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Aspect Ratio
A term that describes a tire’s height-to-width proportion. If a
tire’s sidewall height were 65% of its section width, its aspect ratio would be
65. In the tire size expressed as 205/65-15, the number 65 is the aspect ratio.
(Tire Dimensions
article) |
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Balance
The state in which a tire and wheel assembly spins with all its
weight distributed equally. A wheel balancer is used to place weights
compensating for static and dynamic imbalances that exist in all assemblies. Not
balancing an assembly can result in vibration.
(Tire Balancing article) |
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Bead
A round hoop of steel wires, wrapped or reinforced by steel cords,
placed at the very inside of the tire’s diameter.
(Tire Construction
article) |
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Bias Ply Tire
A pneumatic tire manufactured such that the plies are laid at
alternate angles less than 90 degrees to the centerline of the tread. These
criss-cross plies give the tire its strength, but generate heat during operation
and limit the tire’s wear and performance. |
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Carcass
The tire body beneath the tread and sidewalls; also called
the casing. |
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Contact Patch
The portion of the tread that contacts the road during operation.
Cord
The strands of material forming the plies or layers of tire. Cords may
be made from fiberglass, rayon, nylon, polyester or steel.
DOT Markings
Each tire has a required Department of Transportation number
imprinted on at least one of its sidewalls. That number begins with the letters
"DOT" and may contain up to 12 additional numbers and letters.
The first and last digits are the most important for the tire owner. The first
two letters/numbers identify the manufacturer of the tires. Prior to the year
2000, the last three digits of a DOT number represented the week (two digits)
and the year (one digit) of production.
For example, if the last three digits are 439, the tire was produced in the 43rd
week of 1999. Tires produced after January 1, 2000 have a four digit date code
at the end of the DOT number. The first two digits represent the week of
production and the last two digits represent the last two digits of the year of
production. So, 3500 as the last four numbers indicates that the tire was
produced in the 35th week of the year 2000.
Footprint
The portion of the tire that makes contact with the surface of the
road.
Friction
The resistance of one material (the tire tread) as it moves against
another (the road); this is the force that causes the tire to grip
to the road.
Gross Vehicle Weight (GVW)
The actual weight of a vehicle when fully loaded
with passengers and cargo. |
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Groove
The space between two adjacent tread ribs; also called
tread grooves. |
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Heat Cycling
A method of "breaking in" competition tires prior to initial
use. Heat cycling gradually heats the tire in a controlled environment to gently
stretch the tread compound, resulting in better traction and longer tread life.
Highway Tires
Also called summer tires; designed for dry and occasional wet
weather driving, but not for use on snow and ice.
Hydroplaning
A skimming effect caused by tires losing contact with a surface
covered by water.
Innerliner
The innermost layer of a tubeless tire which prevents air from
permeating through the tire. This thin layer of material replaces the innertube.
Load Index
An assigned number ranging from 0 to 279 that corresponds to the
load carrying capacity of the tire.
(Load Index chart)
M+S, M/S or M & S (Mud and Snow)
Indicates that a tire can reach particular standards for performance in mud and
snow conditions. The tire must meet the Rubber Manufacturers Association (RMA)
definition of a mud and snow tire.
Maximum Inflation Pressure
The maximum air pressure to which a cold tire may
be inflated; found molded onto the tire’s sidewall.
OE and OEM
OE means "Original Equipment" and refers to the tires included
with a new vehicle at the time of purchase. The vehicle's manufacturer selects
these tires to provide the optimal performance based on the performance
characteristics of the vehicle. "OEM" stands for "Original Equipment
Manufacturer."
Overall Diameter
The diameter of an inflated tire without any load.
Overall Width
The distance between a tire's outside sidewalls, including
lettering and designs.
P-Metric
Uniform designation of tire sizes in metric measurements originally
introduced by American tire manufacturers in 1977. Commonly called "P-metric
series." A typical P-metric tire size is P215/70R-15.
Placard
A small label typically located on the edge of the driver's door or
inside the glove compartment of a vehicle. A placard contains information on the
vehicle such as the manufacturer's recommended tire inflation pressure, seating
capacity, and Gross Vehicle Weight (GVW). |
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Plus-Sizing
An option allowing drivers to customize the appearance and
performance of their vehicle by mounting lower profile tires on larger diameter
wheels. One-inch greater wheel diameter is referred to as plus-one, two inches
is plus-two... and so on. Using a lower profile tire with a greater diameter rim
allows the overall diameter to remain about the same.
(Plus Sizing article) |
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Ply
A rubber-coated layer of fabric containing cords that run parallel to
each other; extends from bead to bead and goes between the innerliner and belts
of tread. |
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Ply Rating
A method of expressing load carrying capacity in terms of plies.
A 6-ply rating is now expressed as Load Range C; however, the tire is not built
with six individual plies. Instead, it contains one or two plies of equivalent
strength. The result is a cooler running, longer lasting tire.
PSI
Pounds Per Square Inch. This is the standard unit of measurement for air
pressure within tires.
Radial Ply
Tire construction where the cords in the body run at 90 degrees
to the centerline of the tread.
Ride Matching
A system of balancing a tire and wheel assembly using a
simulated road test. Ride matching provides optimal weight distribution and
eliminates vibrations caused by the combination of minor errors within tires and
wheels. (Ride Matching
article) |
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Rim Width
Distance between the two opposite inside edges of the
rim flanges. |
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Rolling Resistance
The force required to keep a tire moving at a constant
speed. The lower the rolling resistance, the less energy needed to keep a tire
moving. |
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Rotation
Moving tires from side to side or front to rear on a vehicle in a
prescribed pattern to achieve uniform wear on all tires. Rotations should be
performed regularly every 6,000 miles.
(Tire Rotation article) |
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Section Height
The height of a tire measured from the rim to the outer
tread. (Reading the Tire Size
article)
Section Width
The distance between outside sidewalls, not including any
lettering or design. (Reading
the Tire Size article) |
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Series
A numerical representation of a tire’s aspect ratio. For example, 60
Series indicates the tire’s section height is 60% of its section width (See
Aspect Ratio).
Shimmy
Wobbling of wheels from side to side on a vehicle. Improperly
balanced tires, misalignment and bent wheels can cause shimmying. |
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Shoulder
The part of a tire where the sidewall and tread meet. Certain tire
design features shoulder blocks for better traction. |
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Sidewall
The part of the tire between the tread and the bead. |
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Size
An expression that defines a particular tire in terms of its width,
height, rim diameter, aspect ratio and construction type. 205/65R-15 expresses
tire size using the metric system. For more detailed information, visit our page
on reading the tire size.
(Reading the Tire Size
article)
Snow Tire
Also referred to as a winter tire; a special type of tire with a
tread and compound that gives better traction in snow. Identified by M&S, M+S or
M/S on the sidewalls. All season tires also include these designations on the
sidewall. (Winter
Tire FAQ)
Speed Rating
The speed rating of a tire is based on U.S. Government
standards for reaching and sustaining a specified speed. Typically, a tire with
a higher speed rating results in better handling. Speed ratings are determined
via laboratory tests that simulate road performance at various speeds. Tires are
assigned a single letter (such as H or V) to designate speed rating.
(Speed Rating article)
TIre Pressure Monitoring System (TPMS)
TPMS is an automated system that
monitors the air pressure in a vehicle's tires. When air pressure in one or more
tires drops 25 percent or more below the correct pressure, a warning alerts the
driver.
Traction
The friction between a tire and the road surface; the amount
of grip provided. |
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Tread
The part of the tire that comes into contact with the road. The tread
type is distinguished by the design of its ribs and grooves.
(Tire Construction
article) |
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Tread Depth
The distance measured in the major tread groove nearest the
centerline of the tire from the base of the groove to the top of the tread.
According to law, most states legally consider a tire to be worn out when it
reaches a tread depth of 2/32". |
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Treadwear Indicator
Narrow bands, sometimes called “wear bars”, that appear across
the tread when 2/32” of tread remains. |
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Tread Width
The width of a tire's tread.
UTQG (Uniform Tire Quality Grading)
A tire information system that provides
consumers with ratings for a tire's traction (AA to C) and temperature (A to C).
Treadwear is a numeric rating. Ratings are determined by tire manufacturers
using government-prescribed test procedures, and are molded into the tire's
sidewall. These ratings can only be compared within specific manufacturer's
tires and cannot be compared from one manufacturer to another. Our treadwear,
traction and temperature page explains this rating system in much more detail.
(Treadwear,
Traction, and Temperature article)
Valve
A device mounted in the wheel that lets air in or out of the tire.
Valves include caps to keep out dirt and moisture and a valve to prevent air
from escaping. |
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