As a general rule, radials need a bit more caster than bias ply tires because the sidewalls are a little more flexible. Some highway cruisers like big Mercedes cars, etc, run the caster up to 7-8 degrees or more to assist straight-line stability. At that point, tires start to experience some scrub wear in tight turns because of the camber change they experience as they turn away from the straight ahead position.
Jerry
