1. TYRE A tire (American English) or tyre (British English; see spelling differences) is a ring-shaped component that surrounds a wheel's rim to transfer a vehicle's load from the axle through the wheel to the ground and to provide traction on the surface traveled over. Most tires, such as those for automobiles and bicycles, are pneumatically inflated structures, which also provide a flexible cushion that absorbs shock as the tire rolls over rough features on the surface. Tires provide a footprint that is designed to match the weight of the vehicle with the bearing strength of the surface that it rolls over by providing a bearing pressure that will not deform the surface excessively. The materials of modern pneumatic tires are synthetic rubber, natural rubber, fabric and wire, along with carbon black and other chemical compounds. They consist of a tread and a body. The tread provides traction while the body provides containment for a quantity of compressed air. Before rubber was developed, the first versions of tires were simply bands of metal fitted around wooden wheels to prevent wear and tear. Early rubber tires were solid (not pneumatic). Pneumatic tires are used on many types of vehicles, including cars, bicycles, motorcycles, buses, trucks, heavy equipment, and aircraft. Metal tires are still used on locomotives and railcars, and solid rubber (or other polymer) tires are still used in various non-automotive applications, such as some casters, carts, lawnmowers, and wheelbarrows. Wheel support There are 2 aspects to how pneumatic tires support the rim of the wheel on which they are mounted.[16] First, tension in the cords pull on the bead uniformly around the wheel, except where it is reduced above the contact patch.[17] Second, the bead transfers that net force to the rim.[18] Air pressure, via the ply cords, exerts tensile force on the entire bead surrounding the wheel rim on which the tire is mounted, pulling outward in a 360 degree pattern. Thus the bead must have high tensile strength. With no force applied to the outer tread, the bead is pulled equally in all directions, thus no additional net force is applied to the tire bead and wheel rim. However, when the tread is pushed inward on one side, this releases some tension in the corresponding sidewall ply pulling on the bead. Yet the sidewall ply on the other side continues to pull the bead in the opposite direction. Thus the still fully tensioned sidewall ply pulls the tire bead and wheel rim in the same direction as the tread displacement with equal force as that applied to push the tread inward.[17] This sidewall ply to bead tension support was a big reason for older cross-ply cord tire construction using the materials available in the early 19th century. The cross-ply cord arrangement orients the cords to more directly support the bead & wheel rim (like a sling: bead to cord and around below the tread back to the opposite bead, both ways, thus crossing plies of cords). However, with improved combinations of cord, bead, rim materials, and manufacturing techniques, combined with ongoing focus and research on tire efficiency and durability, it became both feasible and desirable to manufacture radial-ply cord tires, which, for many applications (despite higher costs), outperform and more than outlast (reliable-usual-service-life/cost ratio) similar cross-ply cord tire designs by (a) facilitating a flatter contact pattern with more evenly distributed pressure on the momentarily stationary area of contact between tread & ground and (b) lower operating costs over time: due to reduced tire temperature, decreased rolling resistance, lower puncture rates, greater longevity, etc Components[edit] Tire cross section showing components A tire carcass is composed of several parts: the tread, bead, sidewall, shoulder, and ply. Tread Two mountain bicycle tires with different tread patterns The tread is the part of the tire that comes in contact with the road surface. The portion that is in contact with the road at a given instant in time is the contact patch. The tread is a thick rubber, or rubber/composite compound formulated to provide an appropriate level of traction that does not wear away too quickly. The tread pattern is characterized by the geometrical shape of the grooves, lugs, voids and sipes. Grooves run circumferentially around the tire, and are needed to channel away water. Lugs are that portion of the tread design that contacts the road surface. Voids are spaces between lugs that allow the lugs to flex and evacuate water. Tread patterns feature non-symmetrical (or non-uniform) lug sizes circumferentially to minimize noise levels at discrete frequencies. Sipes are slits cut across the tire, usually perpendicular to the grooves, which allow the water from the grooves to escape to the sides in an effort to prevent hydroplaning. Treads are often designed to meet specific product marketing positions. High-performance tires have small void ratios to provide more rubber in contact with the road for higher traction, but may be compounded with softer rubber that provides better traction, but wears quickly. Mud and snow (M&S) tires are designed with higher void ratios to channel away rain and mud, while providing better gripping performance. Tread lug Tread lugs provide the contact surface necessary to provide traction. As the tread lug enters the road contact area, or footprint, it is compressed. As it rotates through the footprint it is deformed circumferentially. As it exits the footprint, it recovers to its original shape. During the deformation and recovery cycle the tire exerts variable forces into the vehicle.[citation needed] These forces are described as Force Variation.[41][citation needed] Tread void Tread voids provide space for the lug to flex and deform as it enters and exits the footprint. Voids also provide channels for rainwater, mud, and snow to be channeled away from the footprint. The void ratio is the void area of the tire divided by the entire tread area. Low void areas have high contact area and therefore higher traction on clean, dry pavement. Rain groove The rain groove is a design element of the tread pattern specifically arranged to channel water away from the footprint. Rain grooves are circumferential in most truck tires. Many high-performance passenger tires feature rain grooves that are angled from the center toward the sides of the tire. Some tire manufacturers claim that their tread pattern is designed to actively pump water out from under the tire by the action of the tread flexing. This results in a smoother ride in different types of weather.[42] Sipes Tread lugs often feature slits—sipes—that improve the flexibility of the lug to deform as it traverses the footprint area. This reduces shear stress in the lug and reduces heat build up. Testing of identical siped and unsiped tires showed measurable improvements in snow traction and ice braking performance, however diminishing and extending braking distances on wet and dry pavement by a few feet on siped tires. Off-road tire enthusiasts have been siping tires for years for greater traction, as many manufacturers now offer already siped off-road-tires. Wear bar Wear bars (or wear indicators) are raised features located at the bottom of the tread grooves that indicate the tire has reached its wear limit. When the tread lugs are worn to the point that the wear bars connect across the lugs, the tires are fully worn and should be taken out of service. Most wear bars indicate a remaining tread depth of 1.6 millimetres (0.063 in) and are tires are deemed "worn out" when the tread is worn down to that point.[43] Bead The tire bead is the part of the tire that contacts the rim on the wheel. The bead is typically reinforced with steel wire and compounded of high strength, low flexibility rubber. The bead seats tightly against the two rims on the wheel to ensure that a tubeless tire holds air without leakage. The bead fit is tight to ensure the tire does not shift circumferentially as the wheel rotates. The width of the rim in relationship to the tire is a factor in the handling characteristics of an automobile, because the rim supports the tire's profile. Sidewall[edit] Uneven sidewall wear, down to fabric plies, due to significant under-inflation The sidewall is that part of the tire that bridges between the tread and bead. The sidewall is largely rubber but reinforced with fabric or steel cords that provide for tensile strength and flexibility. The sidewall contains air pressure and transmits the torque applied by the drive axle to the tread to create traction but supports little of the weight of the vehicle, as is clear from the total collapse of the tire when punctured. Sidewalls are molded with manufacturer-specific detail, government mandated warning labels, and other consumer information, and sometimes decorative ornamentation, like whitewalls or tire lettering. Shoulder The shoulder is that part of the tire at the edge of the tread as it makes transition to the sidewall. Ply Plies are layers of relatively inextensible cords embedded in the rubber[44] to hold its shape by preventing the rubber from stretching in response to the internal pressure. The orientations of the plies play a large role in the performance of the tire and is one of the main ways that tires are categorized. Wheel Tires are mounted onto wheels that most often have integral rims on their outer edges to hold the tire. Automotive wheels are typically made from pressed and welded steel, or a composite of lightweight metal alloys, such as aluminum or magnesium.