What does a zipper have to do with tires?
Zippers have become a very dependable wardrobe accessory. They come in a lot of lengths and styles. Zippers form a tight closure on jackets, pants, shoes and many other clothing accessories. When zippers are closed, they form a good seal against the cold, wind, snow and rain. When opened, they allow things to come apart.
There is another kind of zipper that is related to truck tires. A truck tire zipper can be dangerous and when this type of zipper opens under the wrong conditions it can be fatal.
I will try to explain by telling you how truck tires are made and how they work.
A steel radial truck tire is made with several hundred cables of steel made up of individual strands of steel wound into cables. These cables are then placed side by side and imbedded in rubber to make up the structure of the truck tire. An air tight inner liner molded to the inside of the steel cables holds the air inside the tire.
Tires are designed to hold air. That’s what makes them work. It is only when tires are inflated to working pressure that they are able to carry the weight of the vehicle and the load. Tires properly inflated are rounded in shape with a flatter portion forming the tread or wearing surface. A typical truck tire is inflated to 100 psi (pounds per square inch). With several hundred square inches of internal surface, there are several thousand pounds of internal air forcing the tire to maintain it’s shape as it rolls along, carrying the load.
As a steel radial truck tire loses air, the sidewall begins to lose it’s rounded shape and the sidewalls begin to bulge out, creating an oval shape. As the steel cords flex between the oval shape that is in contact with the road, and rounded shape as it goes through the remainder of it’s rotation, the steel cables begin to break down. Steel is very strong, especially when bundled into cables. But when steel flexes, just like bending a coat hanger, the strands start to weaken and eventually break. As more and more strands break, the cable begins to weaken. As long as the air pressure inside the tire is low, there is little threat.
When air is added to the tire to bring it back to it’s natural rounded shape, the cables begin to fail one by one. An attentive tire technician will hear a popping sound as the cables give way until the pressure is too great and a zipper rupture occurs. The sidewall of the tire explodes as all of the air inside the tire exits the rupture like a shot from a cannon. Anyone or anything in the trajectory zone will suffer the consequences - serious injury or death.
When proper procedures are followed and the tire is being inflated in a safety cage, and no one is standing in the trajectory zone (directly in front of the tire), the blast does no damage. When safety procedures are not followed, the result can be fatal.
Bill Vanderwater, Norcan Training