ROCHESTER HILLS, Mich. (May 22, 2006) — In the last few years, an insidious phenomenon has struck the commercial truck tire industry: Many deaths and serious injuries have resulted when tires just recently mounted on used disc wheels have blown off the rim during or right after inflation.
Besides occurring in tire shops, many of these tragedies occurred on the side of the road during routine emergency tire changes. They've caught green as well as experienced service technicians completely by surprise.
What's going on?
After a thorough investigation of these accidents, it has been found that the bead seats on steel and aluminum disc wheels can shrink when they are exposed to extreme heat. Excessive heat can change the metallurgy of both steel and aluminum, which can cause the wheels to lose strength and change dimension.
The bead seat and rim flange area of these wheels can get smaller and change contour. As a result, the flange area may no longer have the ability to hold the tire bead on the wheel under pressure and the tire can then blow off the wheel during or after inflation with explosive force. When hit by the force of the air blast as well as the jet-propelled truck tire, technicians are always the losers!
Where does this heat come from? Brake malfunctions and abuse are the most common sources of heat. Brakes that drag, are unbalanced or abused by drivers can heat up disc wheels excessively—especially the inside wheel of a dual assembly. That's why excessive heat most often affects the bead seat and rim flange area on the open side—the side opposite the disc face—of aluminum wheels. This heat also can spread to the outside dual in extreme circumstances and damage those wheels.
Tire fires are a significant source of heat as are wheel-bearing failures. Running flat for an extended period of time can generate high levels of heat, too. And let's not forget using improper inflation procedures such as ether, propane or other flammable materials to seat the beads automatically and inflate tires. (Distorted rim flanges are a common result of this prohibited and dangerous practice.)
Distorted rim flange
Tires that have been subjected to excessive heat typically exhibit charring, cracking, brittle or distorted areas in the bead area and are a tip-off to the technician that the wheel may also be damaged by heat.
Wheels that are heat damaged may appear charred, burned or discolored. The valve hole and labels on the wheel, if any, may show evidence of charring, melting, blistering, cracking or burning. In addition, aluminum wheels may discolor from excessive heat. Instead of having a bright finish, they may have a dull grayish color and may not polish to a shiny finish as they should.
Wheels that have had tires mounted with ether or other flammable materials may have black, powdery soot covering the rim area, too. However, some wheels may exhibit no obvious signs of heat and after a routine clean up may appear to be in good condition. These wheels still may have changed dimensionally and may be unsafe to use.
What can you do to prevent yourself from being the next casualty statistic? Be alert and look for signs of heat damage. As you already know, all wheels should be inspected prior to mounting tires. You look for unserviceable wheel conditions such as cracks, worn or wallowed bolt holes, excessive rust and bent flanges.
Now add to your list of things to check for instances of charring or discoloration caused by heat and a rim flange that has a scalloped appearance. Any wheels with these conditions should be placed immediately out-of-service and marked for scrap.
If you find that the tire that was just removed from the wheel has burned or deformed beads or has left rubber crystallization on the rim flange, that is a sure tip-off that this wheel also was exposed to excessive heat. When you remove the wheel from over the brake drum and see that the brake drum has an orange iron oxide color on it, that's a definite sign the brake drum has been exposed to excessive heat and probably the wheel has been, too.
Any time you suspect a tire was run overloaded or underinflated for an extended period of time, pay special attention to inspecting the wheel for heat damage as well. There are a couple of ways to determine whether a wheel has shrunken bead seat(s) and poses a danger.
The first is to roll the wheel on a smooth, flat floor for a minimum of 10 feet. (Obviously this can be done only if you are in a service or garage area where a flat floor is available, not along the side of the road!) Any deviation from rolling in a straight line is an indication that the dimensions of the wheel have changed.
If the wheel has an undersized bead seat, it will usually turn toward the small bead seat side of the wheel. Be aware that under certain circumstances both bead seats may have shrunk and the wheel may not turn when rolled. However, usually on closer examination you will see damage to the rim flanges.
If the wheel fails to roll straight or the rim flanges appear damaged, remove the wheel from service and mark it for scrap. Do not reuse it!
Rim roll test
If you are on the side of the road or still suspect the wheel after conducting a rim roll test, use a framing square to measure the rim flanges and determine if an out-of-service condition exists. That's right, a metal framing square that you can get at any hardware store for under $10. This tool is rugged and will fit easily in your toolbox, tire shop, service bay or service truck.
To use this tool correctly to check disc wheels:
* Place the long leg of the framing square on the disc face and align it through the centers of two opposite bolt holes.
* Drop the short leg of the framing square down in contact with a rim flange squarely across the rim while keeping the long leg firmly in contact with the disc face.
* Check for a gap between the square and either rim flange at multiple locations on the wheel. If any gap is 0.03 of an inch or more as measured by a feeler gauge—about the same thickness as a credit card—place the wheel out of service and mark it for scrap. Do not reuse it!
Shrunken bead seats pose a serious hazard to tire technicians. This is yet another reason why you should always, always, always inflate tire and wheel assemblies in a safety cage and remain outside the trajectory zone both in the service shop as well as on the road. If a tire blows off a heat-damaged wheel, it will knock you literally from here to eternity.
The Technology and Maintenance Council (TMC) of the American Trucking Associations has written a “Recommended Practice” that addresses this problem. For more information request RP241 Tubeless Disc Wheel Inspection for Undersize Bead Seats by calling (703) 838-1763.