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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 wheels can usually be used throughout the lifetime of the car. Uneven or accelerated tire wear can be caused by under-inflation, 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 from 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 safely. A leak may be a slow one, such as when the seal between the rim and tire edge is not perfect. Many leaks in flat tires are caused by nails, screws, caltrops, broken glass or other sharp objects puncturing the tire. 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 located by submerging the pressurized tire in water to see where air bubbles emerge. If submerging a tire underwater is not possible, the leak can be searched for by covering the pressurized tire surface with a soap and water 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, then the protruding ends of the plug can be cut off, and the tire can be refilled with air to the appropriate pressure, and the repaired wheel reinstalled on the vehicle. Patches covering a hole can be glued or rubber-cemented to the interior surface of a tire, 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. It should be noted that a plug-only or patch-only type repair is not an acceptable repair. 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 under-inflated. The heat generated can melt the body cord and an explosive loss of air may occur if the driver continues to operate the vehicle. A tire thus damaged usually must be replaced. 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. Vehicles typically carry a spare tire, already mounted on a rim, to be used in the event of flat tire or blowout. Many spare tires (sometimes called "doughnuts") for modern cars are smaller than normal tires (to save on trunk space, gas mileage, weight and cost) and should not be driven very far before replacement with a full-size tire. A few modern vehicle models may use conventional spare tires. Jacks and for emergency replacement of a flat tire with a spare tire are included with 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. Some modern cars and trucks are equipped with run flat tires that may be driven with a puncture over a distance of 80 km to 100 km. This eliminates the need for an immediate stop, and the associated expensive tow service or tire change. 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 and rear tires with each other) is often done periodically to facilitate uniform 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 tire inflation pressure should be made at least once a month. The proper inflation pressure is located in the owner's manual and on the Tire Placard. Because of slow leaks or changes in weather or other conditions tire pressure may occasionally have to be corrected, usually via the valve stem with compressed 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 ABS 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, and the warning light is lit. However, vehicle operators should not wait for the low pressure warning light to illuminate before they check their tire pressures. In most cars the tire pressure sensing must be reset (typically by holding down a button) whenever the tire pressure is corrected. Tires may gradually lose pressure in all four wheels simultaneously, a situation that the pressure sensing system cannot detect. Road holding and fuel economy may be compromised by a smaller loss of pressure than the sensor is able to detect. An alternate system directly measures the inflation pressure of the tire. |
Crumb Rubber price and steel from recycled tires for export/import |