Truck tire repair has been on my mind a lot lately, probably because I've been working on a new training video that addresses the topic. When I was a (much) younger tech, I quickly developed a comfort level when repairing tires and sometimes found myself standing at a spreader for hours repairing tire after tire. In the old days, we patched everything from the inside because (we thought) that was the way to do it. As it turned out, we were half right.
There are two critical components to every tire repair and a patch is definitely the first. When properly applied, it is molecularly bonded to the innerliner of the tire and serves as a permanent seal after it has been breached. I've seen tires run flat to the point where the belt package was completely gone, yet the repair units that were installed correctly were still in place. It's a process called vulcanization and when all of the ingredients are in place, the bond between the innerliner and the patch will be impossible to break.
But there is more to repairing a tire than stopping the leak, which is why on-the-wheel, or plug, repairs are not recognized by any tire manufacturers. The primary reason is that unless the tire is removed from the rim, damage to the innerliner cannot be seen. Sometimes an object that penetrates the tread may also damage the sidewall. If a tire is returned to service with unseen interior damage, it will fail shortly after it has been installed even when it still holds air after it has been plugged.
By definition, a repair must restore the original condition of the tire. In order to achieve that goal, the damage to the belt package and body cables must be removed. This is another area where the on-the-wheel plug repair fails to measure up. Each belt package is a series of individual cables that are comprised of smaller wires. When the cables are damaged, the wires have a natural tendency to unwind as the tire flexes and adapts to different road surfaces. Plugging the tire from the outside does not remove these damaged cables and as a result, belt separations are not uncommon after the plug is applied.
Likewise, when a patch is installed on the innerliner without removing the damage, a belt separation is imminent if the tire is left in service for a long enough time. To make matters worse, the hole in the tread allows water and moisture to penetrate the steel belts so a small injury can destroy a casing because rust and corrosion spread like cancer.
That's why the second critical component of a tire repair is to remove the damage with a carbide cutter and fill the injury with a rubber stem. The average driver will think the technician is “making the hole bigger,” but the reality is that the cutter trims the cables back to solid rubber and stabilizes the area. And when the rubber stem is vulcanized in the same way as the patch, it becomes a permanent part of the tire.
Restoring the tire back to its original condition after a repair requires a number of steps, each one just as important as the next. Some service providers will attempt to convince their fleet customers that they are doing it right because they are patching the inside of the tire. But if the damage isn't removed and the injury filled with a rubber stem, the tire is either a belt separation waiting to happen or another casing headed to the scrap pile.