Tire & Wheel Glossary

Alignment: The term ?Wheel Alignment? is a general description of the service required to check and correct any of the mechanical devices which from part of the vehicle Steering and Suspension system.
Aquaplaning(CD): A condition in which the tyres in a motor vehicle ride on a film of waiter instead of on the road surface, leading to complete loss of vehicle control.
Aspect Ratio: An expression representing the height of the sidewall of a tire in terms of a percentage of tire's width. Balance: Equal distribution of the weight of a tire and wheel. If a tire & wheel is not balanced it can lead to vibrations or uneven wear. For balancing, weights are attached to the wheel to compensate for uneven weight distribution.
Backspacing: It is the distance from the mounting pad to the back edge of the rim. This is different than wheel offset.
Bead: The area of the mounted tyre which seats against the wheel.
Bead wires: High tensile steel wire in a coil shape, treated to give good adhesion with rubber, and encased in a ?matrix? of hard rubber. The casing plies are turned around the bead wires and are secure bonded to the structure when the built tyre is vulcanized.
Bolt Pattern: The arrangement of the bolt holes on a wheel. A 4 bolt wheel with 100mm between opposite bolt holes would be written as 4x100. Some wheels have more than one bolt pattern on the same wheel to accomodate multiple fitments.
Carbon black: Very fine, specially structured particles of carbon; used in rubber compounds as a reinforcing filler.
Carcass: The structure of the tyre cords locked around wire beads.
Centerbore: The center hole in the wheel that centers the wheel on the hub of the car. Since most wheels are mass produced, they have a large center bore to accommodate several different vehicles. If this is the case, it is recommended that you use a hub ring. Hub rings are hard plastic or metal ring that fits between the wheel and the vehicle. This centers the wheel perfectly on the hub ensuring that there is no run out when the wheel is installed on to the vehicle. Without hub rings it is possible to get vibrations even if the wheel / tire assembly is perfectly balanced.
Cold Inflation Pressure: The measure of air pressure of a tire that is not warm from driving (less than 1 mile or standing for at least 3 hours)
Footprint: The portion of the tread in contact with the road.
High Performance Tire: In the tire industry, those tires with speed ratings of S or greater and aspect ratios of 70 or less.
Hub Centric: A wheel with a centerbore made to match up with a vehicle's hub diameter.
Hub Centric Rings: Hard plastic or aluminum rings mounted on a vehicle's hub before the wheel. They ensure the wheel is perfectly centered on the vehicle's hub. Without hub rings, there is a possibility of getting a vibration even if the wheel & tire assembly is perfectly balanced.
Hysteresis: Actions and reactions:
not all energy applied to a tyre becomes totally absorbed by the latter; there is always an energy loss that transforms into "heat". When dealing with an elastic material like rubber.
Load index: The LOAD INDEX is a numerical code associated with the maximum load a tyre can carry (except for loads at speeds above 210 km/h) at e speed indicated by its Speed Symbol under the conditions specified by the tyre manufacturer.
Mounting: Installing tires onto wheels
Offset: The offset of a wheel is the distance from the mounting surface of the wheel to the true centerline of the rim. A positive offset means the mounting surface of the wheel is positioned in front of the true centerline of the rim / tire assembly. This in effect brings the tire in to the fender well more. Conversely, a negative offset means the mounting surface of the wheel is behind the true centerline of the rim / tire assembly. This will cause the tire to stick out away from the vehicle.
Plus Sizing: Changing from the original stock tire size of your vehicle. Plus sizing your wheel & tire combination was designed to enhance vehicle performance and looks by allowing fitment of larger diameter rims and lower profile tires. The theory is that while making these changes, you keep the overall tire diameter within 3% of the original equipment tires. This is important because larger variances can cause problems with transmission shift points which can decrease fuel mileage. It can also confuse braking system computers which can even lead to brake failure.
Rims Width: The measurement between the flanges of a rim
Rolling resistance: The capacity of a tire to contrast its natural "spinning" movement. The lower the rolling resistance, the more economic it is to roll the tire as less energy will be required to "roll the tire" with a resulting fuel saving and higher mileage.
Service description: In addition to the tire Size Designation a tire may be identified by a Service Description consisting of a load index (or two Load Indices in the case of single/dual fitments) and a speed symbol.
Shoulder: The area of a tire where the tread and sidewall meet.
Sidewall: That portion of a tire between the tread and the bead.
Silica: A reinforcing filler used with the rubber compound to provide higher wearing resistance and low rolling resistance.
Speed category: Indicates tire speed capabilities above 240 km/h and are included within the tire size designation e.g. 215/50 ZR 15. Generally, Service Description is not shown on such tires. For actual speed capabilities and load capacities, consult the tire manufacturer concerned.
Speed symbol: Indicates the max speed at which the tire can carry a load corresponding to its load index (except for speeds above 210 km/h) under service conditions specified by the tire manufacturer.
Tire Compound: A "mix" or combination of raw materials used in the manufacture of the rubber compound of the tire itself. By varying the percentage or the composition of the materials applied, manufacturers can produce different tires for various purposes and with different performances.
Tire Structure: Implies the method used to manufacture the tire carcass. The two main methods are "Radial" (a tire with plies arranged in a way that cords in the body run at 90 degree angles to the center line of the tread) and "X-Ply".
Tread: A portion of a tire which contacts the road surface.
Tread Pattern: The tire design or "sculpture". It varies according to the tire purpose (i.e. rain/wet are more grooves and channels for optimum water dispersion, whilst dry/tarmac tires are less grooved as they function is to maximize the contact area with the ground).
Tubeless: A tire specifically designed for fitment without an inner tube on appropriate rims.
Valve: The device used to let air in or out of the tire and retention.
Wheels Weights: Weights attached to a wheel to balance a tire & wheel. The weights can be on the inside or outside of the wheel and can be clipped, taped or self-adhered to the wheel.
Wear indicator: A rubber nub placed inside a groove of the tire tread to indicate the legal minimum depth of the groove itself.