The bead is constructed of a heavy band of steel wire
wrapped into the inner circumference of the tire's ply
structure. The tread, or crown, is the portion of the tire
that comes in contact with the road surface. It is a pattern
of grooves and ribs that provides traction. The
grooves are designed to drain off water, while the ribs
grip the road surface. Tread thickness varies with tire
quality. On some tires, small cuts, called sipes, are molded
into the ribs of the tread. These sipes open as the tire
flexes on the road, offering additional gripping action, especially
on wet road surfaces. The sidewalls are the sides
of the tire's body. They are constructed of thinner material
than the tread to offer greater flexibility.
The tire body and belt material can be made of rayon,
nylon, polyester, fiberglass, steel, amarid, or Kevlar: Each
has its advantages and disadvantages. For instance, rayon
and cord tires are low in cost and give a good ride, but do
not have the inherent strength needed to cope with long
high-speed runs or extended periods of abusive use on
rough roads. Nylon-cord tires generally give a slightly
harder ride than rayon-especially for the first few miles
after the car has been parked-but offer greater toughness
and resistance to road damage. Polyester and fiberglass
tires offer many of the best qualities of rayon and
nylon, but without the disadvantages. They run as
smoothly as rayon tires but are much tougher. They are
almost as tough as nylon, but give a much smoother ride.
Steel is tougher than fiberglass or polyester but it gives a
slightly rougher ride because the steel cord does not give
under impact, as do fabric plies. Amarid and kevlar
cords are lighter than steel cords and, pound for pound,
stronger than steel.