A tire or tyre (see spelling differences and etymological origins) is a device covering the circumference of a wheel. It is an essential part of most ground vehicles and is used to dampen the oscillations caused by irregularities in the road surface, to protect the wheel from wear and tear as well as to provide a high-friction bond between the vehicle and the ground to improve acceleration and handling. Today most tires, especially those fitted to motor vehicles, are manufactured from synthetic rubber, but other materials such as steel may be used.
The most important feature of tire design is the ply, i.e., the way in which the layers of reinforcing cords in the inner carcass are laid, or arranged. The three main types of arrangements are the bias-ply, the bias-ply belted, and the radial-ply belted. As shown in the , the cords in a bias-ply tire are laid at a “crown” angle of about 50 degrees to the axis of the tire tube, and the cords in successive plies (two or four) cross one another—an arrangement that serves to equalize cord tensions. In a bias-ply belted tire, another set of cords overlies the bias-laid ones. This extra set of cords, called a belt, is typically made of fibreglass. A radial-ply belted tire also has a belt running around the entire tire, but the cords are typically made of steel wire-mesh, hence the term “steel-belted radial” tire.
The pneumatic tire is designed to provide a flexible cover with an impermeable lining to contain and restrain the compressed air. This cover is provided with a rubber tread portion that is designed to withstand the cutting and abrasive wear of road contact and to protect the tire against puncture and loss of air. Such a structure has, as distinct from a solid rubber or cushion tire, no capacity in itself either to carry load or absorb shocks. It is entirely dependent on the contained compressed air to enable it to function.
The first patent for a pneumatic tire was issued to Robert William Thomson in England in 1845 for a hollow leather tire filled with air. Although a set of Thomson’s “Aerial Wheels” ran for 1,200 miles on an English brougham, the same inventor’s solid-rubber tires were more popular; and thus, for almost half a century, air-filled tires were forgotten. The growing popularity of the bicycle in the late 19th century revived interest in tire design, and in 1888 John Boyd Dunlop, a veterinary surgeon of Belfast, obtained patents on a pneumatic tire for bicycles. Pneumatic tires were first applied to motor vehicles by the French rubber manufacturer Michelin & Cie. For more than 60 years, pneumatic tires had inner tubes to contain the compressed air and outer casings to protect the inner tubes and provide traction. In the 1950s, however, tubeless tires reinforced by alternating plies, or layers, of cord became standard equipment on new automobiles. In that decade Michelin introduced the radial-ply tire, which is now standard for all automobiles in developed countries.
Pneumatic tires are usually retained on the wheel through the use of beads (hoops of wire) that are molded into the tire edges. The tire edges are placed in a shaped channel on the wheel rim’s circumference and are held firmly against the channel’s edges by the pressure exerted once the tire is inflated with air. Pressures range from about 30 pounds per square inch (200 kilopascals) for large, soft tires to approximately 100 pounds per square inch (700 kilopascals) for high-pressure, high-load applications. The pressure is carried by layers of cords embedded in a rubber cover that in turn serves to contain the air, protect the cords, and give high friction with the ground. This felicitous combination is the reason why pneumatic tires are so widely used for transportation. More than 200 million of them are manufactured for motor vehicles alone each year, 90 percent for automobiles and 10 percent for trucks.
Pneumatic tires are designed to meet five main goals: low rolling resistance, low vertical stiffness (to cushion the ride), high sliding friction in both wet and dry conditions, high longitudinal and lateral stiffness (to minimize sliding motions in the “contact patch” where the tread meets the road), and resistance to wear and damage such as cutting, puncturing, and abrasion. In order to achieve these goals, tire designers must choose appropriate combinations of materials and structures, such as those described below.
A pneumatic tire is reinforced by layers of relatively inextensible cords that hold the air pressure and restrict deformation and growth of the tire during use. To this end cord materials must have high stiffness, resistance to repeated flexing, high strength-to-weight ratio, and good adhesion to rubber. Tire cords have been made of cotton, rayon, nylon, polyester, and glass, but steel and polyaramid (an extremely hard and stiff synthetic fibre) are currently the dominant materials in use.
Various rubber compounds are used in different parts of the tire. The liner, which is intended to minimize the loss of air, is usually made of butyl rubber because that material has a low permeability to gas. Sidewalls, on the other hand, must resist scraping, flexing, and attack by ozone in the air. A typical formulation for sidewalls (measured in parts by weight of each ingredient) would be 50 parts natural rubber (for resistance to heat buildup), 50 parts butadiene rubber (for abrasion resistance), and 50 parts carbon black (for reinforcement), along with small amounts of processing oil, antioxidant, and protective wax. A tire’s treads must be especially resistant to abrasion. A tread compound might have no natural rubber at all but rather 65 parts styrene-butadiene rubber (for hardness and abrasion resistance), 35 parts butadiene rubber, and as much as 65 parts carbon black.
The addition of carbon black or silica as reinforcing agents in rubber compounds has greatly prolonged tire life. Also contributing to longevity are the abrasion-resistant polymer polybutadiene and improved vulcanization systems. (Vulcanization is the interlinking of rubber molecules in order to create a usable elastic solid. It is conducted by heating the rubber mix along with sulfur and various additives in a tire mold for about 20 minutes at a temperature of approximately 300° F, or 150° C.) These advances, together with the almost universal adoption of radial-ply construction (described below), have extended the life expectancy of some tires to more than 90,000 miles (150,000 km). Still, it must be emphasized that the formulation of rubber mixes for tires is more an art than a science. For instance, why a tread recipe based on butadiene rubber will have greater abrasion resistance than one based on natural rubber or styrene-butadiene rubber is not known.
Automobile tires have numerous rating systems. See tire code. New automotive tires now also have ratings for traction, treadwear, and temperature resistance (collectively known as UTQG ratings); as well as speed and load ratings.
Some tread designs are unidirectional and the tire has a rotation direction indicated by an arrow showing which way the tire should rotate when the vehicle is moving forwards. It is important not to put a 'clockwise' tire on the left hand side of the car or a 'counter-clockwise' tire on the right side. Tire rotation moves tires between the different wheels of the vehicle as front and back axles carry different loads and thus the tires wear differently. Tire tread gauges are small rulers designed to be inserted into tire treads to measure the remaining tread depth. Local legislation may specify minimum tread depths, typically between 1/8" (3.2 mm) and 1/32" (0.8 mm). Wearbars may be designed into the tire tread to indicate when it is time to replace the tire. Essentially, part of the tire tread is shallower than the rest and will show when the tire is worn down to that level.
There is currently an attempt to reinforce the tire with nanomaterial. This is likely to increase the tire life, but may turn out to be a bad idea if the worn out part of nanocarbon deposited on the roads is washed off and ends up in the food chain.
Winter tires are designed to provide improved performance under winter conditions compared to tires made for use in summer. The rubber compound used in the tread of the tire is usually softer than that used in tires for summer conditions, thus providing better grip on ice and snow, but wearing more quickly at higher temperatures. Winter tires often have fine grooves and siping in the tread patterns that are designed to grip any unevenness on ice. Winter tires are usually removed for storage in the spring, because the rubber compound becomes too soft in warm weather resulting in a reduced tire life.
Mud terrain tires are characterized by large, chunky tread patterns designed to bite into muddy surfaces. The large open design also allows mud to clear quickly from between the lugs. Mud terrain tires also tend to be wider than other tires, to spread the weight of the vehicle over a greater contact patch to prevent the vehicle from sinking too deeply into the mud. Depending on the composition and tread pattern, many mud terrain tires are not well suited to on-road use. They can be noisy at highway speeds, and due to the open tread design they have less contact area with the road which limits traction. The large lugs on mud tires tend to tear and chip on roads since they are made from hard rubber compounds that do not bend easily.
All-terrain tires are typically used on SUVs and light trucks. These tires often have stiffer sidewalls for greater resistance against puncture when traveling off-road, the tread pattern offers wider spacing than all-season tires to remove mud from the tread.Within the all-terrain category, many of the tires available are designed primarily for on-road use, particularly all-terrain tires that are originally sold with the vehicle.
These are an attempt to make a tire that is a compromise between one developed for use on dry and wet roads during summer and one developed for use under winter conditions. The type of rubber and the tread pattern best suited for use under summer conditions cannot, for technical reasons, give good performance on snow and ice. The all-season tire is a compromise, and is neither an excellent summer tire nor an excellent winter tire. They have, however, become almost ubiquitous as original and replacement equipment on automobiles marketed in the United States, due to their convenience and their adequate performance in most situations.
Proper inflation is the single most important factor in tire care. The inflation pressure imprinted on the sidewall of the tire is the maximum operating pressure determined by the tire manufacturer. It is not necessarily the correct tire inflation for your vehicle.
Regular rotation extends the life of the tires, saving you time and money in the long run. Each tire and wheel is removed from your vehicle and moved to a different position. This ensures that all of the tires wear evenly and last longer. If no period is specified in your owner's manual, the tires should be rotated every 6,000-12,000 kilometers.
Balancing means compensating for both the weight of the tire and wheel after the tire is mounted. A wheel is out of balance when one area is heavier or lighter than the rest. This can cause irregular treadwear and vibration, and increase the stress on the front-end parts, which may cause them to wear prematurely.
All tires have treadwear indicator bars at 2/32" of remaining tread. When the tread is worn down to 2/32" or where you can see the treadwear indicator bars on any section of you tire, the tire is worn out and should be replaced. Take your vehicle to your local Uniroyal tire retailer for an inspection and have them measure the remaining tread with a tread depth gauge.
A vibration while driving indicates a problem that needs attention. The tires, steering system and suspension system should be checked immediately to help determine the possible cause of the vibration. If the vibration is not corrected, it could cause excessive tire and suspension wear - it could even be dangerous.