Tread
The tread is the outer most part of the tire that everyone sees. It provides the frictional forces to generate grip for accelerating, braking and cornering, so tread compounding plays the most significant role in how well a tire sticks. In addition to tread compounding is tread design. Tread design is yet another balancing act, as the pattern of a tread will control how well a tire evacuates water, snow, or mud to provide traction, how evenly the tread wears, the amount of rolling resistance, amount of road noise generated and overall handling feel. This particular BFGoodrich tire has three seperate tread compound bands for wet and dry traction.
Sub-Tread
Not used in every tire, the sub-tread is a lower hysteresis rubber, which rebounds and releases energy quickly after deforming. Hysteresis in the rubber world describes how well the rubber stores energy, how it's released, and the amount of energy lost. Sub-treads can be used to alter handling characteristics, road noise, and ride quality.
Cap Ply
Higher speed rated tires, namely the ones we put on our vehicles, may have a full width nylon cap ply or plies. They would be placed on top of the belts and wrapped in the circumferential direction. These help to reduce the growth of a tire from the centrifugal forces that a tire experiences in movement durinf rotation, especially at greater speeds.
Belts
To protect the tire from something coming in, like a rock or curb, a stronger exoskeleton has to be used. That's the belts. Usually made from calandered steel wires, as seen above, (in some cases polymer fibers) laid side-by-side and oriented roughly 60 degrees from the centerline of the tire, each belt layer is wrapped on top of the body plies at opposing angles. This serves as a reinforcement to restrict the more flexible body plies from expanding, provides stability to the contact area and serves to protect the tires from impact and punctures. The layout of the belts are usually in pairs, in order to maintain a center-line balance of the tire. If only one diagonal belt is placed onto the tire, the tire will pull in the direction that the steel is aligned to.
Inner Liner
What you see on the inside of an unmounted tire is the inner liner. This is a thin membrane placed on the inside of the tire to improve air retention of the tire by allowing less air to permeate through the body of the tire. Since it plays little role in the structure or shape of the tire, the compound tends to be soft like a balloon with molecules packed tightly together to prevent small gas molecules from passing through and escaping. It's basically an open inner tube with the inside diameter lopped off and integrated into the tire that extends from one bead to the other.
Belt Wedges
Small strips of rubber are placed between the belts near the edges. This is to reduce the shearing forces of the belts against each other and whatever components they are interfacing with as the tire rolls through its contact patch and deflects.
Body Plies
Since the inner liner acts as a balloon that doesn't hold a rigid shape well, a body ply layer is laid on top of the inner liner. The main job of the body ply is to hold the carcass' round shape. If you just rolled an inner tube on the ground, the contact patch would significantly flatten due to the weight of the vehicle. By wrapping the body ply, which is a steel or polymer threads coated in rubber (called calendaring,) around the inner liner, the fabric strength holds the tire in a circular shape and distributes load from the contact patch throughout the tire as the hoop stress. These threads are aligned perpendicular to the bead in a radial fashion to create the carcass and wraps around each bead bundle from one side to the other. So, the body ply layer holds the inner liner in place and in shape, as well as offers some external protection for the tire.
Under Tread
The bonding interface between the tread and the belts is the under tread. It also covers the ends of the belts and helps to reduce belt package movement as the tire rolls through its contact patch and deflects. When a tire delaminates, this is what fails. Delaminating is a mode of failure in tires where the tread will separate from the carcass of the tire. This is usually due to an imperfection during the manufacturing process in which either the tire was cured improperly or some debris or air was trapped in the making process. An improper curing process can cause weak bonding between the rubber molecules and as the tire heats up from excessive work like drifting or under inflation, the heat generated is enough to break these weak rubber bonds.
Shoulder Inserts
Similar to that of the belt wedges, they are placed between the body plies and the belt package in the shoulder areas to reduce the amount of shearing stress that the body plies could be subjected to with the belt package movement. It also helps to maintain a smooth belt contour.
By Samuel Kwa
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