The Parts Of A Tire
When we talk about tires, the discussion typically only involves the tread design and, maybe, compound. That is literally scratching the surface, as final tread design and compounding only plays a part in how a tire works. The tread pattern and tread compound are analogous to your skin. Hidden underneath the treaded skin of the tire, within the sidewalls and behind wheel flanges, are the much more critical structural components of a tire that dictate its characteristics, as your bones, muscles, and ligaments do for your body.
A modern radial tire is constructed of several layers, each with a specific job, such as holding air pressure, supporting the vehicle weight, giving it its shape, providing grip, protecting itself from puncture, or adding a damping component to the suspension. Even though most components are made of rubber, each different part will use different compounds of rubber with different properties to help them perform their job.
What most people don't pay attention to is the inner carcass and sidewall construction. This is where the strength of the tire comes from so that it holds its shape under huge weight, thermal, impact and centrifugal loads. Since even hard rubbers behave in a non-linear fashion, fabrics and steel wires sandwiched inside rubber coatings are used to reinforce the carcass in different directions and add predictability and restrain rubber components from jiggling all over the place. Qualities like noise, tracking, ride, and most importantly, contact patch shape all depend on the design of the inner carcass.
The sidewall is also little understood, even though it can be the most complex structure of the tire. It has to maintain an airtight tire-to-wheel seal, support the tire's load as well as integrate into the carcass structure seamlessly.
It's so complicated because sidewalls have to provide enough strength to support vertical load and be compliant enough to deform during impacts and under lateral cornering. Sidewall construction controls how a tire rolls over, contact patch profiles under cornering, turn in, steering feel and feedback and heat generation under cornering. And this all changes from tire size to tire size as aspect ratios alter each sidewall's behavior.
The easiest part of a tire to comprehend is the actual tread that we can see. Different tread patterns are useful for different handling characteristics. Large tread blocks on the shoulders are good for cornering, tread grooves are for water evacuation, and sipes provide biting edges to break water tension in water and snow. Even the compound can be identified by touch. Tread patterns also play a vital role in noise, tread squirm and communication between the driver and the road.
Even with illustrations, it's hard to imagine all these parts in a tire. If you cut a tire apart, few of these parts are identifiable since everything's still black. So next time we'll look at these parts as a tire is built and see just how complicated tire construction is.
What is a tire supposed to do?
Radial vs. Bias ply
Outside of certain race tires, almost all tires used on the road today are classified as a radial tire as opposed to bias-ply designs that dominated the first part of this century. The primary difference being the materials and design of the carcass or body plies.
Bias ply tires are constructed using polymers fibers and multiple body plies to form the main structure. Radial tires use belts made of steel wires laid side by side and immersed in rubber (a process called calandering.) The body plies in bias ply tires are laid at angles that are substantially less than 90 degrees from bead to bead.
By Samuel Kwa
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