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quote:Originally posted by HomeOnTheRoad
Is the ST and LT article current? My ST tires say radial on them not bias ply. Thank you for the article.
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Upon some futher research I came upon these words of wisdom:
The "ST" Sign
Look at the sidewalls of the tires on your car or truck, and 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 a "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 large load that surpasses the capacity of an ST tire. In those cases a commercial grade truck tire is usually best.
While the tire industry has established this standard for trailer tires—the ST—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.
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, the 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. Steering and/or driving 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, ST tires don't have as deep of a non-skid tread as P and LT tires. A deep non-skid is necessary for driving and steering 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 provides the ST tire with 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, the ST tire also has somewhat stiffer sidewalls, especially in its 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.
Bias or Radial?
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 is 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, however, is better 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.
Under-Inflation
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 under-inflated will cut 25 percent off its lifespan; one 30 percent under-inflated will reduce its term by 55 percent. Under-inflated tires also affect fuel efficiency by as much as 10 percent due to added drag.
More important, under-inflation puts undue stress on the tire, producing irregular wear at best. At worst, under-inflation causes complete failure that can result in an accident. 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 cause the tire plies to separate or result in a blowout.
Merely looking at a tire won't always tell you if it is under-inflated. In fact, some tires can lose up to half of their pressure before it is noticeable from appearance alone. (But you still should check for cracks or other signs of wear.) 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.
Overload
Another major cause of tire failure is overloading. That is why it is important to know the actual load being towed. That includes the boat, trailer and any gear, including the boat fuel in the boat. An overloaded tire will produce an inordinate amount of heat, which, as mentioned above, can cause tire degradation or blowout.
A good way to ensure you don't run your trailer tires either under-inflated 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.
JP