TIRE TERMINOLOGY

PLY RATING - The term “ply rating” is used to indicate an index to the load rating of the tire. Years ago when tires were made from cotton cords, “ply rating” did indicate the actual number of plies in the carcass. With the development of higher-strength fibers such as nylon, fewer plies are needed to give an equivalent strength. Therefore the definition of the term “ply rating” (actual number of cotton plies) has been replaced to mean an index of carcass strength or a load carrying capacity.

RATED LOAD- This is the maximum allowable load that the tire can carry at a rated inflation pressure.

RATED PRESSURE - Rated pressure is the maximum inflation pressure to match the load rating. Aircraft tire pressures are given for an unloaded tire; i.e, a tire not on an airplane. When the rated load is applied to the tire, the pressure increases by 4% as a result of a reduction in air volume.

OUTSIDE DIAMETER - This measurement is taken at the circumferential center line of an inflated tire.

SECTION WIDTH- This measurement is taken at the maximum cross sectional width of an inflated tire.

RIM DIAMETER - This is the nominal diameter of wheel/rim on which the tire is mounted.

SECTION HEIGHT - This measurement can be calculated by using the following formula:
Section Height = Outside Diameter - Rim Diameter/2

ASPECT RATIO- Measure of the tire’s cross section shape. This can be calculated by the following formula: Aspect ratio = Section Height /Section Width

FLANGE HEIGHT - This is the height of the wheel rim flange.

FLANGE DIAMETER - The diameter of the wheel including the flange.

FREE HEIGHT - This measurement can be calculated by using the following formula:
Free Height = Outside Diameter - Flange Diameter/2

STATIC LOADED RADIUS - This is the measurement from the center of the axle to the runway for a loaded tire.

LOADED FREE HEIGHT - This measurement can be calculated by using the following formula:
Loaded Free Height = Static Loaded Radius - Flange Diameter/2

TIRE DEFLECTION - A common term used when talking about aircraft tires is the amount of deflection it sees when rolling under load. The term % Deflection is a calculation made using the following formula: % Deflection = Free Height - Loaded Free Height/Free Height
Aircraft tires are designed to operate at 32% deflection, with some at 35%. As a comparison, cars and trucks operate in the 17% range.

SERVICE LOAD (OPERATIONAL LOAD) – Load on the tire at max aircraft takeoff weight.

SERVICE PRESSURE (OPERATIONAL PRESSURE) – Corresponding pressure to provide proper deflection at service load.

RATED SPEED– Maximum speed to which the tire is qualified.



Bead. A ring of steel wire that anchors the tire carcass plies to the rim.

Belt. An assembly of plies extending from shoulder to shoulder of a tire and
providing a reinforcing foundation for the tread. In radial-ply tires, the belts
are typically reinforced with fine steel wire having high tensile strength.

Bias-ply tire.A pneumatic tire in which the ply cords that extend to the
beads are laid at alternate angles substantially less than 90 degrees to the
centerline of the tread. The bias-ply tire was the predominant passenger
tire in the United States before 1980 but is no longer in common use; it
has been supplanted by the radial-ply tire.

Carbon black.A very fine, nano-size particulate carbon used as a reinforc-
ing filler in rubber compounds to provide abrasion resistance and other
favorable properties.

Carcass or casing. The tire structure, except tread and sidewall rubber,
that bears the load when the tire is inflated.

Coastdown. A process in which a vehicle or test machine is allowed to
slow down freely from a high to a low speed without application of ex-
ternal power or braking.

Coefficient of friction. The ratio of friction force to normal force to
cause sliding expressed as a unitless value (i.e., friction force generated
between tire tread rubber and the road surface divided by vertical load).

High-performance tire. A passenger tire designed for the highest speed
and handling, generally having the speed symbol W, Y, or Z in the United
States.

Hysteresis. A characteristic of a deformable material such that the energy
of deformation is greater than the energy of recovery. The rubber com-
pound in a tire exhibits hysteresis. As the tire rotates under the weight of
the vehicle, it experiences repeated cycles of deformation and recovery,
and it dissipates the hysteresis energy loss as heat. Hysteresis is the main
cause of energy loss associated with rolling resistance and is attributed to
the viscoelastic characteristics of the rubber.

Light truck (LT) tire. A tire constructed for heavy loads and rough ter-
rain that is usually used on medium-duty trucks in commercial service.
LT tires are not regulated as passenger tires and are therefore not examined in this study.

Original equipment manufacturer (OEM). An automobile manufac-
turer.

Original equipment (OE) passenger tire. A tire that is provided as origi-
nal equipment on new passenger vehicles. Such tires are often designed for
particular vehicles to the specifications of the automobile manufacturer.

Passenger tire. A tire constructed and approved for use on passenger
vehicles and that usually contains the prefix P before the metric size
designation on the tire sidewall. Federal Motor Vehicle Safety Standards
and Uniform Tire Quality Grading standards are established specifically
for passenger tires.

Performance tire. A passenger tire intended to provide superior han-
dling and higher speed capabilities and generally having a speed symbol
of H or V in the United States.

Ply. A sheet of rubber-coated parallel tire cords. Tire body plies are
layered.xvi Tires and Passenger Vehicle Fuel Economy

Radial-ply construction. A pneumatic tire construction under which
the ply cords that extend to the beads are laid at approximately 90 de-
grees to the centerline of the tread. Two or more plies of reinforced belts
are applied, encircling the tire under the tread. Radial-ply tires were in-
troduced in Europe during the 1950s and came into common use in the
United States during the 1970s.

Reinforcing filler. Material added to rubber compounds to provide
favorable properties, including resistance to abrasion. The two most
common reinforcing fillers are carbon black and silica.
.
Rim diameter. The diameter of a wheel measured at the intersection of
the bead seat and the flange. The rim diameter is listed in the size desig-
nation on the passenger tire sidewall. Common rim diameters for pas-
senger tires range from 13 to 20 inches.


Rolling resistance. The force at the axle in the direction of travel re-
quired to make a loaded tire roll.


Run-flat tire. A type of pneumatic tire constructed of special materials,
supports, and configurations that allow it to travel for a limited distance
and speed after experiencing a loss of most or all inflation pressure.
While these tires usually have greater weight and resultant rolling resis-
tance, they permit the elimination of storage space and weight associated
with a spare tire and jack.

Sidewall. The portion of the tire between the bead and the tread. The
tire’s name, safety codes, and size designation are molded on the sidewall.

Silane. An organo-silicate compound that is sometimes mixed with sil-
ica to promote dispersion and bonding.

Silica. A very fine, nano-size particle, silicon dioxide, used as a reinforc-
ing filler in rubber compounding.

Speed rating. A letter assigned to a tire denoting the maximum speed
for which the use of the tire is rated (e.g., S =112 mph, H =130 mph).
The speed rating is contained in the tire size designation molded on the
sidewall.

Tire pressure monitoring system (TPMS). A warning system in motor
vehicles that indicates to the operator when a tire is significantly under-
inflated. Some systems use sensors in the tire to transmit pressure infor-
mation to a receiver. Some do not have pressure sensors but rely on
wheel speed sensors to detect and compare differences in wheel rota-
tional speeds, which can be correlated to differences in tire pressure.

Traction. The ability of a loaded tire to generate vehicle control forces
through frictional interaction with a road surface.

Tread. The peripheral portion of the tire designed to contact the road
surface. The tread band consists of a pattern of protruding ribs and
grooved channels on top of a base. Tread depth is measured on the basis
of groove depth. Traction is provided by the tread.

Tread compound.The general term that refers to the chemical formula
of the tread material. The compound consists of polymers, reinforcing
fillers, and other additives that aid in processing and slow degradations
from heat, oxygen, moisture, and ozone.

Tread wear life. Total miles traveled by a tire until its tread wears out,
which is usually defined as a remaining groove depth of 2/32 inch for a
passenger car tire that exhibits even wear.

Uniform Tire Quality Grade (UTQG). A passenger tire rating system
that grades a tire’s performance in tread wear durability, traction, and
temperature resistance. UTQG ratings are required by the federal gov-
ernment for most types of passenger tires and are molded on the tire’s
sidewall. The tread wear grade is a numeric rating, with a higher num-
ber suggesting longer tread wear capability. Most tires receive grades be-
tween 100 and 800. The traction grade is assigned on the basis of results
of skid tests on wet pavements. Tires are graded AA, A, B, or C, with AA
indicating superior wet traction. The temperature grade is assigned to
tires tested at various speeds to determine the ability of a tire to dissipate
heat. Tires are graded A, B, or C, with A indicating an ability to dissipate
heat at higher speeds.

Vehicle fuel economy.The average number of miles a vehicle travels per
gallon of motor fuel (typically gasoline or diesel fuel).

Viscoelastic. A viscoelastic material is characterized by possessing both
viscous and elastic behavior. A purely elastic material is one in which all
energy stored in the material during loading is returned when the load is
removed. In contrast, a purely viscous material stores no strain energy,
and all of the energy required to deform the material is simultaneously
converted into heat. Some of the energy stored in a viscoelastic system is
recovered on removal of the load, and the remainder is dissipated as heat.
Rubber is a viscoelastic material.

Wear resistance. Resistance of the tread to abrasion from use on a nor-
mal road surface.

Wet traction. The ability of a loaded tire to generate vehicle control
forces through frictional interaction with a wet road surface.