Wheels / Rims Glossary

Air Pressure
the amount of the force exterted by the air inside a tire, measured in pounds per square inch (psi) or kiloPascals (kPa)

Alignment
refers to the correct angle settings of suspension components - the 3 alignment settings are caster, camber and toe.

Aspect Ratio
the relationship between a tire's section height and its section width. It is calculated by dividing the section height by the section width. When the section height is half of the section width, the aspect ration is 50. In a tire size description, it would be the 50 in P195/50R15. It is also called the tire's profile or the series.

Asymmetric
tires that have differing tread patterns on each half of the tire.

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
also called rearspacing, it is the distance from the mounting pad to the back edge of the rim. This is different than wheel offset.

Bead
the part of the tire in contact with the wheel flange. It is made of high tensile steel wires shaped to fit the rim and hold the tire on the wheel. The steel wires are wrapped in woven fabric and held in place by the plies.

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

Belted Bias Tires
tires constructed similar to bias tires, but with reinforcing belts between the casing plies and the tread.

Belts
rubber coated cords located between the plies and the tread. They help reinforce the tread, as well as help the tire keep its shape against such forces as: tire inflation pressure, centrifugal force, cornering and braking. These cords are made from such materials as steel, fiberglass, radon, nylon, polyester or other material.

Bias Tire
a tire that is constructed with plies laid out in alternating directions in angles about 30-40 degrees to the center line of the tire. The plies form a criss-cross pattern.

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 4/100. Some wheels have more than one bolt pattern on the same wheel to accomodate multiple fitments.

Camber
abrasion resistant rubber coated material to help prevent the tire's beads from rim damage and chafing.

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)

Compound
the materials used in the construction of the tire's rubber. The main materials used are rubber, carbon black, plasticizers, curing materials and ozone retardants. Different compounding formulas are used to achieve different tire characteristics such as: heat resistance, increased traction, increased treadwear, cut resistance, cold resistance, etc.

Cord
strands of nylon, rayon, polyester, steel or fiberglass that make up the plies & belts of the tire. The strength of a tire & its load carrying capacity is determined by the strength of the cords.

Crown
the center section of the tire's tread

Curb Guard
extra rubber running around the sidewall of a tire. It is there to protect the side of the tire and the wheel face from any damage that may come as a result of hitting a curb.

DOT
stands for Department of Transportation. The 10 digit code appearing after the DOT designation gives information such as the week and year the tire was produced, as well as the manufacturer, plant, tire line, and size.

Footprint
the area of the loaded tire's tread that is in contact with the road. This is also called the contact patch.

Forged
Considered to be the best wheel manufacturing technique, forging allows for the compression of an aluminum billet (one solid piece of aluminum) into an aluminum wheel using over 13 million pounds of pressure combined with heat. This produces a wheel that is both stronger and lighter then your standard aluminum wheel.

Grooves
the space between two tread ribs of a tire

Hub Centric
a wheel with a centerbore made to match up with a vehicle's hub diameter.

Hubrings
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.

Hydroplaning
when a tire loses traction as a result of water on the road. The water accumulates under the tire's footprint and causes the tire to lift from the road surface. Vehicle speed, tread pattern and water depth all affect hydroplaning.

Load Index
a number used to represent the maximum weight a tire can support. The index number corresponds to the actual load carrying capacity. Truck tires use a different system incorporating letter codes to establish a Ply Rating.

Mixing Tires
combining different tire sizes or tire models. This is not recommended as not all 4 tires will respond the same and it may cause unpredictable handling. Some performance vehicles do come stock with different front and rear tire sizes.

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.

Overinflation
when a tire is inflated more than the recommended vehicle air pressure. This might be done for better performance but has negative consequences including: a less comfortable ride, damage to the tires and stress on the suspension.

P-Metric System
a system for specifying tire sizes using the treadwidth (millimeters), the aspect ratio, type or tire construction and the rim diameter (inches). The sizes are written as such: P195/50R15

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.

Ply
layers of cord fabric that give a tire its strength. They are situated between the tire tread and the innerliner, and they run from bead to bead. These cords are rubber coated.

Profile
refer to aspect ratio.

PSI
the most common measurement unit for tire pressure. It stands for pounds per square inch and it measures the force exterted by the air inside a tire.

Radial Tire
tires built with plies running perpendicular (90 degrees) across the crown of the tire. To strengthen the tread, these tires require belt plies going circumferentially around the tire.

Retreading
applying new tread to a used tire casing. This practice is common among medium & heavy trucks.

Ribs
rubber sections of the tread that run around the circumference of the tire

Rim Width
the measurement between the flanges of a rim

Rotation
moving a vehicle's tires from left to right and from front to rear. This is done in a set pattern and should be done periodically. Its purpose is to prevent uneven tire wear and to extend treadlife.

Section Width
the distance between the sidewalls at their widest point of an inflated tire not under load.

Shoulder
the outer edge of the tire tread where it meets the sidewall

Sidewall
the side portion of a tire between the tread and the bead.

Sipes
small slits in a tire's tread that help push water away from the crown of the tire for improved wet traction. They also provide biting edges for ice and snow traction.

Speed Rating
a letter that identifies a tire's high speed durability. A tire's capabilities are tested at preset speeds and the results of these tests determine the tire's speed rating. Speed Ratings include: Q, S, T, U, H, V, Z, W, Y

Tire Placard
a label on a vehicle that identifies the vehicle's stock tire size and its recommended tire air pressure. This label is often found on the inside of the vehicle's door.

Toe
the difference between the distance between the front left & right tire and the distance between the rear left & right tire. Toe-In means that the front of the tires are closer together than the rear. Toe-Out means that the rear of the tires are closer together than the front.

Tread Blocks
individual sections of the tread separated by lateral grooves

Tread Depth
the distance from the top of the tread to the grooves in a tire. This measurement is taken at the centerline of a tire and is measured in thiry-secondths of an inch.

Tread Pattern
the arrangment of grooves, blocks, sipes and channels on the tread.

Tread Shaving
shaving some of the tread from a tire for optimal performance and durability in racing applications.

Tread Wear
also called the tread life, it is the measure of how long a tire lasts. It is measured in miles or kilometers.

Tread Wear Indicators
narrow rubber bars built into the tread grooves that define the tire's legal wear out point. Also called the wear bars, they are even with the tread when 2/32" of tread is left and then the tires are ready to be replaced.

Treadwidth
the width of a tire tread, normally measured in millimeters. In 195/50R15 the treadwidth is 195mm.

Underinflation
a tire with less than the recommended air pressure for a given load. This may lead to tire rollover and deflection.

UTQG
The Uniform Tire Quality Grading rating is a quality rating system developed by the American Department of Transportation. It is designed to tell consumers the relative performance of passenger tires (but does not apply to winter tires).

Wheel 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.