Even with all of the technology that goes into a radial truck tire, it's completely useless unless it's inflated and attached to a truck. Likewise, modern disc wheels are as close to perfect as you can get, but they won't last more than a mile or two without an inflated tire.
And since nobody has found a way to simultaneously manufacture tires and wheels as a single assembly, somebody has to put them together, fill the tires with air and install the entire unit on a vehicle. The key is doing it safely.
Fear factor
Years ago, the dominance of tube-type tires on multi-piece rims placed thousands of techs in harm's way. Separating components often resulted in serious or fatal injuries. The industry even came up with names like "widow-maker" and "suicide" to describe particularly dangerous designs.
But everyone understood the hazards of inflating tires on multi-piece rims, simply because it was easy to imagine a flying lock ring causing damage. As tubeless tires became more popular in the 1980s, the fear factor began to decline. Too many techs looked at the one-piece rim as the safest thing on earth, so the safety cage became less important in their eyes.
They felt there was nothing that could come apart, so they had nothing to worry about.
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Some early methods of seating the beads proved to be hazardous, but even future Darwin Award winners had to know that igniting flammable gases might result in a more serious explosion than originally planned!
About the same time, a new hazard started to surface: When tires were operated in an under-inflated or overloaded condition for extended periods of time, the steel sidewall cables would begin to experience fatigue.
Eventually, the individual wire strands within the cable would begin to break, but the cable, as a whole, would remain intact. However, in its weakened state, it ultimately snapped once the pressure inside the tire was too great.
Since none of the cables were equally fatigued, they snapped at different stages of inflation, which formed a series of small bulges on the sidewall.
When enough of them snapped, an entire section of the sidewall would spontaneously open and release all of the inflation pressure. The resulting rupture resembled a zipper, so the zipper rupture was born.
Since the mid-'90s, the industry has done a remarkable job of educating techs about the hazards of zipper ruptures. Practically everyone knows that if you hear snapping and popping during inflation, a major explosion is imminent if pressurization continues.
Current inflation and inspection procedures call for suspect tires to be over-inflated by 20 psi and left in a safety cage for 20 minutes.
While there has been much debate over which tires fall under this criteria, nobody is going to let a tire sit in a cage for 20 minutes. So most techs just look for bulges, listen for snaps and pops, and then inflate the tire to the recommended or maximum pressure.
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There are still too many people who think that if the warning signs of a zipper rupture aren't there, using a cage is merely an option. Over the last few years, there have been a number of accidents involving the inflation of new radial truck tires on disc wheels where both components were intact.
Heat damage
This is where heat damage to disc wheels enters the picture.
It all starts with some type of event that generates extreme heat at a wheel end position.
Locked brakes, frozen bearings and flat tires appear to be the most common problems that can create enough heat to weaken the metal on the rim. (Damage can occur to both steel and aluminum disc wheels.)
When the temperature of the metal far exceeds normal operating thresholds, it begins to weaken and become more pliable. Pliability is the last thing you want on a wheel.
If both tires are flat, the amount of force applied to the outside rim is significantly greater simply because it's at the end of an axle. Combine excessive force with excessive heat and the diameter of the outside rim flange actually can be reduced by more than one inch.
The weakest part of a disc wheel is the rim opposite the disc -- the part of the outside wheel that is subjected to the most severe forces when both tires are operated in a flat condition.
As the weight of the vehicle compresses the open end of the wheel, it begins to "cone" and become smaller.
When a heat-damaged wheel is placed next to a serviceable wheel, the outer flange will appear to be shorter. When the mating surfaces are placed together, the damaged wheel will lean to one side.
Finally, if a serviceable wheel is rolled on a flat surface, it will travel in a straight line. Heat-damaged wheels will curve in the opposite direction of the disc by as much as several feet.
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Squared up
Unfortunately, road service techs often do not have the luxury of a smooth, flat surface on which to roll a wheel or compare it with another wheel.
The industry came up with a "go/no-go" gauge that any tech could use to check the diameter of the wheel.
We already had one called a ball tape, but it was expensive and people needed one for 22.5-inch wheels and one for 24.5-inch wheels.
After extensive research and development, it was decided that the resulting gauge might be a little less expensive than the ball tape, but it would be so much heavier that nobody would use it. Plus, techs would still need one for 22.5-inch wheels and one for 24.5-inch rims.
The answer to everyone's problems was found in a simple carpenter's square! When the short end of the square is placed flush with the mounting surface, the long end should be flush with both rim flanges.
Disc wheels with heat damage will have a gap between the square and the rim flange opposite the disc. If the gap is larger than a standard credit card, the wheel must be removed from service immediately. (On aluminum wheels, this type of damage also may be accompanied by the bronzing of the area below the outer rim flange.)
Techs who are unaware of this situation can be seriously or fatally injured if the tire is inflated outside a restraining device. Since the damaged flange is opposite the valve stem, the visual warning signs will not be recognized during the inflation process. There's no snapping or popping before the explosion.
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The only thing we really know is that it takes more than a few pounds of air to force the back bead over the rim flange. In fact, when the tire separates at 60 psi, the force is enough to bend a portable safety cage. Unrestrained, it becomes a dangerous projectile.
While the days of flying lock rings and rim components are fortunately coming to an end, the hazards related to inflating truck tires may never cease.
As long as truck drivers operate loaded vehicles on under-inflated radial tires, zipper ruptures will pop up from time to time.
Proper inspection procedures probably will identify most zipper ruptures before they are mounted, but the lack of an affordable inspection device means a few of them are going to slip through.
But in the case of heat damage to disc wheels, a carpenter's square is readily available and fool-proof if used on a regular basis -- kind of like a safety cage.
