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Tire maintenance for motor vehicles is based on several factors. The chief reason for tire replacement is friction from moving contact with road surfaces, causing the tread on the outer perimeter of tires to eventually wear away. When the tread depth becomes too shallow (less than 0.125in./3.2mm), the tire is worn out and should be replaced. The same wheels can usually be used throughout the lifetime of the car. Other problems encountered in tire maintenance include: *Uneven or accelerated tire wear: can be caused by under-inflation, overloading or poor wheel alignment. *Increased tread wear on only one side of a tire: often a sign of poor wheel alignment. *Tread worn away completely: especially when the wear on the outer rubber exposes the reinforcing threads within, the tire is said to be ''bald'' and must 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. Since this bonding may occasionally come loose, new tires are considered superior to recapped ones. 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. See Flat tire for more information. ==Spare tires== Vehicles typically carry a spare tire, already mounted on a wheel rim, to be used in the event of a flat tire or blowout. Spare tires (sometimes called "doughnuts") for modern cars are smaller than regular tires (to save trunk space, weight and cost) and should not be used to drive very far before replacement with a full-size tire. A few vehicle models use conventional size spare tires. Jacks and wrenches for emergency replacement of a flat tire with a spare are included with a new car. Not included, but available separately, are hand or foot pumps for filling a tire with air by the vehicle owner. Cans of pressurized air can sometimes be bought separately for convenient emergency refill of a tire. Some 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 tire change or calling for roadside assistance. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Tire maintenance」の詳細全文を読む スポンサード リンク
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