13 December 2005

The Skinny on Trailer Tires

Trailering isn’t for every hobbyist. Some prefer driving their vintage cars, while others think it’s safer and more convenient to tow.

Trailering may be safer – if you have good trailer tires. However, many people have a habit of putting old car or truck tires on a trailer. They think trailer tires don’t get as much wear or abuse as car tires. But, they are only half right.

Trailer tires usually don’t wear out as fast as car tires because they don’t roll as many miles. You probably don’t use your trailer every day or in the winter, so tread wear will be minimal. However, the real danger to trailer tires is the sun’s ultraviolet rays. When rubber is exposed to sunlight, it begins to deteriorate. Trailer tires are constantly in the sun. Even if you have cycle-style fenders on your trailer, the tires are still exposed to sunlight more than car tires.
 
Ultraviolet rays will cause rubber to crack. Trailer tires use rubber compounds that resist sunlight and ozone exposure, but aging is still more of a problem to trailer tires than wear. You can fight some of the deterioration by using protective tire covers (often seen on motor homes) and parking in the shade, but it’s still likely your trailer tires will crack before they go bald.

To get tires for your trailer, you need to know the Gross Vehicle Weight Rating (GVWR) and Gross Axle Weight Rating (GAWR). The GVWR is the maximum weight for each axle. The GAWR (tongue weight) is the weight on the hitch. If you have a two-axle trailer, add the GVWR of both axles to get the GAWR.

To find your trailer’s GVWR and/or GAWR rating, check your owner’s manual. If you don’t have one, look for a data plate or try to find someone with a similar trailer who knows the rating. These values must be matched to the proper tire load range in order to get the right tires. You can’t just put heftier tires on your trailer and use it to carry a tank.

When you’re ready to buy tires, check the chart at a tire store. It will cross reference tire sizes, load ranges, tire pressures, axle loads and tongue weights. Trailer tires will have an “ST” prefix indicating “Special Trailer.” Trailer tires will usually have a GVWR rating that is 9 percent higher than the rating for similarly sized non-trailer tires. You shouldn’t use standard tires, such as P-Metric tires, with codes starting with “P.” You need tire codes that start with “ST.”

Never use trailer tires on a car. Trailer tires have tread patterns different from those on car and truck tires because they are made to be pulled. The trailer tire will have a solid and continuous center band and tread blocks that maintain road contact to resist irregular wear. In cross-section trailer tires will be more rounded. Nylon reinforcements are used to increase traction.

John “Gunner” Gunnell is the automotive books editor at Krause Publications in Iola, Wis., and former editor of Old Cars Weekly and Old Cars Price Guide.

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