Glossary of Wheels and Tires Terminology

Tire related term

Meaning

Aging

Rubber degrades when exposed to heat, oxygen, ozone, and other elements.

Air Pressure

The amount of air inside the tire pressing outward on each square inch of tire, which is expressed in (bar), 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

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

All-season tires

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

All-terrain tires

All-terrain tires are typically used on SUVs and light trucks. These tires often have stiffer sidewalls for greater resistance against puncture when traveling off-road, the tread pattern offers wider spacing than all-season tires to remove mud from the tread.

Alphanumeric

A term for describing the size of a tire (H78-15, for example) where both letters and numbers are used.

Ackerman Angle

Also known as Toe-Out Turns. A vehicle’s wheels on the inside of a turn follow a smaller radius than the tires on the outside of the turn, because the two front wheels steer at different angles when turning.

Anti-Lock Brake System (ABS)

Under emergency braking using conventional braking systems the wheels can lock up, making the car un-steerable. ABS systems provide continuous monitoring and control of the braking force and in some circumstances can reduce the braking distance while maintaining full car steer ability.

Anti-ozidant

Specialized chemicals designed to retard ozone contamination of rubber preventing premature aging, brittleness and retention of suppleness and pliability.

Anti-slip-control (ASR)

ASR is fitted to vehicles to prevent wheels slipping, spinning on slippery or uneven surfaces.

Aquaplaning or (hydroplaning)

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

Aspect Ratio

The relationship of a tire’s sidewall height to its section width. Numerical term that expresses the ratio between section height of the tire and the cross-section width. Aspect ratio of 70 means that the tire section is approximately 70% as high as it is wide.

Asymmetrical Tread Design (AD)

Different tread patterns featured on either side of the tread that enhance and optimize performance for both wet and dry handling. The inside shoulder has more grooves for water evacuation and massive tread blocks on the outside shoulder make for maximum handling.

Balance/Imbalance

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 part of the tire that is made of steel wires. Wrapped or reinforced by ply cords and that is shaped to fit the rim.

Bead Chafer

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

Bearing

An anti-friction device that commonly uses balls or rollers held between inner and outer races. Designed to reduce friction between moving parts while providing support for the shaft. Common types of bearings are roller bearings and ball bearings.

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. It is usually oriented at opposite angle to another belt.

Belt Wedge

A tire component used by most manufacturers to prevent tread separations. A belt wedge is a piece of rubber inserted inside the tire at the belt edges.

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.

Bleeding a tire

Releasing air to reduce pressure in tire.

Blowout

A bursting of a tire and inner tube., usually associated with a loud popping noise.

Bolt Circle

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.

Braking distance

The distance required for braking depends on the speed of the vehicle, the condition of the road surface and the condition of the tires, in particular the tread. Check the tires tread depth regularly and change your tires when worn down to the "tread wear indicators" located at the bottom of the tread grooves.

Breakaway

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

Bubble

Tire bubbles, also referred to as bulges / bumps / protrusions / carbuncles, occur when the sidewall of the tire has failed, resulting in a protrusion.

Camber

A wheel’s 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.

Camber Thrust

Side or lateral force generated when a tire rolls with camber, which 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 (Casing)

The tire structure, except tread and sidewall rubber which, when inflated, bears the load.

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 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

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

Centerbore

The size of the machined hole on the back of a wheel that helps center it on the hub of a vehicle. Most wheels have large center bores to accommodate several different vehicle fitments. Hub rings can be used to improve the fit of mass produced wheels.

Centrifugal Force

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

Centrifugal growth

A tire rotating at higher speeds tends to develop a larger diameter, due to centrifugal forces that force the tread rubber away from the axis of rotation. This may cause speedometer error. This centrifugal growth can cause rubbing of the tire against the vehicle at high speeds.

Chains

Even modern winter tires can sometimes not help when there are huge amounts of snow and steep gradients. In these situations traction, lateral control and reliable braking require tire chains

Checking

Minute cracking in surface of rubber caused by aging and oxidation.

Chunking

The breaking away of pieces of the tread or sidewall.

Comfort

Indicates the tires ability to minimize road noise, harshness, and vibration as perceived by the vehicle occupants.

Compound

The general term referring to the chemical formula for the tread material.

Contact Patch

The area in which the tire is in contact with the road surface. Also called footprint. This area transmits forces between the tire and the road via friction.

Cord

The strands forming the plies in the tire. Made from steel, fiberglass, rayon, nylon, polyester or other fabrics, cords help determine the strength and load carrying capacity of a tire.

Cord separation

The parting of cords from adjacent rubber compounds.

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.

Cracking

Any parting within the tread, sidewall, or innerliner of the tire extending to cord material.

Criss-Cross Torquing

The recommended sequential tightening of the lug nuts in a pattern across from one another to help ensure even tightening.

Crown

Section between shoulders of the tires.

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.

Cure

To vulcanize; also time and temperature conditions used to vulcanize a tire.

Date of Manufacture

The date of manufacture of a tire is indicated on the tire's sidewall at the end of the DOT serial number.

Deflection

The tread and sidewall flexing where the tread comes into contact with the road. The difference between the unloaded and loaded section heights of a tire at a given load and inflation pressure.

Direction of Rotation

On standard tires with symmetrical tread patterns, it does not matter which way the tire is fitted on the rim and in which position it is fitted on the car.

Some tire manufacturers have, however, started producing tires with specific directions of rotation in order to improve wet grip and optimize noise generation.

The direction of rotation is marked on the side of the tire with an arrow. This side of the tire must be on the outside, and the tire must roll forwards in the direction of the arrow for optimum tire performance.

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.

Directional tread

Tread design in which performance is dependent upon direction of rotation.

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.

DOT Markings (code)

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

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.

Dual kissing

The intermittent contact of tires in dual as they flex due to inadequate dual spacing or overload.

Dual spacing

A measurement, in inches, from the center of the tread of one tire to the center of the tread of the other tire in dual which provides clearance between duals for air circulation.

Durometer

An instrument used to measure hardness. Specific to tires, a durometer typically 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.

ETRTO

The European Tyre and Rim Technical Organization (ETRTO) is the European standards organization "to establish engineering dimensions, load/pressure characteristics and operating guidelines" for tires, rims and valves. It is analogous to T&RA.

Feather edge

Sharp, knifelike feathering along one edge of rib or blocks, due to scrubbing action of misaligned wheel on road.

Flat spot

Irregular wear in an isolated spot or spots around the tire tread. Flat spotting can also result from abrasion of the tire resulting from brake lock.

Flat tire

A flat tire occurs when a tire deflates. This can occur as a result of normal wear-and-tear, a leak, or more serious damage. A tire that has lost sufficient pressure will impair the stability of the vehicle and may damage the tire further if it is driven in this condition.

Footprint

The portion of the tire that makes contact with the surface of the road.

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.

Free Radius

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

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.

Green Tire

A tire which has not been vulcanized or cured.

Groove

The space between two adjacent tread ribs.

Gross Axle Weight Rating (GAWR)

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

Gross Vehicle Weight (GVW)

The weight of the vehicle and its contents (fluids, passengers, 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.

Handling

Handling describes the responsiveness of the tire to steering inputs.

Heat build-up

Increase in heat of tire while in use. The temperature rise of tire can cause tire pressure build-up (increase of air pressure). Under inflation causes excessive heat build-up.

High performance tire

High performance tires are designed for use at higher speeds, and more often, a more "sporty" driving style. They feature a softer rubber compound for improved traction, especially on high speed cornering. The trade off of this softer rubber is shorter tread life.

High performance street tires sometimes sacrifice wet weather handling by having shallower water channels to provide more actual rubber tread surface area for dry weather performance

Highway tires

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

Hub Centric Rings (Hub rings)

Hard plastic or aluminum rings that are mounted on a vehicle's hub before the wheel. Their purpose is to ensure a wheel is perfectly centered on its hub. Without hub ring, a wheel is likely to vibrate even if the wheel and tire assembly has been perfectly balanced.

Hydroplaning (or aquaplaning)

Is what happens when tires lose traction as a result of worn or poorly treaded tires that can no longer effectively evacuate water away from the tread. Water accumulates under the tire footprint and creates a thin layer of water between the tire and road surface. The resulting pressure pushes upward, thus lifting the tire away from the road surface and causing it to lose traction. Vehicle speed, tread pattern and water depth all affect hydroplaning.

Hysteresis

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

Imbalance

The condition that exists when a tire's mass is not evenly distributed around the rolling axis and centerline, causing bounce (static imbalance) or shake (dynamic imbalance).

Inflation

The act of putting air into tires.

Cold tire inflation pressure

Tire pressure when the vehicle has been parked for three hours or more, or has not been driven more than 1 mile or 1.5 km under that condition.

Maximum inflation pressure

The maximum cold inflated pressure to which a tire may be inflated, shown on the sidewall of the tire.

Recommended inflation pressure

Cold tire inflation pressure recommended by the car manufacturer.

Innerliner

The layer(s) forming the inside surface of a tubeless tire that contains the inflating medium within the tire.

Innerliner separation

The parting of the innerliner from cord material in the carcass.

Inner tube

Most bicycle tires, many motorcycle tires, and many tires for large vehicles such as buses, heavy trucks, and tractors are designed for use with inner tubes. Inner tubes are torus-shaped balloons made from an impermeable material, such as soft, elastic synthetic rubber, to prevent air leakage. The inner tubes are inserted into the tire and inflated to retain air pressure.

JATMA

The Japanese Automobile Tire Manufacturers Association (JATMA) is the Japanese standards organization for tires, rims and valves. It is analogous to T&RA and ETRTO.

Kilopascal (kPa)

The metric unit for air pressure. One psi is equal to 6.9 kPa.

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, pickups, passenger vans, or SUVs.

Loaded radius

The measurement in inches from the wheel axle centerline to the ground when the tire is properly inflated for the load.

Load rating

The maximum load that a tire is rated to carry for a given inflation pressure.

Low profile tire

A tire in which the cross-section has a squat appearance. Low profile refers to the fact that the tire's cross-section height is less than 80% -85% of the section width.

Maximum load rating

Load-carrying capacity

The load rating for a tire at the maximum permissible inflation pressure requirements.

Loaded section height

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

Load index

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

Low profile tire

A tire in which the cross section has a squat appearance (section width is wider than the section height), such as in high performance tires.

LT metric

"Light Truck-metric" size that was designed to be used on vehicles that are capable of carrying heavy cargo or towing large trailers. This includes medium and heavy-duty (typically 3/4- and 1-ton load capacity) pickup trucks, sport utility vehicles and full-size vans.

Lug nut

A lug nut is a fastener, specifically a nut, used to secure a wheel on a vehicle.

Lug wrench

A lug wrench is the name for a type of socket wrench used to loosen and tighten lug nuts on automobile wheels. In the United Kingdom, this tool is commonly known as a wheel brace or wheel wrench.

M+S, M/S or

M & S

Mud and snow

All-season rating designation for tires that can perform at certain levels in mud and snow conditions. Meets 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; 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 the ETRTO (European Tire and Rim Technical Organization).

Mixing tires

Fitting tires of different sizes or constructions to a vehicle. Mixing should be avoided (especially on the same axle). Some performance vehicles, however, specify different size tires on front and rear axles.

Mold serial number

Tire manufacturers usually embed a mold serial number into the sidewall area of the mold, so that the tire, once molded, can be traced back to the mold of original manufacturer.

Mounting

This is the act of putting a tire on a wheel and ensuring that the assembly is balanced. When you purchase new tires, they need to be professionally mounted.

Negative camber

Alignment setting where the tops of the tires are leaning toward the centerline of the vehicle (looking from car front); racers use a negative camber angle for maximum cornering potential.

Negative Offset

Negative Wheel Offset

f the hub mounting surface is on the brake side of the center line of the rim, it is considered a negative offset or "deep dish".

Note that extensive negative offset can potentially cause increased steering wheel kick-back and place additional stresses on the vehicle's entire suspension.

Nominal Rim Diameter

The diameter of a tire rim, given in nearest whole numbers (e.g. 15 in.).

OE and OEM (Original Equipment)

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."

Offset

The distance between the plane of the hub-mounting surface of a wheel to the centerline of the wheel and tire assembly. Almost always measured in millimeters, a wheel offset can be positive, zero or negative.

Open splice

Any parting at any junction of tread, sidewall, or innerliner that extends to cord material.

Out-of Round

A wheel or tire defect in which the wheel or tire is not “perfectly” round. This usually results from a mistake or engineering problem in the tire manufacturing process.

Outer diameter Overall diameter

The overall diameter of an inflated new tire, without any load.

Overall width

The linear distance between the exteriors of the sidewalls of an inflated tire, including elevations due to labeling, decorations, or protective bands or ribs.

Oversteer

The tendency for a vehicle, when negotiating a corner, to turn more sharply than the driver intends. The rear end of the vehicle wants to swing toward the outside of a turn. A handling condition in which the slip angles of the rear tires are greater than the slip angles of the front tires. An oversteering car is sometimes said to be “loose,” because its tail tends to swing wide.

Overturning

or rollover

A rollover is a type of vehicle accident in which a vehicle tips over onto its side or roof. Vehicle rollovers are divided into two categories: tripped and untripped. Tripped rollovers are caused by forces from an external object, such as a curb or a collision with another vehicle. Untripped crashes are the result of steering input, speed, and friction with the ground.

Oxidation

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

Passenger car tire

A tire intended for use on passenger cars, multipurpose passenger vehicles, and trucks, that have a gross vehicle weight rating (GVWR) of 10,000 lb. or less.

Plus-sizing

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

Ply

A layer of rubber-coating parallel cords.

Ply Rating

This letter indicates the load carrying capacity of the tire in terms of its construction. A "C" indicates the tire has a 6-ply load carrying capacity. The tire is not actually built with 6 plies, but contains one or two plies of equivalent strength. A "D" is an 8-ply rating, and an "E" is a 10-ply rating. If there is no letter, the tire has a standard 4-ply rating.

Ply separation

A parting of rubber compound between adjacent plies.

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.

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 (looking from car front); 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

Positive Wheel Offset

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

Pressure build up

Increase of air pressure in tire caused by temperature rise of tire.

PSI

The most common measurement unit for tire pressure that stands for pounds per square inch. PSI is intended to measure the force exerted by the air inside a tire.

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

Puncture

Any penetration of a tires air chamber by a foreign object, nail, glass, etc. resulting in the loss of air. Such loss can be rapid with the collapse of the inner tube.

Radial Ply tire

A pneumatic tire in which the ply cords that extend to the beads are laid at substantially 90 degrees to the centerline of the tread.

Rain groove

The rain groove is a design element of the tread pattern specifically arranged to channel water away from the footprint.

R-Compounding

A racing-derived compound optimized for on-track performance and designed for maximum dry grip and repeated heat cycles.

Recap

Another term for a retreaded tire.

Replacement tire

Any tire other than those sold as original equipment.

Retreading

Refers to the practice of applying new tread to a used tire casing. Most commonly used on medium and heavy commercial trucks.

Reused tire

A passenger car tire taken off the rim of a vehicle and placed on the rim of another vehicle without any repair or other form of alteration.

Revolutions Per Mile (RPM)

Also called rpm. 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

A metal support for a tire or a tire and tube assembly upon which the tire beads are seated.

Rim diameter (wheel diameter)

Nominal diameter of the bead seat.

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 size designation

Rime diameter and width

Rim type designation

The industry manufacturer’s designation for a rim by style or code.

Rim width

Nominal distance between rim flanges.

RMA

Rubber Manufacturers Association; The trade association which represents the United States tire manufacturers and the United States based manufacturers of non-tire rubber products.

Road hazard

Injury to a tire sustained in normal operation.

Road shock

Impact of road irregularities transmitted through tires to vehicle.

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

Rolling resistance is the resistance to rolling caused by deformation of the tire in contact with the road surface. It is 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. See your tire warranty for more information on recommended rotation.

Rubber

An elastomer, generally implying natural rubber, but used loosely to mean any elastomer, vulcanized and unvulcanized. By definition, rubber is a material that is capable of recovering from large deformations quickly; and forcibly and can be, or already is modified to a state in which it is essentially insoluble in a boiling solvent.

Run Flat Technology

Tires that are designed to resist the effects of deflation when punctured, and to enable the vehicle to continue to be driven at reduced speeds and for limited distances

Runout

The amount a wheel moves in and out, away from its true center as it is rotated. If runout is excessive, the wheel can be seen to wobble as it rotates.

Scrap tire

A tire which can no longer be used for its original purpose, due to wear or damage.

Section

A slice of a tire from one bead, through the tread to the other bead.

Section height

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

Section width

The linear distance between the exteriors of the sidewalls of an inflated tire, excluding elevations due to labeling, decoration, or protective bands.

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.

Semi-Pneumatic Tires

Rubber tires that have a hollow center, but do not use pressurized air for structural support.

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, or the speed rating. Also known as load index and speed symbol.

Shake

Static Imbalance occurs when there is a heavy or light spot in the tire that prevents the tire from rolling evenly and causing the tire and wheel to undergo an up-and-down motion. The static imbalance creates a hop or vertical vibration.

Shimmy

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

Shock absorber

A fluid type cylinder which stops the car from bouncing after the initial shock has been absorbed by the spring. Most cars have four shock absorbers.

Shoulder

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

Sideslip

A vehicle will skid or slide outwards when the centrifugal force or cross wind exceeds the frictional forces between tires and ground. A vehicle sliding or drifting sideways may hit a curbstone or grass verge which brings the slide to zero in a very short period of time. A sudden lateral deceleration produces an inertia force acting through the center of gravity in addition to the centrifugal force, and together they provide a large overturning couple

Sidewall

That portion of a tire between the tread and bead.

Sidewall separation

The parting of the rubber compound from the cord material in the side wall.

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 then close, breaking the water tension on the road surface and putting rubber in contact with the road to maintain adhesion, increasing wet and snow traction.

Size

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

Skid

To slip or slide on the road when tires lose their rolling grip.

Skid marks

Skid marks caused by tires on roads occur when a vehicle wheel stops rolling and slides or spins on the surface of the road. Skid marks can be analyzed to find the maximum and minimum vehicle speed prior to an impact or incident.

Slip

$slip=\frac{\omega r - v}{v},$

The difference between the linear speed of the vehicle (v) and the rotational speed of the tire (ωr). 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 chains

Snow chains, or tire chains, are devices fitted to the tires of vehicles to provide maximum traction when driving through snow and ice. Snow chains attach to the drive wheels of a vehicle. Chains are usually sold in pairs and must be purchased to match a particular tire size (tire diameter and tread width). Driving with chains reduces fuel efficiency, and can reduce the speed of the automobile to approximately 50 km/h (30 mph).

Snow tire

Snow tires are tires designed for use in colder weather, snow and ice. Snow tires have more sipes than summer tires, increasing traction on snow and ice, but reducing grip on dry and wet roads.

Solid Tires

Rubber tires similar in construction to semi-pneumatic tires, but without a hollow center.

Spare tire

Some vehicles carry a spare tire, already mounted on a wheel, to be used in the event of flat tire or blowout. Minispare, or "space-saver spare" tires are smaller than normal tires to save on trunk/boot space, gas mileage, weight, and cost. Minispares have a short life expectancy and a low speed rating, often below 60 miles per hour (97 km/h).

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.

Spider wrench

Spider wrench, is made in the shape of a cross with different sized sockets on each of the four ends. It used to loosen and tighten lug nuts on automobile wheels.

Spin

The difference between the linear speed of the vehicle (v) and the rotational speed of the tire (ωr). For example, if a tire is spinning on the spot, v = 0 and ωr ≠ 0, spin = ∞.

Sprung weight

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

Stability

Situation in which the driver maintains control of the vehicle.

Staggering

A staggered fitment is putting larger wheels on the back of your vehicle than the front of your 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

A vehicle’s reaction to a driver’s steering inputs. Also the feedback that drivers get through the steering wheel as they make 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.

Stopping (braking) distance

It refers to the distance a vehicle will travel from the point when its brakes are fully applied to when it comes to a complete stop. It is primarily affected by the original speed of the vehicle and the coefficient of friction between the tires and the road surface, and negligibly by the tires' rolling resistance and vehicle's air drag. The braking distance is one of two principal components of the total stopping distance. The other component is the reaction distance, which is the product of the speed and the perception-reaction time of the driver/rider

Suspension

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

Studded tires

Winter tires have better traction on snowy and icy roads than all-season tires, thanks to special tread compounds that stay flexible and grippy even in very cold weather. In some cases, grip and traction can be improved even more with studded tires. These tires feature metal studs that dig into snow or ice, and may improve acceleration and braking in these conditions. Metal studs should only be installed in new tires that are molded for studs. And like all winter tires, studded tires should be used on all four wheels of a vehicle.

Synthetic rubber

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

Take-Off Tire

A tire that has been removed from a vehicle's rim. This includes tires that are used, are candidate casings for repair, retreading, and to be scraped.

Temporary Spare Tires

Temporary spares are designed to carry the same load as the standard size tire on your vehicle and can be applied to any position. Maintain the proper inflation pressure as shown on the sidewall of the tire, it requires a higher inflation pressure than a standard size tire.

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 (Tyre)

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 Deflection

Situation where the tread and sidewalls flex at the point where the tread comes in contact with the road.

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 Maintenance

The practice of establishing and ensuring proper air inflation, tire rotation, balancing, and alignment.

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.

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.

Tire Pressure Gauge

Tool used to properly measure the air pressure in a tire.

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.

Tire Rotation

Process of moving wheels and tires from one position on a vehicle to another in order to promote even tire wear and prolong tread life.

Tire Storage

Tires should be stored in a dry, cool place, away from sunlight and sources of ozone.

T&RA

The Tire and Rim Association (T&RA) is a voluntary U.S. standards organization to promote the interchangeability of tires and rim and allied parts.

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’s wheels on the inside of a turn follow a smaller radius than the tires on the outside of the turn, because the two front wheels steer 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.

Track

Track width, also known as ‘tire stance’, is the distance measured perpendicularly from the center of one wheel (or impression) to the opposite wheel (or impression). The track width of the rear wheels is usually different from that of the front wheels.

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.

TREAD Act

The Transportation Recall Enhancement, Accountability and Documentation Act (TREAD Act) is a United States federal law that sets standards for testing and the reporting of information related to products involved with transportation such as cars and tires.

Tread Blocks

Raised rubber compound segments in a tire’s tread. Blocks of varying sizes, shapes, and designs are typically used for a variety of effects, such as better wet traction, or stronger rock-crawling ability.

Tread Buffing

Scraping rubber off the tread. Also known as shaving.

Tread Depth

The distance measured from the tread surface to the bottom of the grooves in a tire.

Tread Design

The pattern or layout of grooves, blocks, sipes and other tread elements.

Tread Life

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

Tread Lug

Tread lugs provide the contact surface necessary to provide traction.

Tread pattern

The arrangement of blocks, grooves, sipes, and channels designed into the tread to enhance its grip. Also called the tread design.

Tread rib

A tread section running circumferentially around a tire.

Tread separation

Pulling away of the tread from the tire carcass.

Tread void

Tread voids provide space for the lug to flex and deform as it enters and exits the footprint. Voids also provide channels for rainwater, mud, and snow to be channeled away from the footprint.

The void ratio is the void area of the tire divided by the entire tread area. Low void areas have high contact area and therefore higher traction on clean, dry pavement.

Tread wear

There are several types of abnormal tread wear. Poor wheel alignment can cause excessive wear of the innermost or outermost ribs. Gravel roads, rocky terrain, and other rough terrain causes accelerated wear. Over-inflation above the sidewall maximum can cause excessive wear to the center of the tread. Under-inflation can cause excessive wear to the sides of the tread.

Treadwear indicators (TWI)

The projections within the principle grooves designed to give a visual indication of the degrees of wear of the tread.

Tread Width

The width of a tire’s tread.

Tube

An airtight container placed inside tire casing to hold the compressed air.

Tubeless

A tire without a tube. The inner tube is integral within the tire. The valve is permanently fixed to the rim. The assembly is air tight. When punctured the loss of air will be slow.

Turning diameter

Every vehicle has a turning radius or turning diameter which represents the smallest radius that that particular vehicle type can turn in a circle.

Ultra-Low Profile Technology

Specialized sidewall shape, bead area, and bead compound that enhance the durability and mountability of tires with very short sidewalls.

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. Under inflation causes excessive heat build-up and internal structural damage.

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.

Uni-Directional Tread

Also known as directional tread, this is a tire designed to only rotate in one direction.

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.

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.

Valve –or-

Valve stem

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

Valve Cap

The valve cap, although small, has a very important job: it protects the sensitive valve internals from dust, dirt and humidity. Without a functional tire valve cap in place, the tires' air will seep out.

Vertical Bouncing

Vertical bouncing, or static imbalance, exists when the weight is not evenly distributed around the wheel’s axis of rotation. You can feel this through the floor, seat and steering column.

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 lessens the condition by washing the oil away.

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.

Wear bar

Wear bars (or wear indicators) are raised features located at the bottom of the tread grooves that indicate the tire has reached its wear limit.

Wet traction

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

Wheel

Tires are mounted onto wheels that most often have integral rims on their outer edges to hold the tire. Automotive wheels are typically made from pressed and welded steel, or a composite of lightweight metal alloys, such as aluminum or magnesium. A decorative hubcap and trim ring may be placed over the wheel.

Wheel Alignment

Refers to the proper angle settings of suspension components as they relate to the angles of the wheels so they are perpendicular to the ground and parallel to each other so as to help prolong tire life and provide dead straight tracking on straight and level roads. Related terms include camber, caster and toe.

Wheel Balancing

Refers to the balancing of a wheel on vertical axis by using lead weights to compensate for uneven weight distribution on a wheel and tire. Out-of-balance wheels can cause a vehicle to vibrate at certain speeds and produce uneven tire wear.

Wheelbase

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

Wheel Diameter

The physical diameter of a wheel expressed in inches. Whole increments (i.e. 16, 17 or 18) are the most common, however, a few manufacturers offer wheels with diameters of 17.5 inches, though this is rare

Wheel weights

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

Wheel Width

The physical width of a wheel measured inside the outer lip of the wheel, usually in 1/2 increments (i.e. 7.5 or 8.5).

White sidewall (WSW) tires

Or whitewall tires are tires having a stripe or entire sidewall of white rubber.

Winter tire

Winter tires are tires designed for use on ice and snow, and lower temperatures in general. Winter tires start to perform better than summer or all-season tires if temperatures fall below 7 °C (45 °F)

Wires

High tensile, brass plated steel wires, coated with a special adhesion-promoting compound that are used as tire reinforcement. Belts of radial tires plies and beads are common uses.

Wobble

A performance irregularity in tire and wheel assemblies, characterized by a side to side motion.

Zero Pressure™ Technology

Michelin® Zero Pressure™ tires feature a reinforced sidewall that is designed to support the weight of your car even after a loss of air pressure — even with no air in the tire. Michelin® Zero Pressure Technology allows you to continue driving up to 50 miles at 55 mph, so you don’t have to change a tire on the side of a busy highway. Best of all, Zero Pressure Technology has been applied to some of the best-performing Michelin® tires.

Zero Offset

Zero Wheel Offset

When the hub mounting surface is centered within the rim, it is known as a zero offset.

Zero toe (Neutral toe)

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

Tire Construction & dimensions

Tire construction

Tire construction and dimensions

Tire dimensions

Rim dimensions