You know that tires are an important part of your vehicle and you can’t get anywhere without them. However, there is a lot more to this component on your vehicle than you may realize.
What the tire numbers mean
When you go to purchase a new tire, you must provide a set of numbers and letters if you want an exact match. However, many people don’t know what all or part of the set means. Each portion of these numbers and letters is important to your specific tire.
Class of tire: The first letter indicates what class of vehicle you have. For instance, a “P” indicates a passenger car while “LT” indicates that it is a tire for a light truck.
Section width: The first set of numbers is usually three numbers and measures the width of the tire in millimeters from one sidewall to the other. It will say something like “185” or “245.”
Aspect ratio: After a backward slash, you will have a set of two numbers. This number refers to the height of the sidewall of the tire. It is a percentage of the previous number. For example, you might see a 45, which indicates that the height is 45% of the width of the tire.
Speed rating: This is a letter rather than a number because it provides a classification rather than an exact speed to indicate the maximum speed you can travel on the tire. Z is the highest rating.
Construction: The next letter indicates what type of tire you have. An “R” indicates that it is a radial tire, which means that it contains multiple layers of fabric with additional layers provided around the circumference to strengthen the tire. Radial tires are the most common for automobiles. You may also see a “B” for bias belt or “D” for diagonal.
Wheel diameter: The next number tells you what size wheel the tire will fit. Common numbers include 15 or 16 for cars, 16-18 for SUVs and 20 or higher for many trucks. The size is measured in inches.
Load index: This tells you how much weight the tire can bear. It is important to use tires that can handle the required weight.
Speed rating: This letter tells you how many miles per hour you can travel on the tire.
Why tire sizes matter
The diameter of your tire is important because it impacts your vehicle’s traction and stability. Generally, a wider tire will be more stable than one that is narrow. Bigger tires are more susceptible to damage than one that has a smaller size. Tires with shorter sidewalls can create a rougher ride, while the longer sidewalls will enhance your comfort as you travel. For most people, it is a combination of performance and comfort that makes them select a specific size of tire.
Understanding the parts of a tire
The tread or rubber that you see on a tire is only a portion of what makes up the tire. Many other components are hidden beneath this covering.
Bead: The bead consists of steel cable that is coated with rubber, it holds the tire in place on the rim and handles the force needed for installation.
Body: This consists of multiple layers with different fabrics also known as plies. The number of plies a tire has directly relates to how strong the tire is. An average car tire will have two plies. The most common fabric used in vehicles today is polyester cord and coated with rubber to bond with the rest of the components of the tire. When these plies run perpendicular to the tread, they are known as radial. Diagonal bias tires have plies that run at an angle.
Belts: Not all tires are belted, but those that do have steel belts located under the tread to provide reinforcement. They help prevent punctures and help ensure the most contact with the road for added stability.
Cap plies: These are seen on some vehicles to hold other components in place, most often seen in high performance tires.
Sidewall: This component provides the stability for the lateral portion of the tire and protects the body to keep air from escaping.
Tread: The outer layer of the tire, which is created from multiple types of natural as well as synthetic rubber; it starts out smooth until patterns are created. When the components are put together, the tread pattern is created. The depth of the tread impacts the capability of the tire. A tire with a deeper tread pattern has a stronger grip, especially in soft surfaces. A shallow tread pattern provides faster performance but takes away the grip needed for traction. This is why racing tires are illegal on most roads.
Seasonal vs. all-season
Car tires may be designed to be all-season or seasonal. Seasonal tires are developed based on road conditions most often seen in that season. For instance, winter tires are designed to handle snow and ice while summer tires are better on dry pavement. All-season tires are built to handle all kinds of conditions.
Summer tires: These are often considered high-performance tires, feature large blocks of tread that are stiff with wide grooves to expel water; the rubber is designed for warm weather.
Winter or snow tires: These feature softer rubber and tread which produce adequate grip in low temperatures with tread patterns that grip into snow; often feature thin cuts known as sipes that crisscrosses the blocks of tread to further enhance grip.
All-season tires: This kind of tire has medium-sized blocks of tread with some sipes and a rubber that fits a range of temperatures.
Why inflating is important
A tire holds air to give it the correct shape and firmness to carry a vehicle down the road. The amount of air inside the tire is measured by the amount of pressure per square inch or designated as psi. This number comes from the part of the tire that comes in contact with the road, or the contact patch. It is the portion of the tire that is not completely round.
A tire that is properly inflated will look almost round while one that is under-inflated will appear flatter. The number of psi that must be maintained in the tire is what is required for the contact patch to be at the correct size.
A tire that is over or under-inflated is at a higher risk of damage. It also lessens the stability of the vehicle when traveling. For example, a tire with too much air won’t have enough contact with the road and will be more likely to spin or lose control, especially in adverse road conditions.
How tires move
The tires must carry the vehicle along the road, but it requires so much force exerted by the vehicle to accomplish this task. The power required comes from the weight of the vehicle and the speed it is going. The tires require a lot of friction to force them to move. That amount of friction is impacted by the weight of the vehicle, which creates a coefficient of rolling friction. For an average tire, the coefficient of rolling friction or CRF is 0.015, which is multiplied by the weight of the vehicle.
The tire generates heat because of the friction with a higher heat buildup when more force is required to move the vehicle along. The amount of heat also depends on the firmness of the surface. Asphalt creates more heat on the tire while soft surfaces like sand build up less heat. On the other hand, the CRF increases on soft surfaces because it takes more power to move the tires along.
Problems with tires
Tires must be maintained to increase their life and wear. Tires that are overinflated wear more in the center of the tread while under-inflation causes them to wear along the outside of the tire. When tires are out of alignment, they wear unevenly, especially along the inside or outside. Worn areas are more susceptible to picking up sharp objects or getting holes in them when you run over sharp objects.
Tires that are badly worn may not be able to be repaired when they are flat. A certain amount of tread is required for the repair to be made. Another problem is when the steel belt breaks in a belted tire. It is no longer repairable and must be replaced.
Tires come with various warranties based on how many miles they are expected to last. They can range from as low as 20,000 miles to upwards of 100,000 miles. The average tire will last between 40,000 and 60,000 miles if properly maintained. The life of a tire is directly affected by keeping it properly inflated, rotated as needed and the type of surface most often driven on.
This article originally appeared on YourMechanic.com as How Tires Work and was authored by Joyce Morse.