Alignment: The mechanical condition of adjustable components within the vehicle’s suspension. When a vehicle is in alignment, the caster, camber, toe-in, and thrust settings are set to specification. Severe impacts (hitting potholes or curbs) and worn suspension parts are the leading causes of misalignment.
All-Season Tires: Tires designed to provide good traction in a wide variety of road conditions, including wet, dry, mud, and snow. This design limits the tire’s performance in extreme conditions, or when compared to tires built for a particular category.
Aspect Ratio: A term that describes a tire’s height-to-width proportion. If a tire’s sidewall height were 75% of its section width, its aspect ratio would be 75. In the tire size expressed as 295/75R225, the number 75 is the aspect ratio.
Balance: The state in which a tire and wheel assembly spins with all its weight distributed equally. A wheel balancer is used to place weights compensating for static and dynamic imbalances that exist in all assemblies. Not balancing an assembly may result in extreme vibration.
Bead: A round hoop of steel wires, wrapped or reinforced by steel cords, placed at the very inside of the tire’s diameter.
Bias-Ply Tire: A pneumatic tire manufactured such that the plies are laid at alternate angles less than 90 degrees to the centerline of the tread. These criss-cross plies give the tire its strength, but generate heat during operation and limit the tire’s wear and performance.
Contact Patch: The portion of the tread that contacts the road during operation.
Cord: The strands of material forming the plies or layers of tire. Cords may be made from fiberglass, rayon, nylon, polyester, or steel.
DOT Markings: Each tire has a required Department of Transportation number imprinted on at least one of its sidewalls. That number begins with the letters “DOT” and may contain up to 12 additional numbers and letters. The first and last digits are the most important for the tire owner. The first two letters/numbers identify the manufacturer of the tires. The last four digits identify when the tire was produced.
Hydroplaning: A skimming effect caused by tires losing contact with a surface covered by water
Maximum Inflation Pressure: The maximum air pressure to which a cold tire may be inflated; found molded on the tire’s sidewall.
Overall Diameter: The diameter of an inflated tire without any load.
Overall Width: The distance between a tire’s outside sidewalls.
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 size is P215/70R-15.
Ply Rating: A method of expressing load-carrying capacity in terms of plies. A 14-ply rating is now expressed as Load Range G; however, the tire is not built with 12 individual plies. Instead, it contains one or two plies of equivalent strength. The result is a cooler-running, longer-lasting tire.
Radial Ply: Tire construction where the cords in the body run at 90 degrees to the centerline of the tread.
Rim Width: Distance between the two opposite inside edges of the rim flanges.
Rolling Resistance: The force required to keep a tire moving at a constant speed. The lower the rolling resistance, the less energy needed to keep a tire moving.
Section Height: The height of a tire measured from the rim to the outer tread.
Section Width: The distance between outside sidewalls, not including any lettering or design.
Series: A numerical representation of a tire’s aspect ratio. For example, 70 series indicates the tire’s section height is 70% of its section width (see Aspect Ratio).
Shoulder: The part of the tire where the sidewall and tread meet. Certain tire designs feature shoulder blocks for better traction.
Sidewall: The part of the tire between the tread and the bead.
Size: An expression that defines a particular tire in terms of its width, height, rim diameter, aspect ratio, and construction type. 295/75R22.5 expresses tire size using the metric system.
Tread: The part of the tire that comes into contact with the road. The tread type is distinguished by the design of its ribs and grooves.
Treadwear Indicator: Narrow bands, sometimes called “wear bars,” that appear across the tread when 2/32″ of tread remains.
Tread Width: The width of a tire’s tread.
Valve: A device mounted in the wheel that lets air in or out of the tire. Valves include caps to keep out dirt and moisture and a check-valve to prevent air from escaping.