Tire bead failure

The tire bead is a wire structure that aids in securing the tire to the wheel rim. There are three major wire bead designs used in the manufacture of tires: the overlapping splice (weftless) bead, the mono-strand bead and the cable bead. This article deals with the weftless bead, which is prone to damage under certain conditions when mounting a tire. Figure 1 is a cross section of a typical pneumatic tire showing major components.

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Figure 2 is a drawing of the weftless bead design, presently used in many tires. The wire strand size used is typically 0.037 inches in diameter. Several wraps of a ribbon of bead wire (tape) are applied to form a bead. There is a starting point on the bead where the first layer is applied and a termination point where the wire is cut, forming an area called the splice.

This circular bead is often called a grommet before it is manufactured into the tire. The splice is an area of non-uniformity that can cause stress concentration during tire installation and during normal usage. Problems occur during tire installation when the bead splice is subjected to excessive deformation during installation.

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Figure 3 is an example of hang-up where the bead may not fully seat as pressure is applied during installation. This can be a result of a rough surface on the rim such as corrosion causing the bead to stick on the rim because of friction. Typically, tire installers use a soap solution lubricant after assuring that the rim is clean and free of rough surfaces.

Despite these precautions, the bead may still hang up, and if the splice is in this deformed area (arrow Figure 3), then damage to the bead can occur. Assuming the splice has been damaged from excessive deformation during hang up, it’s propensity to fail is random. It can fail while the tire is being inflated or after a period of time. When it does fail, the result is a physical explosion that can cause serious injury or a fatality.

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Figure 4 shows another way that the weftless bead splice can be damaged. The tire is being pulled over the rim with the bead being distorted at the highly stressed area, as indicated by the arrow. If the splice is in this area, distortion occurs along with possible damage to the bead.

Figure 5 shows what a typical weftless splice failure looks like on a radiograph. The bead strands fail at the point where the overlapping tape ends. In one particular accident, a tire parted from the rim in an explosive manner, badly injuring the tire installer. The failure occurred several minutes after the tire was inflated. The installer used the proper lubricant, had checked the rim surfaces prior to installation and had used the proper rim.

Even though tire cages are used during the inflation process, the failure can still occur after the tire is removed from the cage, which it did in this case. There was no evidence that the tire installer had over pressured the tire or used any improper technique to mount the tire.

Over pressurizing the tire, the usage of improper tools and not using a cage can adversely affect recovery. The theory in this case was that the splice, which is prone to failure from deformations associated with mounting of a tire, was either in an area of bead hang-up or at a point of high stress when the tire was mounted as shown in Figure 4.

It should be noted that tire manufacturers do not indicate the location of the splice on the tire. The case settled.

Charles C. Roberts, Jr., Ph.D., P.E., (ccr@croberts.com) is president of C. Roberts Consulting Engineers, Inc., which provides professional engineering services in accident reconstruction, failure analysis, fire causation, explosion analysis, and biomechanics.