The internal construction of a tire is important as it is what helps reinforced and strengthen the tire. The inside of the tire is made up of plies of cord, this network of cords is called the tires carcass and typically are constructed of polyester, steel and other types of textile materials, then reinforced with rubber. How this network of cords is designed is what gives the internal construction of a tire it’s identity.
A radial ply tire means the plies of cord are laid out at 90 degrees from the direction of travel, lip to lip, as if the cords radiate out from the imaginary center of the tire. This type of tire construction is popular for reducing rolling friction of the tire, more breakaway grip, better fuel economy, longer tread life and better steering control.
Bias Ply tires are common in trailer tires and other rough road, heavy hauling applications. The bias ply construction is crisscross, with the cords overlapping each other. Providing a smoother ride on rougher surfaces. The down side of bias ply is the increased rolling resistance and less traction and control at higher speeds.
For trailer tires, bias ply tires have a tougher sidewall and are bruise resistant. Also, they are cheaper. So if your trailer sits more than it is used, bias ply might be the way to go, as many of the benefits of radial trailer tires don’t come into play if the tires are not used regularly.
The diagram below from Wikipedia shows off the two main internal tire construction designs (12 is the radial design and 14 and 16 show the bias design):
Because the internal construction of a tire is so different between radial and bias ply, the tires also handle differently. Be aware of handling turns when driving on bias ply tires, because of the construction design, the treads are typically thicker and the tire will loose some contact with the road on a turn. When using bias ply tires you will want to lead the corner more when driving into a turn.