PR is an abbreviation of "ply rating". This label originally indicated the number of layers of cotton fishing line in the shell. A large number of bearings means a higher load capacity. When stronger materials were introduced for bearings, this term was removed from the actual number of bearings. Currently, a tire with 8 PR can only have four nylon linear bearings. Today, the stability of a controlled tire is expressed in terms of load capacity and speed capacity, and not in terms of the ply rating.