264 Auto Suspension and Steering
The rim design used today is called the drop-center
wheel. The center section of the rim is lowered (or
dropped), which allows the tire to be pulled to one side for
easier removal. Most rims have a small ridge behind the
tire bead to hold the tire in place if a blowout occurs.
These are called safety rims. See Figure 12-35.
Rim Sizes
There are three measurements of rim sizes. The rim
diameter is most commonly used to describe rim size. Rim
width is the size of the rim between the flanges. The flange
height is how far the flange extends above the bead seat.
Figure 12-36 illustrates the three measurements of a rim.
These measurements apply to both steel and custom
wheels.
Tires
Tires perform two jobs: they cushion shocks and
provide traction. In their role as cushioning devices, they
can be considered part of the suspension system. As
traction devices, they transmit engine power, as well as
braking and turning efforts, to the road.
Tire Construction
The external parts of the tire are the tread and the
sidewalls. Tread designs vary depending on the tire’s
application. The sidewalls form the support for the treads.
The tread and sidewalls are a blend of natural rubber and
a synthetic rubber called neoprene.
The internal parts of the tires are composed of plies
and belts. The plies are layers of tire cord that form the
general shape of the tire. Tire cords can be made of various
fabric materials, including nylon, rayon, polyester, aramid,
Kevlar, or fiberglass. The belts are installed directly under
the tread and can be made of the same materials as the
plies. Some belts are made of steel.
All modern tires are radial tires. In a radial tire, the
cords in the plies cross the centerline of the tire at a right
angle to the tread. Older tires were called bias tires. In a
bias tire, the cords cross the centerline on a bias, or a slant.
Almost all radial tires have one or two belts, which reduce
tread squirming, Figure 12-37.
Center section
Lug bolt
hole
Rim
Valve stem hole
Bolt circle
Flange height
Diameter
Rim
width
Figure 12-36. The three most common rim measurements are diameter, rim width, and flange height. Another common
measurement is wheel offset. Wheel offset is measured from the flange edge to the back of the center (spider) section. (Isuzu)
Flange
Tire
Safety ridges
Wheel
Drop center
Figure 12-35. A safety rim holds the tire in place in the event of
a blowout. (Chrysler)