Multi-Piece and Split Rim Explosions

Multi-piece wheels or rims have frequently resulted in serious injury or death to tire mounters employed in service stations and tire stores which explains why they are often referred to as “widow makers.”  Multi-piece rims are available in numerous configurations or designs, but all are potentially life threatening.  Though the Occupational Safety and Health Administration (OSHA) sought to ban multi-piece rims altogether, the tire and wheel industry was successful in avoiding a ban by sponsoring a program of public education in the workplace.  Posters designed for the guidance of workers are now prominently displayed in most service stations and tire stores, but the net result of the educational campaign has been to shift liability from the tire and wheel manufacturers to local businesses, employers, and the injured employees.

Although OSHA guidelines require, among other things, the use of a safety cage during the tire mounting operation, accidents still occur after the wheel is removed from the safety cage, for example, when it explodes as it is being mounted on the vehicle.  The warnings (which are part of the educational program) are not an adequate substitute for a safer design.  The single-piece wheel has been available for all tires for almost 40 years.  Product liability lawsuits resulting from explosions of the multi-piece rims may hasten their removal from the marketplace.

Sidewall Zipper Failures

Injuries to tire installers and bystanders often occur during the inflation process when the sidewall of the tire bursts or explodes.  The patterns of the rupture often resembles an open zipper - thus the phrase “sidewall zipper failure.”

Prior to inflation, there generally are no visual signs of stress in the sidewall and no outward indications that the tire may experience failure during normal servicing.  The deflation of the tire (either in road operation or in a static condition) may, however, cause such stress and lead to a catastrophic blowout when the tire is reinflated.  One particular United States manufacturer seems to have a higher incidence of zipper failures because of its sidewall design and manufacture, although the condition is also known to occur in the case of retreads (where the weakness in the sidewall went undetected during the retread operation).

Though zipper failures are a recognized risk in the tire industry, the response of the manufacturers has been to issue warnings, rather than to make tires less susceptible to the anticipated stresses. Even experienced tire repair people are victims of this problem.

Bead Hang-Ups and Wheel/Tire Mismatch

The bead is that portion of the tire which comes into contact with the wheel.  It is composed of high tensile strength steel formed into loops which function as an anchor for the plies and hold the tire assembly onto the rim of the wheel.  It is formed during the manufacturing process through production of a bead bundle made up of a series of wires which are overlapped and spliced.  Most tire manufacturers use a .037 inch weftless bead configuration in passenger and light truck tires.

Bead failure generally occurs at the splice of the bead bundle, and a separation or fracture of the bead may occur during the inflation process at pressures as low as 38 psi (pounds per square inch).  Historically, bead failures have been prevalent in tires designed to save space in the storage compartment or trunk of a vehicle.

During the development of tires designed to be used only as a spare and only for short distance driving, manufacturers learned that bead strength could be greatly increased by utilizing a continuous bead construction instead of the traditional spliced construction.  Although the continuous bead discovery was made over 20 years ago, splicing is still the design of choice employed by the vast majority of American tire manufacturers.

Bead failures often occur during the inflation and mounting operation, and the resulting “explosion” is often fatal and almost always serious.  This is so because the tire contains compressed air which has enough stored energy to lift a large man into the air.  During the mounting operation, the bead may become hung up on a portion of the rim, thereby stressing the entire bead bundle.  When the bead fractures (usually at the splice joint), and the low pressure explosion occurs, the trajectory of the tire and rim frequently causes the amputation of limbs, the crushing of facial bones, or permanent brain damage.  These bead hang up situations have been documented in patent literature, litigation reports, and industry articles since the early 1950's.

The splice construction and bead hang up problems were greatly exacerbated by the introduction of the 16.5 inch rim.  Prior to the introduction of the 16.5 inch wheel, tires of one size would not fit on wheel rims of a different size.  Unfortunately, the introduction of the 16.5 inch rim changed all that.  A 16 inch tire can be fitted onto a 16.5 inch rim without significant effort.  However, such a mismatch will almost always result in a bead hang up and the potential for an explosion.

While bead failure explosions most commonly involve the .037 weftless bead configuration, there are documented cases involving the .051 weftless, Michelin cable and single-strand bead configurations.  As noted above, those cases are most often seen where the tire size and rim are mismatched, but there have been fractures where the tire and rim are of the same size.  In either case, the failure almost always occurs at the splice joint.  

Solutions to this problem have been known to the tire industry for many years. The bead almost always breaks at the same spot – the inside cut edge of the splice. Increasing the wire size from the standard .038 inch to .050 would almost double the strength of each wire. Moving the inside cut edge of the splice to the outside has also been suggested by industry insiders, but not adopted. Another feasible solution is to make the bead one wire, instead of a bundle of smaller wires, so that the well known weak spot is eliminated.

Ozone Cracking

Though exceedingly rare, tire examiners and experts today occasionally see tire failures resulting from ozone cracking (also known as “ozonolysis”).  Ozone cracking is characterized by the formation of small cracks or fissures on the surface of the rubber which run perpendicular to the direction of strain.

Atmospheric ozone is a natural enemy of natural rubber compounds.  It creates such marked changes in the properties of natural rubber that oxidation itself is considered a chemical modification of the polymer.  At ambient temperatures, oxidation in air is a slow process.  At higher temperatures, the process is speeded up.  At any rate, the effects of the chemical changes are cumulative over time, and since the cracks are perpendicular to the direction of strain, ozone cracking can cause rapid deterioration of tire sidewalls in some areas.

Most rubber compound from which most tires on the market today are made include an antiozonant which protects the polymer against the harmful effects of atmospheric ozone.  Furthermore, the best UV absorber, carbon black, is used in substantial quantities in most high quality rubber articles, and some compounders nowadays actually double the quantity of antioxidants in high-quality black carcass compounds used in tire construction to increase resistance to UV-induced oxidation.  Therefore, any ozone cracking in a tire of today's manufacture is almost always the result of negligence in the manufacturing process - specifically the failure to include antiozonants in the rubber compound.

High Speed Spin-Off Failure

A high speed spin-off failure is a tread separation that occurs in unusual operating conditions.  It is almost never seen in vehicles of more recent manufacture, but, when it does occur, it is usually restricted to snowy environments.

This type of failure may occur when one of the two rear wheels is stationary and the other wheel is able to spin without restraint.  The high centrifugal force of the freely spinning wheel disintegrates the tire.  This type of failure is generally the result of a design defect.