What is the difference between radial and bias ply tires?  A radial tire allows the sidewall and the tread to function as two independent features of the tire.  A bias ply tire consists of multiple rubber plies over-lapping each other.  The crown and sidewalls are independent.  The overlapped plies form a thick layer that is less flexible and more sensitive to overheating.  

A radial tire is a particular design of vehicular tires.  In this design the cord plies are arranged at 90 degrees to the direction of travel, or "radially".

Bias ply tire construction utilizes body ply cords that extend diagonally from bead to bead, usually at angles in the range of 30 to 40 degrees, with successive plies laid at opposing angles forming a criss cross pattern to which the tread is applied.