and Industrial Quality (INMETRO) is Brazil's government agency responsible for certifying car wheels and tires.

The European Tyre and Rim Technical Organisation (ETRTO) is a European group that sets standards for tire and rim sizes, weight and pressure limits, and how tires should be used. All tires sold in Europe after July 1997 must have an E-mark. This mark is either an uppercase "E" or lowercase "e," followed by a number in a circle or rectangle, and then another number. An uppercase "E" means the tire meets European safety rules called ECE regulation 30. A lowercase "e" means the tire meets older European rules called Directive 92/23/EEC. The number in the circle or rectangle shows the country that approved the tire. The final number outside the circle or rectangle is the approval number for that tire's size and type.

The British Rubber Manufacturers Association (BRMA) recommends that tires older than six years should not be used if they are unused. All tires should be replaced ten years after they were made.

The Japanese Automobile Tire Manufacturers Association (JATMA) is Japan's group that sets standards for tires, rims, and valves. It does similar work as the T&RA and ETRTO.

The China Compulsory Certification (CCC) is a required safety check system in China that started in August 2002. This system is managed by the State General Administration for Quality Supervision and Inspection and Quarantine of the People's Republic of China (AQSIQ) and the Certification and Accreditation Administration of the People's Republic of China (CNCA).

Maintenance

To keep tires in good condition, several steps are important: tire rotation, wheel alignment, and, in some cases, retreading tires.

Rotation: Tires can wear unevenly after being on a vehicle for a while. Front-wheel drive vehicles often wear front tires faster than rear tires. Tire rotation involves moving tires to different positions on the car, such as from front to rear, to spread out the wear. This helps tires last longer.

Alignment: Wheel alignment ensures tires move straight, preventing uneven wear. If tires are not properly aligned with the direction of travel, they may wear unevenly. Large alignment issues can cause significant wear if not fixed. Wheel alignment adjusts angles like camber, caster, and toe to meet manufacturer guidelines.

Inflation: Proper air pressure is essential for even tire wear and rolling efficiency. Many vehicles have systems to check air pressure. Passenger cars are usually advised to keep tire pressure between 220 to 240 kilopascals (32 to 35 psi) when tires are not heated by driving.

Specifications: Vehicle makers provide tire pressure guidelines, including cold inflation pressure, to ensure safe use within the tire’s load limits and the vehicle’s capacity. Tires often have a maximum pressure rating, but passenger cars and light trucks usually list recommended pressure on a sticker inside the driver’s door and in the owner’s manual.

Ground Contact: Air pressure affects how tires touch the road. Too much pressure can increase wear on the center of the tire. Too little pressure can cause the tire to flatten, reducing the center contact area, even though the overall contact area becomes larger. Modern tires may wear evenly at high pressures but can wear quickly if underinflated. High pressure may reduce rolling resistance and improve stopping distances, while low pressure increases rolling resistance, tire flexing, and friction. Underinflation can lead to overheating, early tread wear, and, in severe cases, tread separation.

Monitoring: Tire pressure monitoring systems (TPMS) are electronic tools that check air pressure on each wheel and warn drivers if pressure drops below a set level. These systems can measure air pressure directly or use indirect methods, such as detecting changes in tire size caused by low pressure.

Hazards

Tire problems can happen because of tire failure or because the tire loses grip on the road. If a tire breaks, it can cause flat tires or even dangerous blowouts. Some of these issues may be due to mistakes made during manufacturing, which can lead to recalls. For example, many Firestone tires on Ford cars had problems in the 1990s, causing a major issue between Firestone and Ford.

Tires can fail for many reasons, including:

• Belt separation: This happens when the layers inside the tire come apart. This can occur if the tire bends too much, due to high road temperatures, damage from road hazards, or improper care and storage.

• Non-belt separations: These can happen in the tire’s tread, the bead area, the lower part of the tire’s side, between supporting layers, or in the materials that strengthen the tire.

• Other failures include damage from driving without a flat tire, chemical breakdown, cracks, dents, or bulges.

• Melting rubber: When tires heat up from stopping, turning, or speeding up, the rubber can melt. This can make the road surface slippery and leave rubber deposits on the pavement. This effect is more likely in hot weather.

• Hydroplaning: When vehicles or planes move over wet roads at high speeds, tires can lose contact with the road. This happens when water is deep enough (at least 3 millimeters) and the tire moves too fast. Hydroplaning can also occur if the rubber melts briefly, causing slippage. This can leave rubber deposits on runways after planes land. Hydroplaning reduces friction and contact between the tire and the road.

• Snow: Tires gain traction in snow by pressing snow into the tread, which helps grip the road. The snow inside the treads is pushed out so the tread can press new snow on the next rotation. This process helps with moving forward, stopping, and turning.

• Ice: Ice is near its melting point when tires roll over it. This, along with its smooth surface, makes it hard for tires to grip the road during braking, turning, or speeding up.

• Soft ground: Wet soil can become slippery, making it hard for tires to grip the ground during acceleration, braking, or turning. Dry sand also has poor grip because sand particles do not stick together well.

Health impacts

Tires contain small amounts of harmful chemicals, including heavy metals and other substances that help make tires stronger. These substances often include polycyclic aromatic hydrocarbons, benzothiazoles, isoprene, and heavy metals like zinc and lead.

As tires are used in vehicles, the natural wear of the tires creates tiny particles similar to PM0.1, PM2.5, and PM10, which become tire residue. This residue builds up near roads and places where vehicles are used but can also move into the environment through rainwater runoff.

Humans and animals are exposed to these chemicals at places where they build up, such as by walking on road surfaces, and through bioaccumulation in natural environments and food chains. A 2023 study from Imperial College London warned that the toxic chemicals and microplastics from tire wear could cause serious problems for the environment and human health.

Burning tires releases these chemicals into the air, which can harm emergency workers. It also leaves behind harmful materials that can harm nearby communities.

End of use

After tires are no longer used, they become scrap tires. These tires are often reused for purposes like creating barriers for bumper cars, as weights to hold tarps down, or in other practical ways. Landfills do not prefer scrap tires because they take up a lot of space and have 75% empty space inside, which quickly fills up valuable landfill areas. Rubber tires may contain small amounts of heavy metals or other harmful substances, but these are tightly locked inside the rubber material. They are not likely to cause harm unless the tire is seriously damaged by fire or strong chemicals. Some facilities recycle scrap tires by breaking them into pieces and turning them into new products or selling the material to power plants for fuel. Some tires are also retreaded, meaning the worn parts are replaced to make them usable again.

In the United States, about 285 million scrap tires are created each year. Many states control how many scrap tires can be stored at one location to prevent problems like illegal dumping, fire risks, and mosquito breeding. In the past, millions of tires were left in open fields, which created problems. Tires left outdoors can collect water inside and become breeding grounds for mosquitoes, which can spread diseases. They also create fire risks. Tires rarely catch fire on their own, but once they start burning, they are very hard to stop. A large pile of tires has a lot of fuel, and water used to put out fires may not reach deep enough or cool them down. Some tire fires have lasted for months, releasing harmful chemicals into the ground, groundwater, and the air as black smoke.

Using scrap tire pieces in landscaping has raised concerns because metals and other harmful substances can seep into the soil and water. Zinc, which is found in high amounts in tires, can be toxic to plants and aquatic life. Some substances that leach from tires into water may harm fish and cause health issues in animals. Tires are a major source of microplastic pollution. A 2020 study found that tires contribute 78% of the total weight of microplastics found in the ocean. A chemical called 6PPD-quinone, which comes from tire wear and enters water through runoff, has been found to be harmful to certain types of fish.

Tires that are worn down can be retreaded, meaning the worn parts are replaced to make them usable again. This process involves removing the old tread and adding a new one. Two main methods are used: one uses a mold to shape the new tread, and the other uses a pre-made tread piece. Before retreading, tires are inspected for damage. If a tire is in good condition, it can be retreaded multiple times. Tires used for short trips are often retreaded more than those used for long-distance travel.

During retreading, workers check the tire to ensure it is in the best condition possible to avoid failures. Tires with serious damage, such as cracks or broken parts, are not used. In the mold-cure method, new rubber is applied to the tire and shaped using molds. In the pre-cure method, a ready-made tread is placed on the tire and then heated to bond it.

Scrap tires can be recycled into materials like hot melt asphalt, which is used in roads, or as part of concrete for buildings. Shredded tires can be used as rubber mulch on playgrounds to reduce injuries from falls. Some buildings are made using old tires.

A recycling method called pyrolysis heats tires in a special container without oxygen, breaking them down into smaller molecules. Other uses for old tires include:

• Building materials: Tires filled with soil can be used as garden containers, parts of house foundations, or to protect against soil erosion in flood areas. Tires are also used as safety barriers in race tracks.
• Recreational equipment: Tires are used for exercise, such as in drills where athletes run through a zigzag pattern of tires to improve speed and agility. Some tires are turned into swings for children.
• Protests: People sometimes burn tires to create smoke as a form of protest.
• Harmful use: In some cases, tires are used in violent acts, such as being filled with gasoline and placed around a person's neck before being set on fire.