Glossary of Tire Terms

ALIGNMENT
The checking & adjustment of caster camber & 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 & 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 & 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 & 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 b& of multiple tire cords compressed of usually rubberized steel wires beneath the tread & laid at opposing angles that determine the tire's diameter & stabilizes the tread by resisting deformation from cornering, braking & centrifugal forces. These do not tie into the tire beads.

BIAS-PLY
A type of tire construction utilizing plies (usually rubberized fabric cords) that run diagonally from one bead to the other. One ply is set on a bias in one direction, & 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 & 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 & give it shape. Sometimes called the carcass.

CASTER
The angle between the vehicle's steering axis & a vertical line, as viewed from the side.

COMPOUNDING
The combining of five basic ingredients: rubber, carbon black, plasticizers, curing materials & ozone retardants to form the tread & other "rubber" components of a tire.

CONTACT PATCH
The area of the tire's tread that is in actual contact with the ground. (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 difference between a tire's unloaded or free radius & the loaded radius.

DIRECTIONAL STABILITY
The tendency for a tire to roll in its steered direction rather than follow road contours.

DSST
Dunlop Self-Supporting Technology. An advanced tire design that permits limited driving after loss of inflation pressure. Requires a pressure monitoring system.

FOOTPRINT
The area of the tire's tread that is in actual contact with the ground. (See Contact Patch.)
Harmonic markings on wheels & tires that allow match mounting to cancel tire & wheel runout, minimizing vibration.

HOOP STRENGTH
The retention strength inherent in the belt construction of a tire that resists centrifugal force & provides dimensional stability.

IMBALANCE
The condition that exists when a tire's mass is not evenly distributed around the rolling axis & centerline, causing bounce (static imbalance) or shake (dynamic imbalance).

INFLATION PRESSURE
The pressure of the 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, ratings or constructions to a vehicle. Mixing should be avoided. Some performance vehicles, however, specify different size tires on front & rear axles. See vehicle's Owner's Manual.

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 of 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 & rotate in the original direction of travel.

PLIES
The reinforcing members of a tire composed of layers of cord fabric & rubber that provide the strength to contain the air pressure needed to support a load & resist deflection.

RADIAL
The tire construction utilizing plies whose cords run radially from bead to bead under the tread. This construction requires a belt to stabilize the tread & 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 & flange onto which a tire is mounted.

RIM DIAMETER
The diameter of the rim bead seats that support a tire are normally indicated in whole numbers in inches for passenger cars.

ROTATION
The systematic movement of tires from one vehicle position to another to maximize tread life & 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 & the tread.

SLIP ANGLE
The angle between the direction in which a tire is aimed or steered & 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 & rear edges of tires mounted on an axle. Toe-in means the front edges are closer together than the rear edges & tires point inward. Toe-out means the front edges are farther apart than the rear edges & the tires point outward.

TREAD
The region of a tire designed to contact the ground. It is molded of tough rubber for high traction & low wear.

TREAD PATTERN
The arrangement of blocks, grooves, sipes & channels designed into the tread to enhance its grip. Also called the tread design.

TREAD VOID
Areas in the tread, such as grooves & channels, that permit water to drain away from the footprint.

TREADWEAR
The measure of the life of a tire tread, usually described in number of miles.

TUBELESS
A tire construction that uses a rubber inner-liner inside the casing to prevent air leakage & 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, mechanical flexing & heat.

UNDERSTEER
The condition that exists during cornering when the front of a vehicle tends to skid before the rear.

WHEEL ALIGNMENT
See Alignment.