Tire and Wheel Glossary

Word Definition
Air Pressure Force measured in kiloPascals (kPa) or Pounds per Square Inch (PSI) applied by the air inside a tire. The optimum tire pressure recommendation may be found on a sticker on a door jamb, or in an owner’s manual (usually 30-35 PSI).
Alignment Modification of a vehicle’s suspension (the system that connects a vehicle to its wheels) so the angles of the tires make proper road contact.
Aspect Ratio Ratio, stated as a percentage, of a tire’s cross-section height to it’s width.
Asymmetric Refers to the tire with an interior and exterior half’s of the tread pattern differentiating.
Balance Even weight spread across a wheel and tire. For balance to occur weights are attached to the wheel.
Backspacing Space from the mounting pad to the rear edge of the rim.
Bead Space from the mounting pad to the rear edge of the rim.
Bead Seat The brink of the rim that creates a closure between the tire bead and the wheel
Belted Bias Tires Bias tires with strengthening belts between the exterior plies and the tread.
Belts Cords covered in rubber, situated between the plies and the tread. They help strengthen the tread, and help the tire keep its shape against various internal and external forces. The cords are made from steel, fiberglass, radon, nylon, polyester or various other materials.
Bias Tire A tire that is constructed with plies laid out in interchanging directions at angles that are approximately 30-40 degrees to the center line of the tire. The plies form an intersecting design.
Bolt Pattern A tire that is constructed with plies laid out in interchanging directions at angles that are approximately 30-40 degrees to the center line of the tire. The plies form an intersecting design.
Camber Angle of the centerline of a tire and wheel comparative to entirely straight up.
Cast There are two types of pressure when it comes to molding wheels; low pressure and counter pressure. Low pressure casting comprises of Pouring liquid into a mold, whereas counter pressure casting consists of sucking the metal into a vacuum. The counter pressure method decreases impurities producing a stronger wheel.
Caster Angle of the vehicle's steering pivot axis comparative to entirely straight up.
Centerbore The central opening on the back of the wheel that mounts the wheel properly. The opening is created in a way that exactly matches the hub for the wheels to be properly positioned.
Chafer Protective layer made of rubber and fabric inhibiting rubbing between cord body plies and the wheel.
Cold Inflation Pressure Measurement of air pressure within a tire that has not been driven for at least 3 hours.
Compound Various compounding formulas are used to achieve different tire features such as: heat and cold resistance, increased traction, etc., through the use of rubber, plasticizers, preserving materials, and ozone retardants.
Cord Threads of nylon, rayon, polyester, steel or fiberglass that make up the belts and plies of the tire. Cords determine the strength of a tire and its weight carrying capability.
Crown Centre of the tread.
Curb Guard Additional rubber around the sidewall of a tire. It is there to guard the side of the tire from damage.
DOT The 10 digit code appearing after the DOT (Department of Transportation) description provides demographic tire information, manufacturer, tire line, and size.
Footprint When the tread is in contact with the road.
Forged A wheel manufacturing method permitting the compression of an aluminum billet into an aluminum wheel, essentially producing a one-piece wheel that is stronger than others.
Grooves Indents in the tire tread, there to prevent hydroplaning and create a gripping effect.
Hub Centric A wheel with a center piece created to coincide with a vehicle's hub diameter.
Hub Centric Rings (Hubrings) Rings made of plastic or aluminum mounted on a vehicle's hub prior to the wheel, ensuring the wheel is perfectly centered on the vehicle's hub. Lack of hub rings creates the likelihood of receiving a vibration even if the wheel and tire composure is ideally balanced.
Hydroplaning The loss of grip as a result of water on the road, since the water may collect under the tire's footprint and make the tire lift up from the surface of the road. The speed of the vehicle, water depth and the tread pattern are all circumstances affecting the creation of hydroplaning.
Load Index A representative number of the maximum load a tire can support. The index number matches to the actual load carrying dimensions. Truck tires use a different system including letter codes to create a Ply Rating.
M+S Marking that approves the tire for use in mud and snow. Approved by the Rubber Manufacturers Association.
Mixing Tires Joining different tire sizes or models is not suggested since all four tires will react differently and create unpredictable situations. Occasionally performance vehicles may come with different front and rear tire sizes.
Mounting Installation of tires onto wheels.
Offset The space between the surface of the installation of the wheel to the precise centerline of the rim. A positive offset implies that the mounting surface of the wheel is positioned in front of the exact centerline of the rim, or in other words tire assembly, making the tire closer to the fender. On the other hand, a negative offset implies that the mounting surface of the wheel is behind the actual centerline of the rim or tire assembly, causing the tire to extend away from the vehicle.
Over inflation Inflating the tire over the recommended air pressure amount. Sometimes it may be done for higher quality performance, however, this has damaging consequences to the tires and puts stress on the suspension.
P-Metric System Specifies tire sizes by using the tread width, in millimeters, as well as the aspect ratio, type or tire construction, and the rim diameter in inches, all together producing a tire size, for example: P215/70R16.
Plus Sizing Attachment of greater diameter rims to lower sized tires, while keeping the general tire diameter within 3% of the usual tire size. Anything above 3% will create issues with the transmission shift points, leading to an increased fuel consumption, as well as, may complicate the braking system computers which in turn, may eventually lead to a brake failure.
Ply Layers of rubber coated cords that provide a tire with its strength, located between the tire tread and the inner liner.
Profile See “Aspect Ratio”
PSI See “Air Pressure”
Radial Tire Tires made with plies at 90 degree angles across the crown of the tire. Require belt plies to go around the exterior of the tire.
Retreading A practice common among trucks, when there is an application of new thread to an old casing.
Ribs Sections of the tread (made of rubber) that go around the exterior of the tire.
Rim Width The width measurement between rim flanges
Rotation Moving a car's tires in all directions, done periodically in a set pattern, to extend tread life and avoid unequal tire wear.
Section Width The measurement in-between the sidewalls of an inflated tire (not under load) at its widest point.
Series See “Aspect Ratio”
Shoulder The area where the exterior edge of the tire tread meets the sidewall.
Sidewall The area between the tread and the bead.
Sipes Smaller cuts in the tread of the tire, there to assist in repelling water from the top of the tire to improve wet traction. Additionally, sipes create piercing edges for traction on snow and ice.
Speed Rating Letters, such as H, Q, S-W, Z, that identify the high speed of the tire's resilience, tested at speeds set ahead of time, and the outcome of these tests determine the tire's speed rating.
Tire Placard A sticker that is often found on an inside door of the car that shows the car's standard tire size, and its suggested tire air pressure.
Toe-In When the front tires are closer together than the rear.
Toe-Out When the rear tires are closer together than the front.
Tread Blocks Tread’s individual sections parted by horizontal grooves.
Tread Depth The space measurement from the tires top of the tread to its grooves.
Thread Depth A term used to express how long a tire lasts before becoming unusable, measured in km. May also be expressed as a percentage of the leftover thread.
Tread Pattern the organization of, sipes, grooves, and channels on the tread.
Tread Shaving Applied primarily for racing purposes to increase tire robustness and performance by shaving some of the tread from a tire.
Tread Wear See “Tread Life”.
Tread Wear Indicators Thin rubber bars input into the grooves of the tread which state the tire's legal wear out point. They become at level with the tread when 2/32" of the tread is leftover, at which point the tires are ready to be changed to new ones.
Treadwidth Differs from the overall tire width by being narrower, as this is the width of specifically the tread, typically measured in millimeters.
Under inflation A tire that has less air inside than the suggested air pressure for a specified load. May lead to poor tire performance and overall wheel damage.
UTQG Uniform Tire Quality Grading (UTQG), is a rating system created by the American DOT. It is intended to inform customers of the comparative performance of passenger tires. This rating system does not include winter tires.
Wheel Weights Little weights (various grams) attached to a wheel to balance the tire and wheel combination. May be found clipped on either the interior or exterior of the wheel.
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