On radial tires, the fabric carcass layers are laid down perpendicular to the bead rather than at an angle, which makes for a more flexible sidewall. Then, on top of the carcass plies, there’s an added layer of belting, most often made of thin steel cable or aramid thread that’s located only beneath the tread and is specifically there to strengthen that area. So with a radial tire the sidewall and tread have independent constructions, giving those two sections of the tire different flex characteristics. And that’s a big deal if you’re trying to design a tire with very specific behaviors. With a radial, you’re not stuck compromising tread rigidity in favor of sidewall flexibility like you are on a bias-ply, which again has that uniform construction.