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Off Road Round-Rubber Dictionary - Tire & Wheel Special

Posted in How To on November 26, 2008
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Photographers: Courtesy of BFGoodrich

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.

Light-truck tires are labeled on their sidewalls with a load range (i.e. C, D, E) or ply rating ("6 Ply Rated"). The maximum recommended tire pressure is also displayed and is the pressure setting at which the maximum stated load capacity is obtained.

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.

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There are two diverging types of tire construction on the market: bias-ply and radial tires. Each has its pros and cons, but radials are much more often found on cars and light trucks these days than their bias-ply alternative.

Bias-ply tires utilize angled criss-crossing cords of polyester or nylon. They use the same number of plies on the sidewall as they do on the tread area. A derivation of this tire design is the bias-belted tire that uses addition plies under the tread area.

Radial tires use plies that are laid perpendicular to the direction of travel, running from one sidewall lip to the other. Many use a series of steel mesh or other high-strength material belts for added strength. The cross-ply layout also reduces rolling friction whereas bias-ply tires tend to have their plies rub on each other as the tire moves and flexes. Hence, radial tires typically offer better gas mileage on a vehicle, as well as greater tread life.

Today radial tires comprise a large portion of the light-truck tire population. It used to be that many drivers stuck with bias-ply tires on an off-road rig to maintain better sidewall strength. But today, advancements in design and materials have shrunk that difference in weakness.

Once you get a significant mass, such as a tire rotating at high speed, balance becomes important, if not critical, to vehicle behavior. An out-of-balance tire can go from a simple annoyance to a potential safety issue. Also, driving long term with an imbalance can accelerate wear on axle and brake components.

Static bubble balance is the simplest means of checking for tire balance. As its name describes, the tire is placed on a floating hub, and weights are added until the bubble balance shows the tire sits horizontal and true. This method is less often found in modern tire shops.

Dynamic spin balance is the preferred method for checking a wheel-and-tire combo. It used to be difficult to find a tire shop that had a spin balance machine that could balance very large tires, so bubble balance was used. It was also common to see large 1- to 2-pound masses of tire weights attached inside a wheel on a huge mud tire. Tire molds were of poorer quality in days past. Sometimes radical tires rolled so poorly on the highway that shops would spin them on a machine and shave some of the tread rubber off to true the tire. Of course this meant that very expensive new tire rubber and some of your best traction tread ended up on the shop floor. Today off-road shops can better balance large tires, and tire shaving is less of a need.

Alternative methods of balance include using various free-floating materials, including golf balls, liquid balancers, BBs, or ceramic beads inside the tires. Other methods involve the use of balance rings that mount behind the wheel and may use mercury or metal weights immersed in oil to provide a balancing action to the wheel and tire.

There are a number of products on the market that are substances put inside the tire to offer a dynamic balancing effect. In effect, these liquid or granular substances are installed in the tire before installation on a wheel or can be injected into a tire through its valve stem. The material floats freely inside the tire interior and is meant to disperse to the outside of the tire due to centrifugal force and migrate inside to a side opposite that of a heavier spot on the tire. If using such material, ensure that wetting is not an issue that can cause clumping of the material.

One advantage to a dynamic balance method is that it can constantly adjust for tire wear, chunking, mud buildup, rocks in the tread, etc., whereas a traditional lead weight balancing cannot. Lead clip-on weights can get knocked off, and stick-on weights can be torn loose as well. Also, if a tire spins on a wheel while aired to low pressure for off-road use, the balance may be ruined as well.

It's common knowledge that proper tire inflation can help save gas and help maximize tire life. As we mentioned before, under-inflation increases the amount of tire-bulge movement, increasing heat and shortening tire life.

Tire pressure off-road can make all the difference in the world. Low pressures increase sidewall bulge and the tire contact patch, allowing greater traction. At slow wheeling speeds, heat generation from the extra tire squirm is kept to a minimum. In fact, those huge, heavy buildings you see engineers moving can be done on rubber tires due to high air pressures and super-slow rotational speeds. Low pressures and high speeds don't work well together.

When driving an off-road vehicle in sand, the general idea is to float over the surface and not risk sinking into what are usually bottomless grains. Dig in too deep, and you'll soon find your axlehousing or bellypan stuck in the soft stuff. In general, the wider the tire, the better, as this spreads the load of your rig across a greater surface area. And the more mild your tread is, the better you'll float and not dig in. You'd be amazed at how well bald tires work in the sand! Paddles (or even Boggers) can be great sand tires too, but you'll need a ton of power to use them to "paddle" through the sand.

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Debate over what type of tire works best in mud continues. Some drivers prefer a wider tire to help float over the surface, and others want a skinny tire that cuts down quick to harder soil. Each can be right or wrong, and the correct answer is very dependent on the type and depth of mud and the weight and horsepower of the vehicle.

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Off-road-racing tires, such as this BFGoodrich Baja T/A KRT, are built to withstand the extreme rigors of a punishing environment. Tread patterns are meant to provide both traction and reasonably low rolling resistance for high speed.

This race team blew a rear tire in the 2008 Best in the Desert Parker 425 when it hit a sharp rock.

The tire was like new at the beginning of the race and still had fresh tread when it went. The vulnerable sidewall was ripped open, dealing the deathblow.

Racing environments beat a tire mercilessly at high speed with sharp impact in a contracted time period. The carcass can become abused quickly if a substandard tire is used.

The fluffy white stuff is yet another situation where tires can make a big difference as to whether you stay on the trail or road and whether you continue to make forward progress. Like mud, snow composition can vary from ice to wet slush to relatively dry snowpack. Each behaves a little differently, and effective snow tires can vary based on what the snow is like in the area. Again, wide or narrow can be debatable depending on snow conditions and vehicle weight.

Tires can be most vulnerable to punctures or slicing when wet. The water serves as a lubricant on the rubber and allows rocks or other sharp objects to penetrate much more easily.

It's general safe practice that a tire shop will only plug or patch repair a tire if the leak occurs on the tread surface. It is considered unsafe to facilitate a repair on the sidewall area. So if you put a nice slice in the side, then you may be out of luck with that tire. However, in the case of a tire used strictly for relatively low-speed off-roading, it may be possible to have a sidewall slice fixed using a vulcanized patch. In this case, a rubber patch is heated or melted onto the inside (and outside) of the tire and intimately bonded with the tire rubber. This is done using a heating repair machine and can sometimes be found at service shops that work on large equipment or tractor tires.


It's a good idea to have a heavy-duty tire-plug kit with you, especially if you don't have a working spare tire left. You can often repair a puncture in a few minutes at least well enough to get you some place where you can get the tire fully repaired or swap on another tire. It's pretty amazing what these little sticky plug fibers can do. We've seen 1-inch-long sidewall cuts plugged with 8 to 10 plugs, and the leak fixed well enough to hold air to get back to the camp or trailer.

When choosing a tire to work best on your terrain, talk to others in your area to get an idea of what tire works on what vehicle. Dirt, rock, and soil conditions vary widely across the country, and a tire that works well in northwestern woods may not work well on southeastern trails. Don't assume a tire on a 3/4-ton diesel truck will work the same on your lightweight SUV.

Any off-road tire used on a street-driven 4WD will be a compromise. Decide how much time you truly spend on trail, how much off-road performance you need, and how a tire will perform during your highway driving. New advances in tire technology have brought us much better dual-purpose designs that work well in both environments. It's not always an easy choice, but tire manufacturers today have given us tremendous technology and choices to fill our tire needs.

UTV tire choices are expanding rapidly, and you'll find a wide variety of tread patterns and sizes. Like truck tires, UTV tires come in bias-ply and radial designs. However, with improvements in radial construction, it seems bias-ply versions are losing popularity. Newer radial designs offer comfort, good contact patch area for traction, and improved sidewall resistance to punctures.

Flat- and round-tread tires are available. A more-rounded tire will generally provide a softer ride but will tend to roll under during hard cornering. But it usually performs well in softer terrain. Flatter-tread tires tend to work better on firmer ground and have better sidewall stability.

When choosing a UTV tire, like a truck tire, consider the terrain you most often ride and pick a tread pattern best suited to that scenario.

When it comes to hauling heavy loads or pulling bulky trailers, you'll want to ensure your truck tires are up to the task. Extra tongue weight will translate to increased rear tire load and can ultimately lead to tires that run too hot and start to destroy themselves.

Heavy-duty truck and RV tires for heavy weight or towing should be carefully chosen based on their load rating. Many vehicles are run with loads that exceed the tire ratings, causing the tires to heat excessively and risk premature failure. Remember, the maximum load rating is preserved only when the tires are inflated to their maximum sidewall-stated pressure.

Trailer tires should also be chosen for their rated load. It is certainly possible to install passenger-car tires on a heavy-duty trailer and stay within the load ratings. However, there are tires designed specifically for trailer use and designated as "ST" type. These tires typically have stiffer sidewalls to reduce sway and tread designs meant to help a narrower tire run cooler and last longer in a trailer application where the tires are neither driven nor steered. Additionally, the most common cause of trailer tire blowout is heat due to under-inflation. A small leak that causes the tire to deflate can go undetected much more easily on a trailer than on the car or truck you're driving.

When it comes to tackling rock surfaces, an aggressive tire works best in most situations, except possibly slickrock or sandstone, where a soft all-terrain tire may grip better. Open tread lugs grab onto rock edges more readily, and large tread blocks provide greater support and tear out less than tires with a smaller tread design.

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