A tire gets it shape from layers of fabric and adhesive that comprise the body of the tire. The layers are called plies, and belts consist of cords of metal, nylon, kevlar or similar material. The plies are wrapped in an outer and inner layer of rubber. The rubber layers make a tire hold the air, and the ply layers give the shape. To maintain the tire's integrity, all of the layers are permanently bonded together during the manufacturing process.
The underlying cause of a bubble in the side of a tire is a separation between the cords and plies in the body of the tire. The damage to the plies allows air to seep through and form a bubble in the flexible rubber outer layer of the tire. A bubble is a sign of internal damage to the tire. Several factors can cause bubbles, including manufacturing defects, under-inflation and tire impacts.
It is possible that a tire may be manufactured with bad or contaminated materials, or the curing process bonding the components may have not been fully completed. If the cord and plies are not properly bonded together, they will eventually separate, and a tire bubble will result.
An under-inflated tire will flex significantly more than if it is properly inflated. The flexing causes excess heat to build up in the tire, and this heat will break down the materials that hold the plies and cords together. This heat caused separation is another source of bubbles.
Hitting a curb or pothole hard enough will cause internal damage to the plies of a tire. The blow could break cords or cause plies to separate. Once again, when you get this type of separation in the body of a tire, the external sign will be a bubble in the sidewall.
Once you have a bubble in the sidewall of a tire, the only safe solution is to replace the tire. A bubble that is the result of a manufacturing defect should be covered by warranty if the tire is still in the warranty period. Bubbles due to manufacturing problems usually show up within a few months of driving on the tire. Bubbles from under-inflation or impact damage may not show up until months after the abuse occurs.
There are steps you can take to prevent bubbles or prevent a bubble from leading to a tire blowout. First, visually inspect your tires regularly. Walk around your car and give the tires a close inspection. Remember the tires have inner sidewalls too and these need to be inspected regularly. Second, check your tire pressure at least once a week. Have tires with slow leaks repaired promptly. Finally, most impact damage can be avoided by driving with caution near curbs and on bad roads.
