TIRE FAILURE ANALYSIS - BEAD FAILURE
Charles C. Roberts, Jr
Figure 1 is a typical cross-section of a pneumatic tire. The bead is
the inner edge of the tire that contacts the wheel flange and presses
against the bead seat. The bead seals against air loss in tubeless
tires and grips the wheel rim for transmitting various loads such as
steering, traction and braking. Bead failure can cause loss of air
pressure in the tire as well as detachment of the tire from the rim.
Bead failure may cause loss of vehicle control and, possibly, an
accident. Figure 2 is a view of a new tire that sustained a bead
failure after approximately 100 miles of operation. The failure
resulted in tire deflation. The bulge in the tire, caused by the bead
failure, was on the inside of the tire and could not be easily
observed. Figures 3 through 5 show various closer views of the
bead failure. Apparently, the reinforcing plies surrounding the bead
wire failed, causing loss of support and separation from the bead.
The air pressure in the tire caused a bulge and eventual deflation of
the tire. The tire remained on the rim and, when removed from the
vehicle, no evidence of a problem could be found. After the tire
was re-inflated, the bulge caused by the bead failure was apparent.
In this example, the failure is a result of a tire deficiency in
manufacture. Not all bead failures are the result of a defect in the
tire. Other possibilities are impact with a non-compliant object,
over-pressurization of a tire, and improper installation. Figure 6 is
a view of a tire that suffered a bead failure, most likely, because of
an impact with a curb. The deformation to the rim tends to
substantiate the conclusion. When tire disablement is a possible
cause of an accident, investigation as to the probable cause is
warranted. Bead failure is one of many disablement causes. See
Insurance Adjuster Magazine, July 1986, as well as the "Technical
Notebook", for additional information on tire failure analysis.
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