Off Road Round-Rubber Dictionary - Tire & Wheel Special

To the average person, a tire is a tire - simply a necessity to get your vehicle from one place to another. To an off-roader though, a tire is what gets him across the diverse terrain that is his playground. High-horsepower engines, crazy articulations, or long-travel suspensions don't get pushed to full use if the tires used on your rig are ineffective. Your tires and their performance can determine whether you get to the top of a trail hill, get across that dune or mudhole, or allow you to safely haul your boat to the lake for the weekend. There are a lot of variables when it comes to tire talk, but we'll do our best to decipher some of those choices here.

We owe a lot to Charles Goodyear, who invented vulcanized rubber in 1844. In the later 1800s, other inventors started experimenting with the creation of rubber vehicle tires. Since that time, we've seen considerable advancement in tire design and construction. Radial tires were invented as a post-WWII creation, and longer-wearing and higher-performing tires have followed.

Today you can get a simple tire for the family sedan that does little more than get you from here to there in comfort and safety. But you can also buy specialty tires built for high speed and handling, or in the case of off-road, tires specifically suited for mud, rocks, snow, or desert racing. Choosing amongst the many styles can be a dizzying task, but with some understanding of performance considerations, you should be successful at choosing a set of tires that will perform for your purposes.

At first glance, a tire might seem like a relatively low-tech item: simply rubber material molded into a shape made to wrap around a wheel and retain air pressure. However, there is considerable engineering energy poured into the design of a tire, and the results determine how well it handles, wears, and survives the daily road or trail abuse it sees.

The portion of the tire you see outside is the pure "rubber" portion of the tire, but inside, the tire is composed of plies (basically layers) that add strength, shape, and structure to the carcass. The plies also serve to make the carcass tougher and help prevent damage or punctures. Plies can be made from polyester fibers, Kevlar, steel mesh, or various other derivative materials.

Inside the bead of the tire, where the tire forms a lip that mates to the wheel face, are steel cables that add great strength to this area. Here the tire must remain fixed to the wheel and withstand the forces of accelerating and braking that are placed on this interface.

The design and construction of a tire is done such that it has a weight load rating that should not be exceeded. Load ratings are defined such that they are applicable at the maximum inflated pressure as stated on the tire sidewall. It is the pressurized air inside the tire that is called upon to support the load on the tire, and this pressure does not change with added load or weight. Simply put, the higher the pressure you can inflate a tire, the higher its load capacity. Additionally, a larger tire will tend to have a higher load rating than that of a smaller tire of the same type.

As a tire rotates across the ground, it is constantly changing shape as its bulge profile varies. Imagine a point on the tire. As it rotates down toward the road surface, it bulges outward due to vehicle weight, but then as it travels away from the road, the bulge relaxes. This constant flexing movement creates heat in the tire, and heat is one way to shorten tire life. Under-inflation or overload also causes the same phenomena to a greater extent, which leads to premature wear and possible tire failure.

Also, a tire with a lower load rating will often be more flexible, and one with a higher load rating will often be less flexible and thus ride rougher on the road. Tires may be upgraded as needed to reflect the weight and loading of your particular vehicle, assuming the axles and chassis are up to the task.

When choosing a tire, you'll first want to determine size, whether it be the same as the stock size or something larger. Passenger-car tire (sometimes used on light trucks) sizes are designated with a "P" prefix, and light-truck tire sizes are designated with an "LT" prefix.

Wheel width can play a part in tire performance. It affects the sidewall and tread shape and how they flex against the terrain. The wheel width will also determine the amount of sidewall bulge beyond the wheel edge.