How tires work

How tires work

The tires on your car would seem to be simple and straightforward but every year manufacturers spend millions of dollars on research and development to improve the quality and reliability of tires. In this article, we’ll examine some of the things that tire makers must be concerned about.

Tires have two main sections, the tread which is the part of the tire that rolls along the ground, and the sidewalls that connect the tread to the wheel. Although there are many different sizes and tread styles, there are really only two major manufacturing methods for tires. The bias ply tire was used almost exclusively for cars and trucks in the U.S. until the 1970s. Although it was used in Europe for many years, the radial-ply tire did not gain acceptance in the U. S. until its fuel-saving capabilities were recognized in the 1970s. Each type of tire has unique characteristics that make it desirable for certain applications.

The rubber-like compound that tires are made of is a blend of many different substances chosen to produce the exact combination of qualities that the manufacturer wants for a particular tire. Some of the things that tire makers are concerned about are tread life (how long will the tire last), traction (will the tire resist skidding under any conditions) and durability (can the tire resist damage from running over objects or being exposed to heat, chemicals, etc.). Creating a quality tire requires reaching a good compromise on these features because a tire with exceptional tread life may not have good traction and vice versa.

Bias-ply tires consist of layers of nylon or polyester mesh encased in rubber. These sheets of mesh run from one side of the tire to the other at a slight angle. This angle is called the bias and each layer is called a ply thus creating a bias-ply tire. These layers of mesh are placed in a mold and hot liquid tire compound (what we call the rubber) is injected. The outside edge of the mold has the pattern that forms the tread of the tire. The injection, heating, and molding process are sometimes called vulcanizing. When the tire compound cools and solidifies, the mold is opened and the tire is ready for use.

The construction process for a radial ply tire is very similar except that belts of nylon, polyester or even steel mesh are wrapped around the tire under the tread area. Because these belts go around the circumference of the tire they are called “radial plies. The injection and molding process is essentially the same.

Now we’ll look at the different features of tires and examine the differences between bias and radial ply tires. The first feature is tread life. When we buy new tires, one of the first things we ask about is tread life. How many miles will this tire travel before it has to be replaced? There are two main factors that affect tread life, the hardness of the rubber and squirm. If you were able to watch a tires tread as it moves down the road, you would see that it doesn’t remain straight and flat on the ground. It moves from side to side and moves up and down. This is called squirm.

The effect on the tire is the same as the effect on a rubber eraser when you rub back and forth on a piece of paper. The rubber is scrubbed off. If you make the tread from hard rubber, it is more difficult for it to be scrubbed off but you pay a price in loss of traction. Tires with hard rubber tread skid much more easily than ones with soft rubber. Put an eraser on a table and push it sideways. Now put a checker or domino on the table and push it sideways. The checker moves easier because it is harder and has less traction. The other way to increase tread life is to prevent squirm. This is an advantage of the radial-ply design.

The belts under the tread in a radial-ply tire hold it steady and minimize squirm. A typical bias ply tire will last 10 to 20,000 miles whereas radial tread life starts at about 40,000 miles. An added advantage is that reducing squirm increases gas mileage significantly.

The next important tire feature is traction. Here again there are several things that affect traction. The first is size of tread and design. Under the ideal conditions of a straight, hard, dry road, maximum traction is achieved by putting the largest possible amount of rubber in contact with the road. This is why race cars use wide tires with no tread (slicks). Unfortunately on a wet road these tires act like water skis, lose contact with the road and skid uncontrollably.

Passenger car tires must perform well on wet, dry, muddy or snow-covered roads and treads are designed to accommodate all of these. Another thing that affects traction is rubber hardness. As we discussed above, softer treads have better traction but lower tread life so the tire designer must compromise. And the last factor is squirm. Here again, radial tires have the advantage since the belts hold the tread in better contact with the road and therefore provide better traction.

The last major design consideration is resistance to road hazards. As a tire travels down the road, it is constantly flexing. This movement creates heat. Carefully feel of your car tire after you drove several miles and you’ll see what I mean. Tires must withstand heat, cold, gas, oil, chemicals, and being driven over bumps and through potholes. By varying the construction techniques and the formulation of tire compounds, manufacturers are able to create tires whose characteristics vary so that the right tire can be used for each particular application. There is much more to be said about tires but this should give you an idea of how amazing those tires on your car are.

The last major design

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