Definitions

Listed below is common wheel and tire terminology:

Aspect Ratio: Also called ‘tire profile’ or ‘series’, aspect ratio is the height of a tire – from the bead to the tread (a.k.a. crown) – in relation to its section width. Calculated by dividing section height by section width. An aspect ratio of 50, for example, means that a tire’s section height is half that of its section width. In a tire size description, it would be the ’50’ in P215/50R17. In general, tires with a lower aspect ratio have shorter, stiffer sidewalls that resist cornering forces and improve lateral stability.

Asymmetric: Refers to a tire with a different tread patterns on both halves of the tire.

Backspacing: The distance (in inches) from the mounting surface to the back edge (or inside lip) of a wheel. Backspacing is different from, but closely related to wheel offset.

Bead: The innermost edges of a tire where it mounts to the rim of a wheel. The bead is reinforced with concentric steel wires embedded in the rubber that are designed to hold the tire to the wheel and prevent damage during installation and/or removal. Every tire has two beads.

Bead Seat: The edge of a rim that creates a seal between the tire bead and the wheel.

Belted Bias Tires: Tire construction type where a ply cut at an angle is reinforced with belts that wrap around the circumference of the tire on top of the bias or radial ply.

Belts: Layer(s) of rubber-coated cords that run circumferentially around a tire beneath its tread. Their purpose is to reinforce the tread and help maintain a tire’s shape under a wide variety of conditions including tire pressure variations, centrifugal forces and cornering and braking g-forces, for example. Belts also improve tread wear and help resist damage from impacts and penetration.

Bias Ply Tire: Tire construction type where the plies are cut at an angle and laid across one another in a criss cross fashion.

Bolt Circle (Pattern): For standardization purposes, bolt circle is also known as PCD (pattern circle diameter). It refers to the diameter of an imaginary circle that goes through the center of the bolt holes. A four-bolt wheel with 110mm between opposing bolt holes could be written as 4/110, for example. A wheel can have more than one bolt pattern to accommodate multiple vehicle fitments, however, the bolt pattern must first match the requirements of the hub before it can be installed. In other words, it’s impossible to install a wheel with 5/110 bolt circle when the hub calls for a 4/110 PCD.

Camber: The angle of the centerline of a wheel and tire relative to its vertical axis. Measured in degrees.

Carcass (Casing): Also called the casing, the carcass is the main component of a tire consisting of wire beads and body plies. The tread or sidewalls are not part of the carcass.

Cast: A wheel manufacturing technique that requires molten (liquid) metal be poured into a mold that will yield the final shape of a wheel.

Caster: The angle between the vehicles steering pivot axis and completely vertical.

Centerbore (Hub Bore): 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.

Chafer: Abrasion-resistant rubber-coated material that helps protect the beads from rim damage and chafing.

Cold Tire Pressure: The measure of air pressure inside a tire that has not been driven on for three to four hours, or has been driven on for less than one mile (after the first condition has been met).

Compound: Refers to the chemical composition (materials used) of rubber used in the construction of a tire. Rubber, carbon black, plasticizers, curing agents and ozone retardants are the main ingredients in most tires. Different compounds are used to manipulate tire characteristics (i.e. heat resistance, level of traction, tread life, cut resistance, cold resistance, etc.) and influence performance.

Cords: Strands of fabric that form the plies or layers of a tire. Made from steel, fiberglass, rayon, nylon, polyester or other fabrics, cords help determine the strength and load carrying capacity of a tire.

Crown: The outer perimeter or contact surface of a tire.

Curb Guard: An extra-thick band of rubber that helps protect the tire sidewall as well as the wheel face from damage as a result of hitting a curb.

DOT: Label on a tire sidewall that certifies it complies with all applicable safety standards established by the Department of Transportation (DOT). An alphanumeric code (or serial number) up to 12 digits long appears next to every DOT label containing such information as when and where it was made, by which manufacturer, what plant, assembly line and its size

Euro-Metric System: Or just plain Metric, this European-developed system for specifying tire sizes uses the basic same formula as the P-Metric System but does not use a “P” in the size. Euro- and P-metric tires of the same size (P215/50R17 and 215/50R17, for example) are identical in their physical dimensions with slight variances in their load capacity calculations and inflation pressure tables.

Footprint: The area of a tire tread that touches the road surface at full load. Commonly referred to as the contact patch.

Forged: A wheel manufacturing technique that uses intense heat and pressure to transform a solid “block” of alloy material into the final shape of a wheel.

Hub Centric: Wheel with a centerbore that matches a vehicles hub diameter and provides a snug, centered fit. Adapters can help achieve this. See ‘Hub Centric Rings‘.

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 rings, a wheel is likely to vibrate even if the wheel and tire assembly has been perfectly balanced.

Hydroplaning: 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.

Inner Liner: A layer of specially-formulated rubber that forms the inside of a tubeless tire. Its purpose is to maintain tire pressure.

Load Index: A numbering system that corresponds to the maximum load (in pounds) a tire can support when properly inflated. This number is found on the tire sidewall. Maximum load in pounds and in kilograms is found elsewhere on the sidewall. Truck tires use a different system incorporating letter codes to establish a Ply Rating. See ‘Load Range‘.

Load Range: Is the carrying capacity of a given tire at specific air inflation pressures. Refer to chart. ‘Load Range’ is gradually replacing the term ‘Ply Rating’; and, is expressed as Standard Load (SL) and Extra Load (XL) for auto tires; and, Load Range C, D and E for light truck tires.

Tire Load Ranges: Inflation Pressure Assigned For “Maximum Load” Rating P-Metric (passenger cars) Cars: Standard Load (SL) 35 psi Extra Load (XL) 41 psi Light Truck: Load Range C (LRC) 50psi Load Range D (LRD) 65 psi Load Range E (LRE) 80 psi

M+S: A sidewall marking that indicates a tire is approved by the Rubber Manufacturers Association for ‘Mud & Snow’ use. This marking generally appears on “all-season” tires.

Mixing Tires: Practice of combining different tire sizes and/or tire models in the front and rear of a vehicle.

Mounting: The process of installing a tire(s) onto a wheel(s).

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.

Overinflation: A condition that exists when a tire has more than its recommended air pressure for a given load. This can yield better performance, but may have negative side effects such as a less harsh ride, tire damage and excessive stress on the suspension.

P-Metric System: A system for specifying tire sizes using the tread width (in millimeters), the aspect ratio, type or tire construction as well as the rim diameter (inches). The sizes are written as such: P215/50R17. In this example, the first character is a ‘P’ and, thus, means this is a passenger car tire. It also says this is a ‘P-Metric’ tire that’s engineered to standards set by the U. S. Tire and Rim Association (T&RA). If there is no ‘P’ (as in 215/50R17) it means the tire is engineered to European Tire and Rim Technical Organization (ETRTO) standards and is an Euro-Metric tire. Fortunately, T&RA and ETRTO standards have evolved together through the years and are virtually interchangeable.

Plus Sizing: The practice of altering the wheel diameter in conjunction with tire aspect ratio to improve both the look and performance of a vehicle without changing the stock tire footprint. Theoretically, plus sizing should not change the overall tire diameter of the original equipment tires too drastically, which is important since larger variances can cause problems with transmission shift points and other important systems.,

Ply: Layers of heat- and impact-resistant cord fabric that gives a tire its strength and rigidity. Running from bead to bead, plies are usually made of nylon or polyester and sandwiched between the tread and inner liner to form the body of a tire.

Profile: (See aspect ratio)

PSI: A common unit of measurement for tire pressure. Short for pounds-per-square-inch, it’s a measure of the force exerted by the air inside a tire.

Radial Tire: Tire construction type where the plies run perpendicular across the tire from bead to bead (90 degrees to the direction of travel). Similar to ‘Belted Bias Tires’, the plies are reinforced with belts that wrap around the circumference of the tire on top of the radial ply.

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

Ribs: Straight rubber sections of the tread that encircles a tire in its direction of travel.

Rim Width: The distance between the inner and outer flanges of a wheel.

Section Width: Distance between the inner and outer sidewalls at their widest point of an inflated tire not under load.

Series: (See aspect ratio)

Service Description: Marking that identifies a tires load index and speed rating. Located on a tire sidewall after its size listing.

Shoulder: The rounded outer portion of the tread where it meets sidewall.

Sidewall: The side portion of the tire between the shoulder and beads. Every tire has two sidewalls.

Sipes: Small slits cut into tread blocks that help evacuate water away from the crown of the tire for improved wet traction. Sipes also provide “biting edges” for improved traction on ice- and snow-covered roads.

Speed Rating: A letter-rating system for passenger cars only that corresponds to a fixed maximum speed capability for any given tire. Each and every tires capabilities are tested at preset speeds. The results of these tests determine its speed rating. Refer to the chart. (Note: Light truck tires (LT) are not speed rated.)

Rating: Maximum Speed (mph) 99-S, 112-T, 118-U, 124-H, 130-V, 149-W, 168-Y, 189-Z, Above 149-T

Tire Placard: More appropriately called a vehicle information placard, this small sticker (often found on the inside of driver side door) contains key information about a vehicle, including the stock tire size, recommended tire pressures, weights and more.

Tire Pressure: The air pressure inside a tire. Expressed as pounds per square inch (psi) or kiloPascals (kPa).

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

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.

Toe: The difference between the front and rear edges of tires mounted on an axle. Toe-in means the front of the tires are closer together than the rear edges and tires point inward. Toe-out means the front of the tires are farther apart than the rear edges and the tires point outward.

TPMS (Tire Pressure Monitoring System): A tire-pressure monitoring system (TPMS) is an electronic system designed to monitor the air pressure inside the pneumatic tires on various types of vehicles. TPMS report real-time tire-pressure information to the driver of the vehicle, either via a gauge, a pictogram display, or a simple low-pressure warning light.

Tread: Made from tough rubber for high traction and low wear, tread is the part of a tire designed to make contact with the road surface. Tread designs can vary greatly depending on the specific purpose of the tire.

Tread Blocks: Individual sections of the tread separated by lateral grooves.

Tread Depth: Distance from the bottom of the tread grooves in a tire to the crown along the centerline of a tire. Measured in thirty-secondths (1/32) of an inch.

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

Tread Grooves: The space or void between two tread ribs.

Tires: In short, tires are precisely-engineered pieces of rubber, chemicals, fabric and metal that are designed to provide traction, cushion road shock and carry a specified load under all kinds of conditions.

Tires (Used): Tires that have about 35% - 70% of tread remaining.

Tires (Take-Off): Tires that have about 75% - 95% of tread remaining.

Tires (New Take-Off): These tires have very low miles on them. They generally came off brand new cars prior to being sold. They are essentially new, except they have been mounted.

Tread Pattern: The arrangement of grooves, blocks, sipes and channels on the tread that permit water to drain away from the footprint.

Tire Rollover: A condition where a tire sidewall rubs the road surface during hard cornering.

Tread Shaving: Most common in racing applications, this is when parts of the tread are physically removed (by cutting, shaving or otherwise) for optimal performance and durability in specific situations.

Tread Wear: Similar to tread life, this is a measure of how long a tire should last in miles or kilometers.

Tread Wear Indicators: Also called wear bars, these narrow rubber bars are built into the tread grooves and clearly show when a tire tread has past its legal expiry point. Wear bars only become visible after tread depth has worn down to 2/32-inch; at which point the tire must be replaced.

Treadwidth: The width of a tire tread measured in millimeters. Given P215/50R17, the treadwidth is 215mm.

Underinflation: A condition that exists when a tire has less than its recommended air pressure for a given load. Underinflation can cause tire rollover and deflection.

UTQG: Acronym for the Uniform Tire Quality Grading rating system developed by the American Department of Transportation. UTQG ratings for tread wear, traction and temperature to assist consumers in comparing the relative performance of any given passenger tire at a glance. (Note: does not apply to winter tires.)

Valve Stem: A self-contained valve or filler neck that opens to allow gas into an enclosed chamber and then automatically closed and sealed by the pressure in the chamber, a spring, or both.

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 (Spin / Computer / Dynamic): The wheel is placed onto the balancer's spindle through the center bore, and a metal cone is inserted to ensure the wheel is perfectly centered. The machine spins the assembly at very high speed to determine the heaviest point, and signals the operator where and how many weights to place on the opposite side to compensate. Out-of-balance wheels can cause a vehicle to vibrate at certain speeds and produce uneven tire wear.

Wheel Balancing (Road Force): Road force balancers, in addition to performing a traditional spin balance, measure both the wheel and tire by pressing a large roller against the tread of the spinning tire. The roller applies 1,200 to 1,400 pounds of pressure to simulate the weight of the vehicle on the tire as it rolls down the road. A computer in the machine, along with various sensors, determines variations of tire stiffness, radial runout and anything in the tire’s construction (such as inconsistencies in the belt package) that would prevent the tire from rolling smoothly when it is weighted by the car. By measuring both the wheel and tire, the road force balancer tells the technician where to move the tire around on the wheel until the effective high spot of the tire (when it is rolling on the car) matches the low spot of the wheel – a more sophisticated method of match mounting. Use of a road force balancer can provide consumers with the same level of ride comfort previously only available to OEMs. An additional benefit to the tire dealer is that many road force balancers are able to optimize the corrective balance weights, resulting in considerable (upwards of 30%) savings in the amount of weight used.

Wheel Center: The physical center of a wheel in relation to its width (on a vertical axis).

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: These are weights (usually lead) added to a wheel to properly balance a tire and wheel assembly. The weights can be installed on the inside or outside of a wheel and can be clipped, taped or self-adhered to a wheel.

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).