Tire & Wheel Definitions

Have you come across some tire or wheel lingo that you aren't familiar with? Browse our handy, alphabetized glossary that includes brief definitions of the terms you might encounter when you’re shopping for or researching new tires and wheels.

A
Air Check
During an air check, a driver or technician uses a pressure gauge to determine the amount of air currently inflating a tire. You should perform an air check every other time you fill up with gas, each time your tires are rotated, or once a month. Learn more about Air Pressure.
Air Pressure
Refers to the amount of air inflating your tires. Vehicles have a recommended air pressure that optimizes tire performance for that specific vehicle. Air pressure is measured in PSI (pounds per square inch).
Airtight Synthetic Rubber
Formulated with virtually impermeable butyl rubber, a material used to replace the inner tube in a modern, “tubeless” tire.
Alignment
Alignment refers to the mechanical condition of the suspension’s adjustable components. In alignment, a vehicle has three settings set to specification: caster, camber, and toe-in. Severe impacts, such as hitting potholes or curbs, and worn suspension parts can cause misalignment. Learn more about Wheel Alignment.
All-Season Tires
All season tires are designed to provide good traction in a wide variety of road conditions, including wet, dry, mud, and snow.  This tire’s versatile design limits its performance in extreme conditions, or when compared to tires built for a particular category.
All-Terrain Tires
All-terrain tires are designed to provide good performance during both off and on-road driving. These tires are often equipped on trucks, SUVs, and four-wheel-drive vehicles that may occasionally need to go off-road.
Asymmetric Tread
Tires with an asymmetric tread have a continuously unique design across the width of the tread. The pattern elements oriented towards the outside of the tire are focused more on handling and dry traction, while the elements closest to the inside of the tire focus on wet and winter traction. This changing pattern allows the tire a wider range of performance attributes in various conditions.
Alphanumeric
Alphanumeric is a form of tire sizing designation used until the late ‘70’s that incorporated both letters and numbers.
Aspect Ratio
Aspect Ratio describes the proportion between a tire’s height and width. It appears after the slash in the tire size code, and tells you that the sidewall is a certain percentage of the width. Consider a tire sized P205/65R16. This tire’s aspect ratio is “65,” meaning that the sidewall is 65% of the width. Learn more about Aspect Ratio.
Asymmetric Tread
Tires with an asymmetric tread have a continuously unique design across the width of the tread. The pattern elements oriented towards the outside of the tire are focused more on handling and dry traction, while the elements closest to the inside of the tire focus on wet and winter traction. This changing pattern allows the tire a wider range of performance attributes in various conditions.

B
Backspacing
Backspacing describes the distance between a wheel’s mounting surface and innermost lip. This measurement helps a technician understand how a wheel fits into a wheel well. Learn more about Backspacing.
Balance
Occurs when a tire and wheel assembly spins with evenly distributed weight. To achieve balance, technicians use a wheel balancer to determine the location of static and dynamic imbalances that may cause ride vibrations. They then strategically place weights on the wheel, compensating for the imbalances to produce a smooth ride. Learn more about Balancing Tires.
Bead
A round hoop of steel wires, placed at the very inside of a tire’s diameter. The bead is an important component of tire construction, helping it to stand and support weight.
Bead Chafer
A key tire component that acts as the contact point between the tire and wheel. The bead chafer is designed to withstand the force the wheel exerts on the tire during mounting, as well as the dynamic forces of driving and braking.
Bead Filler
Component that transfers propulsion and braking torque from the wheel rim to the road surface contact area.
Bead Tension Structure
Two sidewall plies wrapped around each bead wire in opposite directions, providing lateral stability and flex to absorb road irregularities.
Belt
Refers to the rubber-coated layer of cords located between the tire’s body plies and tread. Belts are commonly made of steel, but may also be constructed of fiberglass, rayon, nylon, polyester, or other fabrics.
Bias Ply Tire
These pneumatic tires possess plies situated at angles less than 90 degrees to the tread centerline.  Their criss-cross plies provide extra strength, but generate heat during operation and limit the tire’s wear and performance.
Bolt Circle/Bolt Pattern
The circular arrangement of wheel bolts that keep the wheel attached to the hub. It refers to the diameter of the imaginary circle drawn through the exact middle of each bolt.
Breakaway
Term used to describe the loss of traction that occurs when negotiating a curve or accelerating from a standing start. During breakaway, tires slide against the road surface, instead of gripping it.
Butyl Rubber
A synthetic rubber used to create today’s tires. It is virtually impenetrable to water and air.

C
Camber
Measurement, in degrees, of the wheel’s inward tilt when viewed from the front. Camber is one of the settings adjusted during alignment.
Camber Thrust
The centrifugal force generated when a tire rolls with camber, which can add to or subtract from the side force generated by a tire.
Carbon Black
A reinforcing filler that can be incorporated into the tire rubber compound. It can give the tire higher resistance to wear.
Carcass Ply
Made up of thin textile fiber cables bonded into the rubber. These cables are largely responsible for determining the strength of a tire.
Carrying Capacity
Refers to how much weight a tire is designed to carry at a given air pressure. Check a load inflation table to determine how much air a particular size tire needs to support the vehicle axle load. Learn more about Carrying Capacity.
Casing
Refers to the tire body beneath the tread and sidewalls. The carcass consists of rubber coated fabric plies that contribute to carrying capacity and shock absorption.
Caster
Caster refers to the angle of an imaginary line drawn through a wheel’s centerline and steering axis. Caster is adjusted during alignment.
Casting
Wheel manufacturing process in which liquid metal is poured into a mold that determines the wheel’s final shape.
Centerline
An imaginary line down the center of the vehicle. This line aids in measuring a vehicle’s alignment.
Centrifugal Force
Physicists call centrifugal force a pseudo or fictitious force because it doesn’t really exist. You experience centrifugal force when sitting in a turning car. The turn produces the sensation of movement, even though there’s no force being applied to your body.
Cold Inflation Pressure
Refers to the amount of air pressure in a tire, measured in pounds per square inch (PSI), before a tire experiences heat buildup from driving.
Compliance Cushion
An added rubber component between the tire tread and belt. It absorbs road irregularities for a smoother ride.
Contact Patch
Refers to the tread portion that makes contact with pavement during driving.
Cord
Cord is the name for strands of material forming the plies or layers of tire. Cord can be made from fiberglass, rayon, nylon, polyester, or steel.
Cornering Force
Refers to the force exerted on a tire when the vehicle turns.
Crown Plies
These plies act as rigid base for the tread, allowing good fuel economy.  They also give the tire centrifugal and lateral rigidity, and can flex sufficiently for a comfortable ride.
Curb Weight
Describes the weight of a production vehicle with full fluid reservoirs, all normal equipment in place, and no passengers.

D
Deflection
The area where the tread and sidewall flex when the tread makes contact with the road.
DOT Markings
This alphanumeric code can be found on every tire that complies with Department of Transportation requirements for highway use. It begins with the letters “DOT,” and may contain up to 12 additional numbers and letters that provide tire information.  .
Drift
Occurs when a vehicle deviates from a straight-line path without the driver’s steering input.
Dry Traction
Describes the tire’s ability to grip the road under braking and acceleration. 
Dry Zone
Large tread blocks placed along the outer edges. They help provide confident handling on dry roads.
Dual Tread Compound
Describes a tire with two different compound types incorporated in the tread, an outside one for dry traction and an inside one for wet traction.
Duals
Refers to two tires placed immediately adjacent to each other on an axle, meaning that the axle holds four tires instead of the usual two.  Duals increase traction and carrying capacity.
Dynamic Imbalance
When the weight of a tire and wheel assembly is equally distributed around its circumference and on either side of its centerline. If a tire and wheel assembly goes out of dynamic balance, it will produce a wobbling effect or side-to-side shake.

E
Eccentric Mounting
Occurs when a tire and wheel assembly is mounted so that the assembly’s center of rotation isn’t aligned with hub’s center of rotation.
ECE Symbol
ECE stands for “Economic Commission of Europe,” a European organization that develops motor vehicle requirements. The ECE symbol means that a tire meets ECE standards for physical dimensions, branding requirements, and high-speed endurance.
Extra Load (XL)
A designation indicating that a given tire can carry a higher load. Tires receive this designation if they have a maximum inflation pressure higher than the standard maximum.

F
Filament at Zero
Refers to individual, spiral-wrapped nylon or aramid/nylon reinforcing filaments that may be precisely placed in specific portions or across the entire tread area atop the steel belts banded at zero degrees. This helps retain tire shape, while enhancing steering precision and ride quality.
Fore-and-Aft Weight Transfer
Occurs when weight is transferred between the front and rear axles. Acceleration causes a weight transfer from front to rear, while braking causes a weight transfer from rear to front.
Four-Wheel-Drift
Occurs when a car’s front and rear tires slide in a controlled manner. To keep the vehicle on a prescribed path, a driver can correct course with the throttle and steering wheel.
Forging
Wheel production process in which heat and pressure reshape a block of alloy material into the wheel design.
Free Radius
The radius of a tire and wheel assembly when not deflected under load.
Friction
Refers to the resistance of one material (the tire tread) as it moves against another (the road). Every tire uses friction to grip the road.

G
Gross Axle Weight Rating
Refers to the maximum weight that can be distributed among the tires on a given axle. Sometimes abbreviated as GAWR.
Gross Vehicle Weight
The actual weight a vehicle when fully loaded with passengers and cargo.
Gross Vehicle Weight Rating
The maximum weight that a vehicle can support, including its own.

H
Handling
Describes a tire’s responsiveness to handling inputs.
Heat Cycling
A method of “breaking in” competition tires prior to initial use. Heat cycling gradually heats the tire in a controlled environment. This heat buildup creates a more consistent traction and longer tread life. Learn more about Heat Cycling.
High Flotation Sizes
A tire sizing system designed especially for light trucks. These tires have lower sidewalls and wider treads for better traction on sand, soft soil, mud, or other in off-road situations.
High-Performance Tires
Tires that can handle, grip, and corner better than standard tires. These tires are rated for operation at higher speeds than their standard counterparts. High-performance tires can also be called maximum performance or ultra-high-performance.
Highway Tires
Designed for dry and occasional wet weather driving, but not for snow and ice. Also called summer tires.
Hub Cap
Hub caps are used to shield the lug nuts from debris, while sometimes offering aesthetic appeal to the wheels they’re equipped on. Hub caps are usually made of affordable materials such as plastic or aluminum, and are generally simple to remove when needed.
Hub-centric
Wheels are manufactured to fit either the hub or the lugs. Hub-centric wheels match perfectly to the diameter of the vehicle’s hub.
Hydroplaning
Hydroplaning refers to the loss of control that can happen while driving in wet conditions. This skimming effect occurs when a tire loses contact with a water-covered surface.

I
Imbalance
Describes when the tire and wheel assembly’s distribution of mass becomes uneven.
Indentation
A normal, safe occurrence where overlapping splices of fabric cords create indentations in the tire sidewall. Indentations don’t occur on treads because of steel cable implementation.
Inflation
The act of putting air in tires.
Innerliner
An innerliner is the innermost layer of rubber a tubeless tire. The innerliner serves to reduce air loss through the tire itself.
Interlocking Sipes
S-shaped sipes that interlock for greater length, providing extra tractive grip.

K
Kilopascal
The metric unit for pressure. One PSI is equal to 6.9 kPa.

L
Lateral Runout
When a wheel wobbles side-to-side during rotation, causing vehicle vibration.
Lateral Weight Transfer
Describes the transfer of weight occurring whenever a vehicle travels through a curve. During a lateral weight transfer, centrifugal force acts on the vehicle to transfer weight from the wheels on the inside of the curve to those the outside.
Light Truck (LT)
Automotive industry term for smaller trucks, pickups, passenger vans, or SUVs.
Light Truck (LT) Tires
This more rugged tire design can resist the abuse caused by conditions commonly encountered by trucks and SUVs.
Load Carrying Capacity
Indicates how much weight a tire is certified to carry at maximum inflation pressure.
Load Index
This assigned number varies from 0 – 279, and corresponds to the tire’s load carrying capacity. Learn more about Load Index.
Load Range
Defines a range of maximum loads a tire can carry at a defined pressure.
Loaded Radius
The measurement, in inches, from the wheel axle centerline to the ground when the tire is properly inflated for the load.
Loaded Section Height
The height of the tire section that makes contact with the road.
Lug-Centric
Wheels are manufactured to fit either the hubs or the lugs. Lug-centric wheels perfectly match a vehicle’s lug pattern.

M
Match Mounting
Technique that matches the tire’s harmonic high point with the wheel’s low point, ensuring optimal ride performance. Learn more about Match Mounting.
Maximum Inflation Pressure
A PSI limit indicating the maximum pressure to which a cold tire can be inflated. Find it molded onto a tire’s sidewall.
Metric Tire Size System
A system describing tire size. It is used by ETRTO, the European Tire and Rim Technical Organization.
Misalignment
Condition occurring when a vehicle’s front or rear suspension is not properly aligned.
Mounting
The act of putting a tire on a wheel and making sure the assembly is balanced. New tires need to be professionally mounted.
Mud and Snow (M + S, M / S, or M & S)
These characters indicate that the tire tread is focused on performing well in severe conditions.  Tires with a Mud and Snow rating must exceed a certain tread to void ratio.

N
Negative Camber
Alignment setting is where tire tops lean toward the vehicle’s centerline. Racers use negative camber for maximum cornering potential.
Negative Offset
Describes when the wheel mounting face is positioned closer to the brake side of the wheel, moving the tire and assembly out of the fender well.
NHSTA
National Highway Traffic Safety Administration.
Nominal Rim Diameter
Diameter of a tire rim given in the nearest whole numbers.

O
Off-Road Tires
Tire type designed to perform on non-paved surfaces, including dirt and sand. These tires will typically have large blocky tread patterns for improved off-road grip.
Offset
The distance between a wheel’s mounting surface and centerline. This measurement helps technicians understand how the wheel fits in the wheel well.
Off-the-Car Balancing
Process where the assembly is taken off the vehicle and balanced to eliminate side-to-side shimmy and hopping up and down. This process helps achieve an optimal weight balance between tire and wheel.
Original Equipment (OE)
Original equipment refers to the tires a manufacturer equips on a brand new vehicle. The manufacturer chooses tires that will provide optimal performance based on the vehicle’s characteristics.
Original Equipment Manufacturer (OEM)
Original equipment manufacturer refers to the maker of a piece of original equipment.
Overall Diameter
The diameter of an inflated tire when it isn’t carrying load.
Overall Width
The distance between a tire’s outside sidewalls, including lettering and designs. On most sizing systems, overall width is measured in millimeters.
Overinflation
Overinflation occurs when tires are filled with too much air. Overinflation can cause harsh ride, irregular wear, and increased stopping distance.
Oversteer
A vehicle’s tendency to turn sharper than the driver intends while negotiating a corner. An oversteering car is said to be “loose,” since its tail tends to swing wide.
Oxidation
The rusting process that occurs when moisture makes contact with the tire’s steel belt, which usually happens because of damage. Oxidation can result in the tire becoming unserviceable before normal replacement time.

P
P-Metric
Refers to the tire size designation system introduced by American tire manufacturers in 1977. This sizing system uses metric measurements and meets US based standardizing organization requirements.
Passenger Tires
Tires built in sizes used by passenger car vehicles. These tires focus on value and proficiency in most driving conditions.
Performance Tires
Tires built in sizes typically used on higher performance passenger car vehicles.  These tires usually focus more on performance and higher speed usage.
Penny Test
A simple, commonly used test where one uses an upside-down penny to check tire tread depth.
Permeation
A natural process by which a liquid or vapor escapes out from inside a solid. Tires are slowly and constantly losing air because of this process.
Placard
A small label typically located on the edge of the driver door or doorframe. The placard provides important vehicle information, such as the manufacturer’s cold tire inflation pressure, seating capacity, and gross vehicle weight.
Plus-Sizing
Plus-sizing allows a driver to customize the look and performance of their vehicle. Learn more about Plus Sizing.
Ply
A rubber-coated fabric layer containing cords that run parallel to each other. These layers are situated between the innerliner and tread belts, and extend from bead to bead.
Ply Rating
A letter indicating tire’s load carrying capacity in terms of its construction. “C” indicates that the tire has a 6-ply load carrying capacity. Instead of actually having six plies, the tire has one or two plies of equivalent strength. “D” corresponds to an 8-ply rating, while “E” represents a 10-ply rating. No letter indicates that the tire carries a standard 4-ply rating.
Pneumatic Tire
A tire designed to be filled with air.
Positive Camber
Alignment angle where the tire tops are farther apart than the tire bottoms, which causes the tires to tilt out from the vehicle’s centerline.
Positive Caster
Alignment setting where the steering axis is inclined rearward at the top.
Positive Offset
When the wheel’s mounting face sits closer to the wheel’s street side, moving the tire and wheel assembly in toward the vehicle.
Proprietary Blended Compounding
Technology that creates a uniform compound blend providing outstanding all-around performance in wet and dry conditions.
PSI
The abbreviation for “pounds per square inch.” PSI is the standard unit of measurement for air pressure within tires.
Pull
A condition where a vehicle swerves to one side without being steering in that direction. Pull results from irregular tire wear, improper front or rear wheel alignment, or worn or improperly adjusted brakes.

R
Radial Ply
In this type of tire construction, the cords in the body run at 90 degrees to the centerline of the tread.
R-Compounding
A racing-derived compound optimized for on-track performance. These compounds are designed for maximum dry grip and repeated heat cycles.
Revolutions Per Mile
Measured number of revolutions for a tire traveling one mile, which can vary with speed, load, and inflation pressure. Abbreviated as RPM.
Ribs
The pattern of tread features aligned around a tire’s circumference. There are usually multiple ribs across a tire’s tread area.
Ride Height
The distance between the road and vehicle frame. Provides a reference point for alignment measurements.
Ride Matching
A process of ensuring a wheel and tire assembly is as close to perfectly round as possible by matching the low spot of the wheel with the high spot of the tire.  This process aids in reducing vibration due to the assembly being out of round.
Rim
The part of the wheel where the tire is mounted.
Rim Diameter
The diameter of the rim bead seats supporting the tire.
Rim Flange
The part of the wheel’s rim surface that contacts with the side of the tire bead.
Rim Width
The distance between the two opposite inside edges of the rim flanges.
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.
Runout
Describes how a wheel moves in and out – away from its true center – during rotation. Excessive runout can cause the wheel to visibly wobble during rotation.

S
Section Height
The height of a tire measured from the rim to the outer tread, measured in millimeters.
Section Width
The distance between outside sidewalls, excluding lettering and design. The section width is measured in millimeters. Learn more about Section Width.
Series
A numerical representation of a tire’s aspect ratio. For example, 60 series indicates that the tire’s section height is 60% of its section width.
Service Description
Numbers and letters molded into the sidewall indicating the load-carrying capacity, load index, and the speed rating.
Shimmy
Describes the wobbling, side-to-side motion that may occur at the front of the vehicle, felt through the steering wheel.  Misalignment, worn steering components, or dynamic imbalance can cause shimmy.
Shoulder
The area where the tire’s sidewall and tread meet. Certain tire designs feature shoulder blocks for better traction.
Sidewall
The part of the tire between the tread and the bead. Find important tire information written on the sidewall.
Sipes
Special slits in a tread block that can increase traction, especially in wet or snowy conditions. Sipes open as the tire rolls onto the contact patch, and then close to break surface tension on wet roads. This process puts more rubber in contact with the road, maintaining adhesion and increasing traction on wet or snowy roads. Learn more about Tire Sipes.
Size
The alphanumeric expression that defines a tire in terms of width, height, rim diameter, aspect ratio, and construction type. The example 205/65R-15 expresses tire size using the metric system. Learn more about Reading Tire Sidewall.
Slip
The difference between a vehicle’s linear speed and the rotational speed of a tire. For example, a tire that is locked and sliding while a vehicle isn’t moving is operating at -100% slip.
Slip Angle
The difference between the direction the wheel is traveling and the direction a vehicle is traveling.
Speed Rating
A letter system identifying the maximum speed at which a tire can be driven, as determined by laboratory testing. Learn more about Speed Ratings.
Spoke
Wheel spokes extend from the center hub of a wheel to its outer edges and connect to the rim. A wheel can have a wide-ranging number of spokes, of which aftermarket wheels generally feature unique designs, colors and finishes.
Sprung Weight
The car parts supported by its springs, including the frame, engine, and body.
Stability
A tire’s ability to respond to steering inputs and external forces.
Staggering
Equipping your vehicle with larger wheels in the back, and smaller ones in the front.
Standard Load
The amount of weight a particular size tire can carry at a recommended air pressure.
Star Pattern
A pattern for tightening the lug nuts when mounting the tire and wheel assembly to the vehicle. This pattern assures uniform pressure, prevents misalignment, and helps keep the wheel centered.
Static Balance
Exists when the weight mass is evenly distributed around the axis of rotation.
Static Loaded Radius
Distance from the wheel axis of rotation to supporting surface at a given load and stated inflation pressure.
Steel Belt
A combination of rubber-covered steel cords that form a strip or belt under the tire tread. Steel belts ensure uniformity during rotation and help prevent flats.
Steering Response
A vehicle’s reaction to a driver’s steering inputs. Can also refer to the feedback drivers receive through the steering wheel as they make inputs.
Steering System
The entire mechanism enabling the driver to guide and direct the vehicle. Includes the steering wheel, steering column, steering gear, linkages, and wheel supports.
Step Groove
A design feature at the grove base. It generates an additional gripping mechanism in deeper snow.
Structure
Refers to the a tire casing’s construction. Radial structure tires, which account for most of today’s tires, are identified by the letter R.
Studs
Studs are metal inserts placed in winter tire treads. They provide additional traction in hard packed snow and soft ice conditions. Because they cause damage to the road, some jurisdictions restrict studded tires to the most inclement winter months or prohibited them completely.
Studded Tires
Winter tires that incorporate metal studs for improved traction in deep snow.
Suspension
The various springs, shock absorbers, and linkages used to suspend a vehicle’s frame, body, engine, and drivetrain above the wheels.
Symmetrical Tread
Uniform tread pattern on both sides of the tread. Symmetrical tread designs generate better performance in specific conditions on specific roads.
Synthetic Rubber
Man-made rubber. Today’s tires are mostly made of synthetic rubber, with little to no natural rubber in their construction.

T
T&RA
Acronym for Tire and Rim Association
Teardrop Siping
Small teardrop shapes in the ends of each sipe. These ease the shear force at high stress points that can crack the sipe, also allowing the tread block to flex more easily.
Tensile Strength
An object’s resistance to stretching or breaking when placed in tension. Steel belts are compared by tensile strength.
Tire Construction
Describes how a tire’s plies are assembled. “R” stands for radial, which means the plies run radially across the tire. “B” stands for bias, meaning the plies run diagonally across the tire. Learn more about Tire Construction.
Tire Designation
An alphanumeric code molded into the tire sidewall. It describes aspects of the tire’s size, including width, aspect ratio, rim diameter, load index, and speed rating. Most designations use the P-metric system.
Tire Mixing
When tires of various brands, types, or sizes are mixed on a vehicle. Tire mixing can lead to variations in the vehicle’s ride and handling characteristics.
Tire Pressure Gauge
Tool used to properly measure the air pressure in a tire.
Tire Pressure Monitoring System (TPMS)
This automated system monitors the air pressure in a vehicle’s tires. The system generates an alert on the vehicle dash when air pressure drops below the correct level by 25% or more.
Tire Rotation
A maintenance procedure that helps achieve uniform wear across all tires. During rotation, a technician moves from tires from side to side, or front to rear, in a prescribed pattern. Learn more about Tire Rotations.
Toe
Describes the symmetry between two horizontally situated tires, when viewed from above. Toe-in means that the tire front ends veer inward toward the engine. Toe-out refers to tire front ends that veer outward toward the road. Toe is adjusted during alignment.
Toe-Out Turns
Describes how a vehicle’s wheels on the inside of a turn follow a smaller radius than those on the outside. This occurs because the two front wheels steer at different angles when turning.
Torque
Turning or twisting effort. Usually measured in lb-ft or Newton meters.
Torque-Locking Sipes
Sipes with vertical undulation for added rigidity during cornering.
Torsion Bar
A long, straight bar fastened to the vehicle frame at one end and a suspension part at the other. This component acts like a coiled spring that absorbs energy by twisting.
Touring Tires
Tire type offering a balance of ride quality and performance attributes. Touring tires occupy the middle ground between passenger and performance tires.
Track
The distance between the outside tread edges of two tires on the same axle.
Traction
Describes the friction between the tire and the road surface. Traction also describes the level of grip a tire maintains on the road.
Tramping
Condition where a vehicle bounces up and down abnormally.
Tread
The outermost part of the tire that makes contact with the road. The tread is composed of a durable but wearable rubber layered on top of the tire carcass.
TREAD Act
Federal law that increased vehicle safety regulations. It made tire pressure monitoring systems mandatory for all vehicles produced after 2007.
Tread Buffing
Scraping rubber off the tread. Also known as shaving.
Tread Depth
A measurement of the usable tread left on a tire. Find tread depth by measuring the tread groove in the tire centerline from the groove base to the tread top. Most states consider a tire legally worn down when the tread depth measures 2/32”. Learn more about Tread Depth.
Tread Grooves
Grooves are the spaces between tread blocks in circumferential, or lateral, directions.
Tread Life
The life of a tire before it is pulled from service.
Tread Pattern
Tread pattern refers to the design of the rubber on the tire’s tread rubber.  Tread pattern designs incorporate grooves, sipes, and blocks.  The shape and arrangement of these tread pattern elements give a tire certain qualities and performance aspects.
Tread Rib
The tread section that runs around the circumference of the tire separated by tread grooves.
Treadwear Indicator
These narrow bands, sometimes called wear bars, appear across the tread when 2/32” of it remains.
Tread Width
The width of a tire’s tread.
Tri Side-by-Side Compounding
A process that makes it possible to precisely place three different rubber compounds across the tread of the tire.

U
Ultra-Low Profile Technology
Specialized sidewall shape, bead area, and bead compound that enhance the durability and mountability of tires with very short sidewalls.
Ultra-High Performance Tires
Tire type engineered for the most sophisticated and capable sports cars. UHP tires provide the maximum level of performance attributes, including optimal cornering response and high speed stability.
Underinflation
Underinflation occurs when tires aren’t filled with enough air, and can happen naturally over time. Underinflation can cause premature tire wear and reduce fuel economy. Find more detail about tire inflation on our Determining Correct Air Pressure page.
Understeer
Handling characteristic in which the front tires break loose because they are running a larger slip angle than the rear tires.
Undertread
Material between the bottom of the tread rubber and the top layer of steel belts. It acts as a cushion that enhances comfort.
Uni-Directional Tread
A tire designed to only rotate in one direction. Also called directional tread.
Unsprung Weight
The weight of the vehicle’s components not supported by its springs, including wheels and tires, outboard brake assemblies, the rear axle assembly, suspension members, springs, shock absorbers, and anti-roll bars.
UTQG (Uniform Tire Quality Grading)
This tire information system provides consumers with ratings describing a tire’s traction (AA to C) and temperature (A to C). Tire manufacturers use standardized test procedures to determine ratings, which are displayed on the tire sidewall. These ratings can only be used to compare tires produced by the same manufacturer, not from one manufacturer to another. Learn more about UTQG.

V
Valve
A device mounted in the wheel that permits or restricts airflow out of the tire. Valves include caps that maintain cleanliness, keep out moisture, and prevent unwanted air escape.
Variable Angle Sipes
Tread blocks with siping set at three different angles to enhance lateral grip.
Variable Contact Patch
A system that maximizes the contact patch area during cornering through a combination of asymmetrical tread patterns and underlying belts.
Variable Integrated Pitch
Process where different-sized tread blocks are placed around a tire’s circumference to minimize noise.
Variable Thickness or Bidirectional Sipe
Two types of sipe technology that increase the tread’s rigidity under loaded conditions, such as cornering, braking, or accelerating. This technology allows tread block surfaces to lock together in both latitudinal and longitudinal directions, resulting in precise and responsive steering, short braking distances, and improved transmission of engine torque to the driving surface.
Vertical Bouncing
Occurs when a vehicle’s weight isn’t evenly distributed around the wheel’s axis of rotation. You can feel this through the floor, seat, and steering column.
Viscous Planing
Road condition that can occur when rain wets a formerly dry surface, causing oil on the road surface to rise up and sit on the fresh moisture. Can be very slippery, even when the moisture is very thin. Continuing rain lessens this condition by washing the oil away.
Vulcanization
The irreversible process of heating rubber under pressure to improve its strength and resilience.

W
Wander
A vehicle’s tendency to stray or wander from its intended direction of travel as a result of steering abnormalities, worn tires, suspension misalignment, crosswinds, or pavement irregularities.
Wet Traction
Describes a tire’s ability to provide grip and vehicle control on wet roads, as well as resistance to hydroplaning.
Wheelbase
The longitudinal distance from the front wheel center to the rear wheel center on the same side of the vehicle.
Wheel Weights
Small weights attached to the wheel during balancing. Some wheel weights clip onto the lip of the wheels. Others, especially those commonly needed on alloy and aftermarket wheels, have an adhesive strip to be applied to the wheel barrel itself.
Winter Tires
Also called snow tires, these tires possess a special tread and compound that provides better traction in snow. Snow tires are designated M&S, M+S, or M/S on their sidewalls. Learn more about Winter Tires.

Z
Zero Offset
When the mounting face of the wheel directly aligns with the wheel’s centerline.
Zero Toe
When tires on the same axle are parallel. The fronts and rears of the tires are equidistant.