Alignment The checking and adjustment of caster, camber and toe angles in a vehicle's suspension to maintain specifications engineered by the vehicle manufacturer for optimum performance. Aspect ratio The relationship between the section height and section width of a tire expressed as a percentage of section width. If the section height is one half the section width, the aspect ratio is 50%. Balance The equal distribution of the mass of the tire and wheel assembly for smooth driving. Balance is achieved by fitting weights to the wheel rim to offset uneven weight distribution of the tire or wheel. Bead An inextensible hoop of high tensile steel wires which anchors the plies and conforms to the rim seat to hold the tire onto the wheel rim. Bead seat The inner ledge portion of the wheel rim where the tire bead rests adjacent to the flange. Belts The plies of tire cords beneath the tread that determines the tire's diameter and stabilizes the tread by resisting deformation from cornering, braking, and centrifugal forces. Bias-ply A type of tire construction utilizing plies that run diagonally from one bead to the other. One ply is set on a bias in one direction, and succeeding plies are set alternately in opposing directions crossing each other. Sometimes called a cross-ply tire. Camber The angle between the centerline of the tire and a vertical line as viewed from the front. Camber thrust A cornering force generated by the tire's camber. Casing The tire body, composed of plies which form the tire's structure and give it shape. Sometimes called the carcass. Caster The angle between the vehicle's steering axis and a vertical line, as viewed from the side. Compounding The combining of five basic ingredients: rubber, carbon black, plasticizers, curing materials, and ozone retardants to form the tread and other "rubber" components of a tire. Contact patch (See Footprint) Cornering force The lateral frictional force generated by a cornering tire, acting in opposition to the centrifugal force. Crown The center area of a tire's tread. Deflection The deference between a tire's unloaded or free radius and the loaded radius. Directional stability The tendency for a tire to roll in it's steered direction rather than follow road contours. Footprint The area of the tire's tread that is in actual contact with the ground. (See Contact Patch) Harmonic marking Markings on wheels and tires that allow match mounting to cancel tire and wheel runout, minimizing vibration. Hoop strength The retention strength inherent in the belt construction of a tire that resists centrifugal force and provides dimensional stability. Hydro planing The accumulation of water in a film under the footprint which causes a tire to lift from the road surface, losing traction. Hydroplaning is affected by vehicle speed, tread pattern, and water depth. Imbalance The condition that exists when a tire's mass is not evenly distributed around the rolling axis and centerline, causing bounce (static imbalance) or shake (dynamic imbalance). Inflation pressure The pressure of air inside a tire which applies a tensile stress to the tire cords permitting them to carry the vehicle's load. Liner or inner-liner The thin layer of halobutyl rubber inside a tire that contains the inflation air, sometimes called the inner-liner. All Dunlop passenger tires are manufactured with an inner-liner. Mixing tires Fitting tires of different sizes or constructions to a vehicle. Mixing should be avoided. Some performance vehicles, however, specify different size tires on front and rear axles. Mounting tires The act of fitting tires to wheel rims. Overall diameter The diameter of an unloaded, inflated tire measured from the crown on one side to the crown on the opposite side. The free radius equals one-half the overall diameter. Sometimes called the outside diameter. Over inflation The condition that exists when a tire is inflated beyond the pressure corresponding to the actual load or beyond the vehicle manufacturer's recommendation . Oversteer The situation that occurs in cornering when the rear of a vehicle tends to skid before the front. Plies The reinforcing members of a tire composed of layers of cord fabric and rubber that provide the strength to contain the air pressure needed to support a load and resist deflection. Radial The tire construction utilizing plies that run radially from bead to bead under the tread. This construction requires a belt to stabilize the tread and define the tire diameter. Revolutions per mile The measured number of revolutions made by a tire traveling one mile. Rim The portion of a wheel incorporating the well, seats, and flange onto which a tire is mounted. Rim diameter The diameter of the rim bead seats that support a tire, normally indicated in whole numbers in inches for passenger cars. Rollover The condition that occurs during hard cornering when a tire sidewall rubs the road surface. Rotation The systematic movement of tires from one vehicle position to another to maximize tread life and minimize irregular wear. Runout The measure of the out of roundness of the tire causing a vibration which cannot be balanced. Rim width The distance between rim flanges. Section A slice of a tire from one bead, through the tread to the other bead. Section height The vertical distance from the bead edge to center of the crown in an unloaded tire. Section width The distance between a tire's sidewalls measured at the widest part of the tire. Each size of tire is measured on a specific rim width. Series A designation of a tire's aspect ratio. A tire with an aspect ratio of 60% is a 60 series tire. Shoulder The edge of a tire's tread where it joins the sidewall. Sidewall The portion of the tire between the bead and the tread. It is flexible to soak up bumps yet stiff to limit tire rollover. Slip angle The angle between the direction in which a tire is aimed or steered and the actual direction of tire travel. Speed rating A letter designation identifying the tire's high speed durability on an indoor test wheel. Refer to ECE 30 European Indoor Wheel Test Standards. Toe The difference between the front and rear edges of tires mounted on an axle. Toe-in means the front edges are closer together than the rear edges and tires point inward. Toe-out means the front edges are farther apart than the rear edges and the tires point outward. Tread The region of a tire designed to contact the ground. It is molded of tough rubber for high traction and low wear. Tread pattern The arrangement of blocks, grooves, sipes, and channels designed into the tread to enhance its grip. Also called the tread design. Tread void Areas in the tread, such as grooves and channels, that permit water to drain away from the footprint. Tread wear The measure of the life of a tire tread. Tubeless A tire construction which uses a rubber inner liner inside the casing to prevent air leakage and eliminate the need for an inner-tube. Under inflation The condition that exists when there is not sufficient air pressure in a tire to support a specific load. This causes the tire to operate with excessive deflection and rollover. Understeer The condition that exists during cornering when the front of a vehicle tends to skid before the rear. Wheel alignment See Alignment.