A

Air Pressure

The amount of air inside the tire pressing outward on each square inch of tire; expressed in pounds per square inch (psi) or kiloPascals (kPa), the metric designation for air pressure.

Airtight synthetic rubber

Formulated with virtually impermeable butyl rubber, this material replaces the inner tube in modern, tubeless tires. Check you air pressure monthly, as some air loss occurs over time.

Alignment

Adjusting all wheels on the vehicle so that they are pointed in the optimum direction relative to the road and each other. Adjusting all wheels on the vehicle so that they are pointed in the optimum direction relative to the road and each other.

All-season Tires

Provide a good balance of traction in rain or snow with good tread life and a comfortable, quiet ride.

All-season high-performance tires

Deliver a measure of traction on snow and ice without sacrificing dry performance driving capabilities.

All-seasonTraction

Indicates the tire's ability to provide a balance of traction in wet, dry, and winter conditions.

Aquaplaning

An extremely dangerous situation where water builds up in front of the tires resulting in the tires losing contact with the road surface. The vehicle is skimming on the water surface and is completely out of control at this point. Also called hydroplaning.

Aramid

A synthetic fabric used in some tires that is (pound-for-pound) stronger than steel.

AspectRatio

The dimensional relationship of a tires section height to its section width expressed as a percentage.

B

BalanceImbalance

The state in which a tire and wheel spin with all their weight distributed equally. To correct an imbalance, a trained mechanic will add weights on the interior or exterior of the wheel.

Bead

The section of the tire that sits on the wheel. Inside, there is a round hoop of steel wires, wrapped or reinforced by body ply cords, that clamps the tire firmly against the wheel rim.

BeadChafer

A key component of the tire that is the contact point between the tire and the wheel, designed to withstand the forces the wheel puts on the tire during mounting and the dynamic forces of driving and breaking.

BeadFiller

Responsible for transferring propulsion and braking torque from the wheel rim to the road surface contact area.

Belt

A rubber-coated layer of cords that is located between the body plies and the tread. Cords are most commonly made from steel but may also be made from fiberglass, rayon, nylon, polyester or other fabrics.

BoltCircles

The diameter of an imaginary circle drawn through the center of each lug nut hole and then measured from two holes that are directly across from each other. The measurement is used in selecting the proper wheel for replacement.

BrakingTorque

A technique practiced by drag racers and road testers to improve their off-the-line acceleration; applying the brake and throttle at the same time, increasing the engine rpm until release of the brake.

Breakaway

A term used to describe a loss of traction when negotiating a curve or when accelerating from a standing start. The tires slide, instead of grip, against the road surface.

C

Camber

It reflects the degree of inward or outward tilt from vertical, measured in degrees. The camber angle is adjusted to keep the outside tires flat on the ground during a turn.

CamberThrust

Side or lateral force generated when a tire rolls with camber; can add to or subtract from the side force a tire generates.

Carbon black

This is a reinforcing filler which, when incorporated into the tire rubber compound, gives it a high resistance to wear.

Carcass

The supporting structure of the tire consisting of plies anchored to the bead on one side and running in a radius to the other side and anchoring to the bead. Also called casing.

Carcass ply

Made up of thin textile fiber cables bonded into the rubber. These cables are largely responsible for determining the strength of the tire.

Carrying capacity

At a given air pressure, how much weight each tire is designed to carry. For each tire size, there is a load inflation table that it used to ensure the inflation pressure used is sufficient for the vehicle axle load.

Caster

The angle between a line drawn vertically through a wheel's centerline and the axis around which the wheel is steered; improves a car's directional stability and on-center feel.

Centerline

Imaginary line down the center of the vehicle. Alignment tracking is measured from this line.

Centrifugal force

The sideways acceleration, measured in g of an object in curvilinear motion. As a car traverses a curve, centrifugal force acts on it and tries to pull it outward. To counter act this, the tires develop an equal and opposite force acting against the road. Also called lateral force.

Cold inflation pressure

The amount of air pressure in a tire, measured in pounds per square inch (psi) before a tire has built up heat from driving.

Contact patch

Area in which the tire is in contact with the road surface. Also called footprint.

Cord

The strands of fabric forming the plies or layers of the tire. Cords may be made from polyester, rayon, nylon, fiberglass or steel.

Cornering force

The force on a turning vehicle's tires - the tire's ability to grip and resist side force - that keeps the vehicle on the desired arc.

Crown plies

Provide the rigid base for the tread, which allows for good gas mileage. The plies also provide centrifugal and lateral rigidity to the tire, while being designed to flex sufficiently for a comfortable ride.

Curb weight

Weight of a production vehicle with fluid reservoirs (including fuel tank) full and all normal equipment in place, but without driver or passengers.

D

DOT markings

A code molded into the sidewall of a tire signifying that the tire complies with U.S. Department of Transportation motor vehicle safety standards.

Deflection

Free radius minus loaded radius.

Directional stability

The ability of a vehicle to be driven safely and with confidence in a straight line and at high speed without being affected by pavement irregularities, crosswinds, aerodynamic lifting forces, or other external influences.

Dog tracking

Track is the width between the outside tread edges of tires on the same axle. Tracking, or more specifically Dog Tracking; refers to a condition in which the vehicle is out of alignment, and the rear wheels do not follow in the path of the front wheels when the vehicle is traveling in a straight line. Also called tracking

Drift

Drift refers to a vehicle deviating from a straight-line path when no steering input is given. Also called pull.

Duals

Tires placed side by side on an axle to increase both carrying capacity and traction capability; four tires across an axle.

Durashield

Contructed with Rubber coated pyramidal steel belts, Insulating belt edge strips and Heavy gauge innerlined for a cool running tire that provided long tread life and exceptional durability.

Durometer

An instrument used to measure hardness. Specific to tires, a durometer is usually measures the hardness of the tread compound. Durometer can also refer to the hardness result, as in "The tire's durometer is 60".

Dynamic balance

Exists when the weight is equally distributed both around its circumference and on either side of its centerline. A tire and wheels assembly that is out of dynamic balance will produce a wobble effect or a shaking from side to side.

E

E.C.E. symbol

Economic Commission of Europe develops motor vehicle requirements; ECE approved tires must meet standards for physical dimensions, branding requirements and high-speed endurance regulations.

Eccentric mounting

Mounting of a tire wheel assembly in such a way that the center of rotation for the assembly is not aligned with the center of rotation for the vehicle's hub.

Extra load

Tires that are rated to carry a higher load by virtue of having a maximum inflation pressure higher than the standard maximum.

F

Fore-and-aft weight transfer

Transfer of weight from the front axle to the rear axle (or vice versa) caused by acceleration or braking. Acceleration causes weight transfer from the front axle to the rear axle. Braking causes weight transfer from the rear axle to the front axle.

Four-wheel-drift

A handling term describing a car with its front and rear tires sliding in a controlled manner. The driver uses both throttle and steering to keep the vehicle on a prescribed path.

Free radius

The radius of the tire/wheel assembly that is not deflected under load.

G

Groove

The space between two adjacent tread ribs; also called tread grooves.

Gross Axle Weight Rating

The maximum weight that can be distributed among the tires on a given axle.

Gross Vehicle Weight

The weight of the vehicle and its contents (fluids, passenger, and cargo).

Gross Vehicle Weight Rating (GVWR)

The maximum weight allowed for the vehicle and its contents. This value is established by the vehicle manufacturer and can be identified on the vehicle door placard.

H

Handling

The tire's ability to navigate the twists and turns of everyday driving conditions, as well as the ability to provide good "on-center" steering feel and true tracking for highway driving.

High flotation sizing system for light trucks

Lower sidewall, wider tread tires that yield better traction on surfaces like sand and soft soil found in watery, off-road situations.

High-performance tires

Also called;maximum performance;ultra-high-performance, etc., offer a superior degree of handling, grip, and cornering ability than standard tires. High-performance tires are also rated for operation at higher speeds than non-high-performance tires.

Highway tires

Also called summer tires; designed for wet-and-dry weather driving, but not for use on snow and ice.

Hub-centric

Wheels are manufactured to fit either the hub or the lugs. Hub-centric is matching the hub hole of a custom wheel perfectly to the diameter of the hub of the vehicle.

Hysteresis

When rubber stretches and compresses it does not render all the energy applied to it. Energy is lost due to internal friction. The mechanical energy is transformed into thermal energy and the heat produced leads both to damage and to energy loss.

I

Inflation

The act of putting air into tires.

Inner liner

The innermost layer of a tubeless tire, compounded with virtually impermeable butyl rubber. Some air loss over time will occur. Check your pressures monthly to ensure safe reliable operation of your tires.

K

Kilopascal

The metric unit for air pressure. There are 6.9 kPa to one psi.

L

Lateral runout

Side-to-side wobbling of a wheel as it rotates; a shimmy.

Lateral weight transfer

When a vehicle travels through a curve, weight is transferred from the wheels on the inside of the curve to the wheels on the outside of the curve. This is a result of the centrifugal force, or lateral force acting on the vehicle.

Light truck

Automotive industry term for smaller trucks, pick-ups, vans, or SUVs.

Linearity

A term used to characterize steering response.

Load Index, Speed Symbol

The load index is an assigned number ranging from 0 to 279 that corresponds with the load-carrying capacity of a tire. Most passenger car tire load incides range from 75 to 105. Using an example 205/60R15 91V, the load index of 91corresponds to a load carrying capacity of 1356 lbs at maximum inflation pressure. The V speed symbol corresponds to a maximum speed of 240 km/h.

The speed rating of a tire indicates the highest speed at which the tire can carry a specified load under specified conditions. Lettres from A to Z symbolize a tire's certified speed rating, ranging from 5 km/h to above 300 km/h. This rating system described the top speed for which a tire is certified. It does not indicate the total performance capability of a tire.

Load index

An assigned number ranging from 0 to 279 that corresponds to the load carrying capacity of a tire.

Load range

Defines a range of maximum loads that tires can carry at a defined pressure.

Load-carrying capacity

Indicates how much weight a tire is certified to carry at maximum inflation pressure.

Loaded radius

Distance from wheel axis of rotation to supporting surface at a given load and stated inflation pressure. Also called Static loaded radius.

Loaded section height

The height of the section of the tire that is making contact with the road.

Lug-centric

Wheels are manufactured to fit either the hub or the lugs. Lug-centric is matching the lug holes of a custom wheel perfectly to the lug pattern of the vehicle.

Luxury performance touring tires

Generally designed for Luxury sedans, this breed of tires blend performance handling with a comfortable, smooth ride.

M

M+S, M/S or M & S

All-season rating designation for tires that can perform at certain levels in mud and snow conditions. Meets Rubber Manufacturers Association (RMA) definition of a mud and snow tire.

Match mounting

Technique that matches the harmonic high point of a tire with a low point of the wheel to insure optimal ride performance.

Maximum Load

Maximum weight that can be carried and the maximum pressure needed to support that load.

Maximum inflation pressure

The maximum air pressure to which a cold tire may be inflated; can be found molded onto the sidewall.

Metric tire size system

One system used to describe a tire's size. It is the standard system of ETRTO (European Tire and Rim Technical Organization).

Mounting

This is the act of putting the tire on wheel and ensuring that the assembly is balanced. When you purchase new tires, they will need to be professionally mounted. It is also standard for the tire dealer to charge a nominal fee for a valve stem.

Mud Traction

The tire's ability to provide grip and traction for starts and stops in off-road conditions such as mud, turf, etc.

N

NHTSA

National Highway Traffic Safety Administration

NailGard

A unique patented material that automatically seals most tread punctures and prevents air leaking. As soon as a puncturing object 3/16" or less in diameter, such as a nail, passes through the tire tread its instantly coated with the NailGard sealant.

Negative camber

Alignment setting where the tops of the tires are leaning towards the centerline of the vehicle; racers use a negative camber angle for maximum cornering potential.

Negative offset

The wheel mounting face is closer to the brake side of the wheel moving the tire and wheel assembly out of the fender well.

O

Off-the-car balancing

To achieve the optimal weight balance between the tire and the wheel, the assembly can be taken off the vehicle and balanced to eliminate both side-to-side shimmy and hopping up and down.

Offset

The offset of the rim is what locates the tire/wheel assembly in relation to the suspension. A wheel with zero offset has a mounting face that directly aligns the wheel's centerline.

Original equipment

Tires selected by a vehicle manufacturer that best matches tire performance to vehicle performance characteristics. Also known as OE.

Original equipment

Tires selected by a vehicle manufacturer that best matches tire performance to vehicle performance characteristics. Also known as OE.

Overall diameter

The diameter of the inflated tire, without any load.

Overall width

The distance between the outside of the two sidewalls, including lettering and designs.

Overinflation

Too much air in the tire, resulting in premature wear in the center of the tread.

Oversteer

Condition when the rear wheels are carving a larger arc than the front wheels or the intended line of the turn. Rear "slip angles" exceed those of the front tires. This is often described as a "loose" condition, as the car feels like it may swap ends, or be "twitchy."

Oxidation

Rust process that takes place in the steel belts when moisture, via damage, is allowed to get inside the tire. Can result in the tire becoming unserviceable before normal replacement time.

P

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 is P205/70R14 93S.

PSI

An abbreviation for pounds per square inch (psi).

Plies

One of the rubber-coated layers, that make up the structure of a tire. Plies are usually reinforced with either textile or steel cords.

Plus-sizing

An option allowing drivers to customize their vehicles by mounting low-profile tires on wider rims of one or two inches greater diameter, usually enhancing vehicle appearance, handling, and performance.

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 or tread.

Pneumatic tire

A tire designed to be filled with air.

Positive camber

Alignment angle that makes the top of the tires farther apart than at the bottom; tires are tilted out from the centerline of the vehicle.

Positive caster

Alignment setting when the steering axis is inclined rearward at the top.

Positive offset

The mounting face of a wheel is towards the wheel's street side, moving the tire and wheel assembly in towards the vehicle.

Product Code

Identification number related to the line and size of a specific tire used by tire retailers to order products.

Pull

A condition in which a vehicle swerves to one side without being steered in that direction, as a result of irregular tire wear, improper front and/or rear wheel alignment, or worn or improperly adjusted brakes.

Q

Quiet Ride

The tire's ability to minimize road noise, harshness, and vibrations as perceived by the vehicle occupants.

R

Radial ply tire

A type of tire with plies arranged so cords in the body run at 90-degree angles to the center line of the tread.

Revolutions per mile

Measured number of revolutions for a tire traveling one mile. This can vary with speed, load, and inflation pressure.

Ribs

A pattern of tread features aligned around the circumference of a tire. There are usually multiple ribs across the tread area of a tire.

Ride height

The distance from the ground to a fixed reference point (differs by automaker) on the vehicle's body. This dimension can used to measure the amount of suspension travel or the height of the body from the ground.

Rim

That portion of a wheel to which a tire is mounted.

Rim diameter

Diameter of the rim bead seats supporting the tire.

Rim drop

Also called "drop center", a change (drop) in the rim profile between the rim flanges in which the bead area of a tire is placed during the mounting process. This allows the tire to be mounted on the rim.

Rim flange

Surface of the rim of the wheel that contacts the side of the tire bead.

Rim width

Distance between the two opposites inside edges of the rim flanges.

Rolling circumference

The linear distance traveled by a tire in one revolution (its circumference). This can vary with load and inflation. Rolling circumference can be calculated as follows: 63,360 divided by revolutions per mile = rolling circumference in inches.

Rolling resistance

The force required to keep a tire moving at a uniform speed. The lower the rolling resistance, the less energy needed to keep a tire moving.

Rotation

The changing of tires from front to rear or from side-to-side on a vehicle according to a set pattern; provides even treadwear. Rotating your tires on a regular basis (every 6,000-8,000 miles) is a simple way to add miles to their life.

Rubber compound

A combination of raw materials blended according to carefully developed procedures. The rubber compound is specially adapted to the performance required of each type of tire.

Runout

Radial runout is an out-of-round situation producing a vibration as the wheel spindle moves up and down.

S

Section height

The height of the tire, measured from rim to the outer tread.

Section width

The distance between the outside of a tire sidewalls, not including any lettering or designs.

Self-aligning torque

When the tire is cornering, torque created at the road contact patch acts at a point somewhat to the rear of the actual wheel center due to pneumatic trail. This has the same effect as positive caster and tends to force the wheel back to the straight-ahead position.

Series

Tires with the same aspect ratio, or relationship of section height to section width.

Service description

Numbers and letters molded into the sidewall indicating the load-carrying capacity, load index, and the speed at which the tire can carry a load under specified conditions, the speed rating. Also known as load index and speed symbol.

Shimmy

Wobbling of wheels from side-to-side on a vehicle. Shimmying can be caused by a variety of factors, including improperly balanced tires, poor alignment, and bent wheels.

Shoulder

The area of a tire where the tread and sidewall meet.

Sidewall

That portion of a tire between the tread and the bead. Protects the tire against impacts with curbs, etc. This is also where the sidewall markings can be found which tell you important information regarding the tire.

Silica

In 1992, Michelin discovered how to incorporate this new reinforcing filler into the rubber compound. The discovery paved the way for compounds that provided resistance to wear, low rolling resistance, and good road holding.

Singles

One tire mounted on each side of an axle (two tires per axle).

Sipes

Special slits within a tread block that open as the tire rolls into the contact patch and close, breaking the water tension on the road surface and putting rubber in contact with road to maintain adhesion; increase wet and snow traction.

Size

The combination of tire width, construction type, aspect ratio, and rim size used in differentiating tires.

Slip

The difference between the linear speed of the vehicle and the rotational speed of the tire. For example, if a tire is locked and sliding (e.g., not rotating) while the vehicle is still moving then it is operating at -100% slip.

Slip angle

The difference between the direction the wheel is traveling and the direction the vehicle is traveling.

Snow tire

Also called winter tire; a special type of tire with a tread pattern and compound that gives better traction in snowy and icy conditions; identified by the M+S, M&S, or M/S on the sidewalls. Quality winter tires bear the RAC Mountain Snowflake symbol to indicate that they meet snow traction performance requirements.

Speed rating

An alphabetical code (A-Z) assigned to a tire indicating the range of speeds at which the tire can carry a load under specified service conditions.

Sprung weight

The parts of a car that are supported by its springs, including the frame, engine and body.

Squirm

Flexing of the tread blocks between the belt package and the road surface. Less squirm means better steering response, more squirm means worse steering response.

Stability

Situation in which the driver maintains control of the vehicle.

Standard load

The amount of weight a given 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 imbalance can be detected from vibrations through the seat, floor and steering column.

Static loaded radius

Distance from the wheel axis of rotation to supporting surface at a given load and stated inflation pressure.

Steel belt

The combination of steel cords covered with rubber that forms a strip or belt placed under the tread rubber and on top of the casing (carcass); ensures uniformity when the tire is rotating and helps prevent flats.

Steering response

Vehicle reaction to a driver's steering inputs. Also the feedback that a driver gets through the steering wheel as he makes steering inputs.

Steering system

The entire mechanism that allows the driver to guide and direct the vehicle; includes the steering wheel, steering column, steering gear, linkages, and wheel supports.

Structure

The way in which a tire carcass is constructed. Radial structure tires can be identified by the word radialor by the letter R; and today account for the majority of vehicle tires.

Suspension

The various springs, shock absorbers and linkages used to suspend a car & frame, body, engine, and drivetrain above its wheels.

Synthetic rubber

Man-made, as opposed to natural, rubber. Most of today's passenger car and light truck tires have a relatively small content of natural rubber.

T

T&RA

Tire and Rim Association

Tensile strength

An object's resistance to stretching or breaking when placed in tension. Steel belts in a tire are characterized and compared based on their tensile strength.

Tire

Also called "pneumatic tire", a precisely engineered assembly of rubber, chemicals, fabric, and metal, designed to provide traction, cushion road shock and carry a load under varying conditions.

Tire designation

An alphanumeric code molded into the sidewall of the tire that describes the tire's size, including width, aspect ratio, rim diameter, load index, and speed rating. Most designations use the P-Metric system.

Tire mixing

A situation in which tires of various brands, types, or sizes are mixed on a vehicle. This can lead to variations in the vehicle's ride and handling characteristics. This is not an ideal situation and should be avoided if possible.

Tire placard

A metal or paper tag permanently affixed to a vehicle, which indicates the appropriate tire size and inflation pressures for the vehicle. The placard can ordinarily be found on either the driver's doorpost, the glove box lid, or the fuel filler door.

Toe

The difference in distance between the front and rear of a pair of tires mounted on the same axle.

Toe-in

The fronts of two tires on the same axle are closer than the rears of the tires.

Toe-out

The fronts of two tires on the same axle are further apart than the rears of the tires.

Toe-out turns

Also known as Ackerman Angle. A vehicle' wheels on the inside of a turn follow a smaller radius than the tires on the outside of the turn, the two front wheels steering at different angles when turning.

Torque

Turning or twisting effort, usually measured in lb-ft or Newton meters.

Torsion bar

A long straight bar fastened to the frame at one end and to a suspension part at the other; acts like an uncoiled spring that absorbs energy by twisting.

Touring tires

Generally offer increased tread life, comfort, and all-season traction.

Track

The distance between the outside tread edges of two tires on the same axle.

Traction

The friction between the tires and the road surface; the amount of grip provided.

Tramping

A state in which a vehicle bounces up and down abnormally.

Tread

That portion of a tire that comes into contact with the road. It is distinguished by the design of its ribs and grooves. Provides traction in a variety of conditions, withstands high forces, and resists wear, abrasion, and heat.

Tread buffing

Scraping rubber off the tread. Also known as shaving.

Tread depth

The depth of usable tread rubber measured in 32nds of an inch. If a tire comes new with 10/32nds of rubber, you have 8/32nds of usable rubber. Tires have to be replaced when the wear bars are visible at 2/32nds.

Tread life

The life of tire before it is pulled from service; mileage.

Tread rib

The tread section that runs around the circumference of the tire separated by the tread grooves.

Tread width

The portion of the tread design which comes in contact with the road.

Treadwear indicator

Narrow bands, sometimes called "wear bars," that appear across the tread of the tire when only 2/32 inch of tread remains.

U

UTQGS

Also known as Uniform Tire Quality Grading Standards. A government-sponsored tire information system that provides consumers with ratings (from AA to C) for a tire's traction and temperature. Treadwear is normally rated from 60 to 700.

Underinflation

Operating a tire without sufficient air pressure to support the weight of the vehicle with occupants and additional load; could cause failure of the tire when heat is generated inside the tire to the point of degeneration of components.

Understeer

The handling characteristic in which the front tires break loose because they are running a larger slip angle than the rear tires. Also known as plowing.

Undertread

Material between the bottom of the tread rubber and the top layer of steel belts; acts a cushion that enhances comfort.

Unsprung weight

The weight of the parts of a vehicle 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.

V

Valve

A device that lets air in or out of a tire. Fitted with a valve cap to keep out dirt and moisture and a valve core to prevent air from escaping.

Variable integrated pitch

The process of varying the size of tread blocks around the circumference of a tire to minimize the noise generated by the tire as it rolls.

Viscous planing

Can occur just after a rain shower wets down a dry road surface. Oil on the road surface migrates to the top of the layer of moisture, and can be very slippery, even when the layer of moisture is very thin. Continuing rain mitigates the 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

Indicates how efficiently the tire disperses water to combat aquaplaning, and how well it grips wet roads in low-speed driving.

Wheel weights

Small weights attached or secured to the wheel to balance the tire and wheel assembly.

Wheelbase

The longitudinal distance from the center of the front wheel to the center of the rear wheel on the same side of the car.

Winter Traction

The tire's ability to provide grip and traction for starts and stops in winter conditions such as snow.

Z

Zero offset

The mounting face of the wheel directly aligns with the wheel centerline.

Zero toe

When tires on the same axle are parallel; the fronts and rears of the tires are equidistant.