Boating World

Under Pressure

It's not your trailer that carries the brunt of the weight when trailering your boat; it's the trailer tires. Make sure you have a set that's up to the challenge.

April 1, 2003

Boaters readily acknowledge that not all boats are created equal. There is great disparity in quality and purpose. For instance, aside from being pointy in front and square in back, a small bowrider has little resemblance to a large cruiser. And a tournament ski boat is far different from a tournament fishing boat. Even within the same category, there is often a huge diversity between brands. Some use expensive, high-tech construction materials and methods, while others are built cheaply and employ inferior materials and processes.

Although much less understood and appreciated, the same holds true for the trailer tires that help get your boat to and from water. But far too often boaters conclude that a tire is a tire; that there really isn't much difference between them other than size. However, the first time you experience tire failure and find yourself panic-stricken by an out-of-control trailer, the learning curve climbs pretty quick. Here is a better, less adrenaline-driven way to learn that trailer tires are a distinct breed.

If you look at the sidewalls of the tires on your car or truck, you'll likely notice a "P" or "LT" designation. P stands for "passenger" while LT indicates "light truck," meaning they are designed for those specific types of automobiles. A look at your trailer tire sidewall, on the other hand, will likely reveal an "ST" designation, which stands for "special trailer." ST tires are made specifically for use on trailers and differ considerably from automotive tires. Therefore, tire manufacturers - at least those that bother to make ST tires - strongly discourage the use of anything but ST tires on trailers. About the only exception is when an ST tire is not available for a particular extra-heavy application, such as single-axle trailers with an unusually heavy load that surpass the capacity of an ST tire. In those cases a commercial-grade truck tire is usually best.

While the tire industry has established ST as standard for trailer tires, trailer manufacturers are not required to use them. As long as the tire meets the trailer's load and speed requirements, it is allowable by law. However, the majority of trailer manufacturers do use STs because of the tire's inherent trailering benefits.

One of the benefits of an ST tire when it comes to trailering is that it has about 10 percent more load capacity than an equivalent LT tire and nearly 40 percent more than a P tire when each is filled to its maximum psi rating. Furthermore, an ST's mold shape is designed to accommodate trailer wheels, which are typically narrower than P and LT wheels. Being narrow is an advantage on trailer tires, but a disadvantage on wheels that steer and wheels that are attached to the drive axle. Steer and/or drive tires benefit from a larger footprint, which provides the extra traction and performance demanded of them. A free-rolling trailer tire, on the other hand, isn't called on to turn, swerve and grab the pavement like a steer or drive tire. Since it doesn't need such a large footprint, it performs its task better by being narrower.

For the same reasons, an ST tire does not have as deep of a non-skid tread as P and LT tires. A deep non-skid is necessary for drive and steer tires in wet environments because it provides them with better traction. Again, since a trailer tire is simply free-rolling it doesn't need the same degree of traction on wet pavement and, in fact, it would be detrimental to its performance. Having a shallower tread depth offers the ST tire two benefits: (1) it doesn't wiggle as much, which can help reduce sway, and (2) it rides cooler, which adds to its longevity.

Generally, ST tires also have somewhat stiffer sidewalls, especially in their lower section. This reduces sidewall flexing, helping it to track straighter and diminish the risk of trailer sway. The stiffer sides also lessen the risk of sidewall blowout.

Years ago, nearly all ST tires were bias-ply tires, which have crisscrossing cords of polyester or nylon, with the same number of ply on the sides as there are on the tread. Occasionally bias-belted tires were used. These are the same as bias ply, but with added belts, or layers, under the tread. Since then, however, radial tires have swept the tire industry. Radials feature plies that run perpendicularly across the tire, with belts (some of which can be steel) running under the tread. Today, many ST tires are radials. Is bias better than radial, or visa versa? Actually, each has its pros and cons. In making a decision, it's good to talk to the tire manufacturer about your particular application.

Generally speaking, a bias tire has the advantage when it comes to load and high-speed performance. A radial has the upper hand, however, when it comes to tread wear. On average, a bias tire's tread will last approximately 12,000 miles, whereas a radial typically lasts 40,000 miles. There are exceptions, of course, but this is an average comparison.

Regardless of whether you choose radial or bias, it is crucial that you provide regular tire maintenance. The most common cause of tire problems is under-inflation. It is estimated that a trailer tire that is 20 percent underinflated will cut 25 percent off its lifespan; one 30 percent underinflated will reduce its term by 55 percent. Underinflated tires also affect fuel effi-ciency by as much as 10 percent due to added drag.

An underinflated tire puts undue stress on the tire, producing irregular wear at best and complete tire failure that could result in an accident at worse. Abnormal tire flexing occurs when a tire doesn't have enough air, which can generate an excessive amount of heat internally. Too much heat will exceed the tire's capabilities and could result in a blowout.

Merely looking at a tire won't always tell you if it is underinflated. In fact, some tires can lose up to half of their pressure before it is noticeable from appearance alone. And with each pound of pressure loss there is a corresponding loss of load-
carrying ability. Therefore, you should check the tire's pressure regularly with a good quality gauge. And for accuracy, always check the pressure when the tire is cold. Driving only a few miles will heat the tire and increase the pressure reading.

Another major cause of tire failure is overloading. That's why it is important to know the actual load being towed. That includes your boat, trailer and any gear, including boat fuel, that you put in the boat. An overloaded tire will produce an inordinate amount of heat, which, as discussed above, can cause tire degradation or blowout.

A good way to ensure you don't run your trailer tires either underinflated or overloaded is to install a quality tire monitoring system. Many such systems monitor tire pressure only. A few, however, also monitor heat, which can rise to dangerous levels in a properly inflated tire that is overloaded.

A final word on tire care is to use the right kind of cleaners, or dressings, as they are often referred to. Many of these products are designed strictly to enhance the tires' aesthetics by making them shine. In reality, however, they often contain petroleum products, alcohol or silicones that deteriorate the protective antioxidants the manufacturer puts in the tire during the construction process. Read the tire manufacturer's warranty and you may see that using such products render it void.