every 3-4 years.
Trouble with dry rot is by time you see it, its too late. These are trailer tires, dont confuse tread wear on a passenger/light truck tire with tread wear on a trailer tire. Tread wear is NOT an indicator of wear on a trialer tire. They are designed not to wear out. Trailer tires are not subjected to power or steering forces like passenger/light truck tires. A trailer tire is designed to carry a load, they have stiffer sidewalls than passenger and light truck tires.
the number one reason trailer tires fail is due to heat. They are designed to go a max speed of 65pmh. If your trailer tires are smaller in diameter than your truck tires it is traveling faster than your speedometer. If your doing 65 in the truck on 30" tires and towing a trailer with 20" tires the tires are going a hell of a lot faster than 65 mph.
Here's a link to help you better understand.
Trailer Tire Facts - Discount Tire
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