Friction from moving contact with the road causes the tread on the outer perimeter of the tire to eventually wear away. When the tire tread becomes too shallow, the tire is worn out and should be replaced. The same tire rims can usually be used throughout the lifetime of the car. Uneven or accelerated tire wear can be caused by bad wheel alignment. More wear on a tire facing the outside or the inside of a car is often a sign of bad wheel alignment. When the tread is worn away completely and especially when the wear on the outer rubber exposes the reinforcing threads inside them, the tire is said to be bald. A bald tire should be replaced as soon as possible. Sometimes tires with worn tread are recapped, i. e. a new layer of rubber with grooves is bonded onto the outer perimeter of a worn tire. Because this bonding may occasionally come loose on the tire, new tires are superior to recapped tires.
Sometimes a pneumatic tire gets a hole or a leak through which the air inside leaks out resulting in a flat tire, a condition which must be fixed before the car can be driven further safely. A leak may be slow in a few cases, such as is sometimes observed when the seal between the rim and tire edge is not perfect. Many leaks in flat tires, though, are caused by nails, screws, caltrops, broken glass or other sharp objects puncturing the rubber tire wall. If the hole is small and not elongated, the tire can often be repaired by using plugs from a tire repair kit. A leak in a tire can often be found by submerging the tire, pressurized with air, under water to see where air bubbles come out. If submerging a tire underwater is not possible, the leak can be searched for by covering the pressurized tire surface with a soapy solution to see where leaking air forms soap bubbles. A puncturing object, such as a nail or a screw, can be pulled out using pliers. Then a plug coated with a semi-liquid form of rubber can be inserted into the hole with a special tool. The rubber covering the plug solidifies rather quickly, after which the protruding ends of the plug can be cut off, the tire can be refilled with air to the appropriate pressure, and the repaired wheel replaced on the vehicle. Patches covering a hole have been glued or rubber-cemented to the interior surface of a tire also, particularly if a hole is too elongated for a simple plug. Tire repair with such patches requires the tire to be taken off the rim and then remounted after the patch is applied. Sometimes a more serious rupture of the tire material occurs resulting in a blowout. The damaged tire typically must be replaced after that. A leaking valve stem may occasionally be the cause of a leak, necessitating valve stem replacement. This replacement means the tire will have to be taken off the rim and remounted after the valve replacement. Occasionally, other types of damage require replacement of a tire.