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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.