Tire Construction
A lot goes into the construction of a tire. It can be made of over 100 separate components, consisting of natural and synthetic rubber, steel, nylon, polyester, carbon black and many other materials. These are all engineered and combined to provide reliable performance for you and your vehicle.
Inner Liner – this is the innermost layer of the tire and is designed to contain the air inside and prevent air loss.
Bead – this is a hoop made of high tensile strength steel wires and surrounded by a hardened rubber compound that holds the tire securely on the wheel.
Carcass (or body) ply – the rubber coated sheet of fabric (typically made of polyester) that has cords running radially from bead to bead (at 90 degrees to direction of travel for a Radial tire). This is the main construction component for determining the strength of the tire. There can be one, two or even three carcass plies in a passenger or light truck tire.
Apex (or bead filler) – the portion of harder rubber compound that sits above the bead bundle and fills the lower sidewall area, providing stiffness and stability.
Sidewall – the rubber layer on the outside of the tire, providing protection to the inner components. It also provides the surface for the lettering and decorations on the tire.
Belts – the belt package is constructed with woven strands of high strength steel fibers, which are bonded to rubber. They cover the inner tread area and provide the rigid base for the tread rubber.
Cap ply (or overlay) – a nylon (or sometimes aramid) ply placed on top of the belt package to contain and protect the belt package at higher speeds.
Tread – the outer rubber section of the tire in contact with the ground that provides traction and is designed to resist wear, abrasion and heat.
Tire Profile Design
The shape of the tire’s profile (also known as the mold cavity shape) is very important. It affects many factors in a tire’s performance, including treadwear, handling, rolling resistance, and even noise. The basic dimensions of the profile are dependent on the tire’s designated size and are within a standard design “window”. But there are slight changes that can made to the different parts of the profile that can significantly affect tire performance.
Tread Arc (or Tread Design) Width – this is the dimension of the tread face of the tire, effectively measured from the midpoint of each tread shoulder. This is often specified to equal a set percentage of the tire’s section width.
Non-skid Depth (Tread Depth) – the amount of tread depth varies by tire type, with standard passenger tires having less than off-road light truck tires, for example.
Tread Arc (or Radius) – the outer surface of the tread profile is made up of circular arcs or radii that can be flatter or rounder, depending on the type of the tire and its intended performance. Near the center of the tread, the radii are typically very flat, while in the shoulder they are much smaller diameters (and more round).
Curing Rim (or Molded Base) Width – this is the width at the inner diameter of the tire. It’s designed to allow the tire to mount easily on the rim but fit securely on the range of rim widths designated for that tire size.
Rim Guard – this is a thicker section of rubber molded into the lower sidewall area of the tire designed to help protect the edge of the rim from scrapes.
Elements of a Tread Pattern
The tire’s tread pattern design is made up of different elements that all work together to provide the performance intended for the type of a tire.
Tread Block – the large elements in the tread design that do most of the work, providing traction and grip.
Tread Grooves – the larger slots in the tread block that typically run laterally across the tread face, providing wet and snow traction.
Circumferential Grooves – the main channels in the tread face that allow water to drain from the contact patch.
Sipe – the small, thin slits or tiny grooves in the tread block that provide the “biting edge” for enhanced wet and snow traction.
Rib – a group of tread blocks or solid section in a circumferential row going around the tire.
Tread Shoulder – the area at the edge of the tread pattern when it merges into the sidewall. This area comes into contact with the road surface in heavy handling maneuvers.
Void Ratio – this is the amount of open space in the tread. It’s defined as the sum of the groove and sipe area divided by the total tread face area. It can also be referred to as the “sea-land ratio” or “net-to-gross”.
Treadwear Indicator (TWI) – the treadwear indicators are humps molded into the circumferential grooves that indicate the tire is worn out and needs to be replaced if the tread becomes even with that level. They are spaced around the tire in multiple locations and their location is typically marked on the tread shoulder with a “TWI” or triangle symbol.
Tread Pattern Design
The tread pattern design from one tire to another can look very different but they all tend to share some basic characteristics. The design is very important, as it is the only part of the tire in contact with the road. Therefore, it impacts many areas of tire performance – wet and dry traction, handling, noise, ride comfort and treadwear.
For passenger and light truck tires, there are a few different types of tread designs.
Symmetric Design – both the left and right side of the tread are the same (a mirror image rotated around a center point). This is the most common type of design.
Asymmetric Design – this features a design that changes across the tread face from left to right. This type may have one side of the tread with more grooves or smaller tread blocks for increased wet traction, while the other side has larger tread blocks with less grooves for more dry traction.
Directional Design – this type of tread pattern is designed to only rotate in one direction (and must be positioned on a specific side of the vehicle). Ultra-high performance and Winter tires are often directional designs.