Maintenance of automobile tires

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 & 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 underinflation, overload or 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 & 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 & 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 & 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, & 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 & then remounted after the patch is applied. It should be noted that a plug only or patch only type repair is "NOT AN ACCEPTABLE REPAIR". Ref.

Sometimes a more serious rupture of the tire material occurs resulting in a blowout. A "blowout" may also be caused by running at highway speeds while the tire is significantly underinflated. The heat generated can melt the body cord & an explosive loss of air occur if the driver continues to operate the vehicle in this condition. 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 & remounted after the valve replacement. Occasionally, other types of damage require replacement of a tire.

Vehicles typically carry a spare tire, already mounted on a rim, to be used in case a flat tire or blowout occurs. These days, most spare tires (sometimes called "doughnuts") for cars are smaller than normal tires (to save on trunk space, gas mileage, & cost) & should not be driven very far before replacement with a full-size tire. Years ago, full-size or conventional spare tires were used. A few modern vehicle models may use conventional spare tires also. Jacks & tire irons for emergency replacement of a flat tire with a spare tire are included when buying a new car. Not included, but sometimes available separately, are hand or foot pumps for filling a tire with air by the vehicle owner. Cans of pressurized "gas" can sometimes be bought separately for convenient emergency refill of a tire.

Alternatively, many modern cars & trucks are equipped with run flat tires that may be driven with a puncture - or perhaps are even self-repairing for moderate sized holes.

Front tires, especially on front wheel drive vehicles, have a tendency to wear out more quickly than rear tires. Routine maintenance including tire rotation, exchanging the front & rear tires with each other, is often done periodically to even out tire wear. There are simple hand-held tire-pressure gauges which can be temporarily attached to the valve stem to check a tire's interior air pressure. This measurement of the tire inflation pressure should be made at least once a month. The properinflation for the vehicle is located in the owner's manual & on the Tire Placard. Because of slow leaks or changes in weather or other conditions, tire pressure may occasionally have to be adjusted, usually by refilling through the valve stem with some pressurized air which is often available at service stations.

Some modern cars now incorporate automatic tire pressure sensing with a warning light indicating when tires have become dangerously deflated. These systems use the measurements from the wheel speed sensors at each wheel. Since a partially deflated tire has a slightly smaller diameter than a correctly inflated tire, the car computer can check that all four wheels make approximately the same number of rotations when averaged over many miles of driving. If one wheel consistently makes more rotations than the others then it must be deflated so the warning light is lit. However, vehicle operators should not wait for the low pressure warning light to come on before they check their tire pressures. In most cars the tire pressure sensors have to be reset (typically by holding down a button) whenever you top up the air in your tires. Tires will gradually lose pressure in all four wheels together - a situation that the pressure sensing system cannot detect - & in any case, road holding & fuel economy may be compromised by a smaller loss of pressure than the sensor is set up to warn about. An alternate system, prefered by some, directly measures the inflation pressure of the tire.