Design News for Mechanical and Design Engineers

The tire inflation process stores enough PV work in the compressed air to raise an automobile engine to a height of about 100 ft. This work also corresponds to the kinetic energy of said engine dropped from 100 ft. Watch out below! Such an amount of energy may do grievous damage to such a frail structure as a human body, however applied.

In this case a young man, soon to be the plaintiff, mounted a customer's freshly recapped tubeless tire on a rim and started to inflate it. He claimed that at only about 25 psi the tire came off the rim and blew 12 ft in the air, mangling his left hand and wrist in the process. He hired an attorney who had tire x-rays made, which showed a region where all the wires had fractured. The bead could thus easily slip over the rim flange during inflation. The attorney thought, "defect," and hired me to investigate the metallurgy of the bead.

Bead wire is steel, typically hard drawn to a diameter of 37 mils with a minimum breaking strength of 285 lbs. This strength corresponds to about 260,000 psi, about five times the value for annealed mild steel. (Try to bend a wire and you will find out just how strong it is.) There were about 20 wires in each bead for a breaking strength of over 10,000 lbs. By contrast the force on the beads due to a 25-psi internal pressure is less than 1,000 lbs. The factor of safety is huge. Something had clearly gone seriously wrong to cause the observed failure.

Several causes of failure came to mind. Improper mounting could have fractured/kinked the wires enough to give failure during inflation. Or, improper operation, as while over or under inflated could have given fatigue failure.

Microscopic examination showed that the wires were indeed heavily cold worked. They had necked heavily and finally failed by a ductile cup-cone mechanism. There was no sign of kinking or of fatigue markings. The wires had simply been stretched beyond their breaking strength. Many air compressors put out about 100 psi of pressure. Inflation to such a pressure would reduce the factor of safety enough to make failure plausible.

A retired navy man assisted me on the microscopy. He put the failure scenario as follows: "These kids, they fill the tire out of the high-pressure tank when they shouldn't. Then they get to looking at some [attractive young woman] walking by, don't watch the pressure, and BANG." I accepted the old salt's scenario, but couched my report in conventional terms.

I submitted my report and a bill for my time and expenses. This bill and a series of reminders brought no response. Apparently the attorneys felt no need to pay since my report did not support their client's case. I for one have never worked on contingency and consider it unethical for a forensic expert to do so. Not being paid displeased me and being ignored made me plain mad. I sued and finally got my fee, but the case left a foul taste in my mouth.