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 tyre 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 tyre and wheel assembly for smooth driving.
Balance is achieved by fitting weights to the wheel rim to offset uneven weight
distribution of the tyre or wheel.
Bead
An inextensible hoop of high-tensile steel wires that anchors the plies and conforms to
the rim seat to hold the tyre onto the wheel rim.
Bead seat
The inner ledge portion of the wheel rim where the tyre bead rests adjacent to the
flange.
Belts
The plies of tyre cords beneath the tread that determine the tyre's diameter and
stabilize the tread by resisting deformation from cornering, braking, and centrifugal
forces.
Bias-ply
A type of tyre 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
tyre.
Camber
The angle between the centerline of the tyre and a vertical line as viewed from the
front.
Camber thrust
A cornering force generated by the tyre's camber.
Casing
The tyre body composed of plies that form the tyre'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
tyre.
Cornering force
The lateral frictional force generated by a cornering tyre, acting in opposition to the
centrifugal force.
Crown
The center area of a tyre's tread.
Deflection
The difference between a tyre's unloaded or free radius and the loaded radius.
Directional stability
The tendency for a tyre to roll in its steered direction rather than follow road
contours.
Footprint
The area of the tyre's tread that is in actual contact with the ground.
Harmonic marking
Markings on wheels and tyres that allow match mounting to cancel tyre and wheel run out,
minimizing vibration.
Hoop strength
The retention strength inherent in the belt construction of a tyre that resists
centrifugal force and provides dimensional stability.
Hydroplaning
The accumulation of water in a film under the footprint that causes a tyre 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 tyre'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 tyre that applies a tensile stress to the tyre cords,
permitting them to carry the vehicle's load.
Liner or inner-liner
The thin layer of halo butyl rubber inside a tyre that contains the inflation air,
sometimes called the inner-liner. All tubeless tyres are manufactured with an
inner-liner.
Mixing tires
Fitting tyres of different sizes or constructions to a vehicle. Mixing should be avoided.
Some performance vehicles, however, specify different size tyres on front and rear
axles.
Mounting tires
The act of fitting tyres to wheel rims.
Overall diameter
The diameter of an unloaded, inflated tyre 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 tyre 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 tyre 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 tyre 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 km
The measured number of revolutions made by a tyre traveling one km.
Rim
The portion of a wheel incorporating the well, seats, and flange onto which a tyre is
mounted.
Rim diameter
The diameter of the rim bead seats that support a tyre, normally indicated in whole
numbers in inches for passenger cars.
Rim width
The distance between rim flanges.
Rollover
The condition that occurs during hard cornering when a tyre sidewall rubs the road
surface.
Rotation
The systematic movement of tyres from one vehicle position to another to maximize tread
life and minimize irregular wear.
Run out
The measure of the out of roundness of the tyre causing a vibration that cannot be
balanced.
Section
A slice of a tyre 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 tyre.
Section width
The distance between a tyre's sidewalls measured at the widest part of the tyre. Each
size of tyre is measured on a specific rim width.
Series
A designation of a tyre's aspect ratio. A tyre with an aspect ratio of 60% is a 60 series
tyre.
Shoulder
The edge of a tyre's tread where it joins the sidewall.
Sidewall
The portion of the tyre between the bead and the tread. It is flexible to soak up bumps
yet stiff to limit tyre rollover.
Slip angle
The angle between the direction in which a tyre is aimed or steered and the actual
direction of tyre travel.
Speed rating
A letter designation identifying the tyre's high-speed durability on an indoor test
wheel. Refer to the speed-rating chart.
Toe
The difference between the front and rear edges of tyres mounted on an axle. Toe-in means
the front edges are closer together than the rear edges and tyres point inward. Toe-out
means the front edges are farther apart than the rear edges and the tyres point outward.
Tread
The region of a tyre 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.
Treadwear
The measure of the life of a tyre tread.
Tubeless
A tyre construction that uses a rubber inner liner inside the casing to prevent air
leakage and eliminate the need for an inner tube.
Underinflation
The condition that exists when there is not sufficient air pressure in a tyre to support
a specific load. This causes the tyre 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.
