Trailer, or "ST", tires are considered a special category in the current global design standards of Tire and Rim. The major difference is that trailer tires are required to carry 10% more load than an equivalent P-metric size tire.
LT type tires are actually on a lower formula and would require a significant size or load change to meet the trailer load requirements.
In the design process, we evaluate what works best under trailer applications/conditions. We typically find that a narrower tread width and a shallower non-skid (tread depth) provides a better overall performance in a free rolling position. This is one case where wider is not better.
There are significant design differences with these tires and this explains why trailer tires are in a special
category. In addition, that is why we recommend using trailer designed or trailer specific tires in trailer towable applications.
Sincerely,
Steve McKay
Consumer Relations
Goodyear Tire and Rubber Company
TRAILER-TIRE CONSTRUCTION
Commonly encountered tires are P (passenger car), LT (light truck) and ST (special trailer). While each tire series shares basic construction methods, such as bias or radial belts, a bead, tread plies and so on, the details vary meaningfully among the three. Considerations in designing tires used on trailers include the need to carry heavy loads, the relative lack of cornering loads, long duty cycles (the tires can be expected to sit for a year; then rotate for days on end during long trips). Furthermore, because the design of most trailer suspensions dates back many decades, ST tires are designed for as soft a ride as possible so they don't transmit too much shock to the trailer and its contents.
Trailer-tire sidewall stiffness is a compromise between P and LT designs. The desire for stiffer sidewalls is still occasionally cited as the reason for choosing a bias-belted trailer tire. While passenger-car tires are nearly all radials these days, ST tires are still available in bias-belted construction. Radial trailer tires are superior in all respects to bias-belted tires except in sidewall stiffness. Reduced tire heat, lower rolling resistance and softer ride are among the benefits of radials, not to mention extended wear. On the road, ST tires share some characteristics of passenger-car tires, but are closer to the design of light-truck tires. Trailer tires typically employ heavier steel or poly-ester cords and somewhat lighter sidewall construction than light-truck
TRAILER-TIRE SAFETY
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tires, and trailer tires typically run lower air pressures than their truck counterparts. This gives ST tires good load-carrying capacity; but with the desired softer ride. ST tires also have the advantage of rubber compounds that are specifically designed to resist deterioration from the elements, including sunlight and ozone, during extended storage.
