POSSIBLE CAUSES
Severe operating conditions may cause local reinforcing fabric nylon cord to rupture. When such tires are deflated, small bulges (i.e. surface swelling) may appear on the tread surface. This phenomenon is due to the stretching of rubber around the ruptures in the reinforcing fabric and does not pose any problems once the tire is retreaded as the reinforcing fabric is replaced.
However, bulges and blisters appearing while the tire is inflated and in service indicate a separation or damaged tire.
RECOMMENDATION
Mark the area and remove the tire immediately.
