"Bias ply" is actually a poor term. "Cross ply" would be a better description. In a bias-ply tire, the belts are arranged at crisscross angles, i.e. "on the bias." With a bias-ply tire, the sidewalls have the same construction as that of the crown. When a bias-ply tire flexes, the crossed plies "fight" each other. This internal struggle produces heat. The heat can break the tire down faster. Another byproduct of the cross-plies resistance is that the tire doesn't absorb small jolts as easily as a radial. What advantages do bias-ply tires have? Bias-ply tires are less expensive to produce, so they're easier on the wallet. Bias-ply tires tend to have tougher, thicker sidewalls compared to radials. Finally, the "dead" feeling of a bias-ply tire means that it won't bounce as much, which can be an advantage on steep climbs in the rocks.