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With the Technology & Maintenance Council (TMC) introducing two new Recommended Practices, focusing on tire zippers and inflation maintenance, are we finally going to give tire pressure the attention it deserves?

Several years ago, I was hired to present "Tech Week," a week-long technical training seminar series for a fresh crop of newly hired commercial salespeople. The first part of the seminar was held in a classroom. The last couple of days included scrap tire analysis and fleet tire inspections in real world conditions. We partnered with a salesperson from a local tire dealership and in exchange for full access to several of his fleet customers, we agreed to provide detailed reporting on tire performance, tire failure analysis and overall fleet tire condition.

One of the fleets chosen was a regional sand and gravel hauler that was, according to the salesperson, a test fleet. He explained that they are running a wide variety of brands and models to see which one would do the best job. It sounded like a perfect opportunity.

It was an unusually hot July afternoon in Phoenix. We arrived at the yard about 20 minutes before the dealer salesperson and as soon as we arrived, the shop manager came out of the shop and walked briskly toward us. He said that he was very happy to see us and hoped we could tell him why his tires keep "popping." The tire dealer salesperson didn't mention anything about chronic tire failures, so I asked him to tell us more. As we talked, the trucks were rolling in at the end of their day. After the 6th or 7th driver finished his post-trip and headed home, I looked at the shop manager and said, "I haven't inspected anything yet but I can already tell why you're having a problem."

Incredulous, he said, "How's that?"

His problem is the biggest maintenance problem in the trucking industry: he wasn't keeping enough air in his tires. We watched as each driver parked his truck and walked around it thumping each tire with a "Tire Billy." Each of these trucks were loaded to the legal limit and driven through the desert at (or above) the 75 mph speed limit for hundreds of miles each day in ambient temperatures over 110 degrees. There was no margin for error. With each rash of tire failures he became increasingly frustrated and switched brands hoping to find a tire that would hold up. Through this trial-and-error approach to resolving the problem, he was now running dozens of different tire brands.

The tire dealer salesperson said this was a "test fleet"! Here's a tip: if you've had the exact same problem with more two or more brands of tires, the problem probably isn't the tire.

AIR CARRIES THE LOAD

Why is inflation pressure so important? The fact is, the tire does not carry the load, the air inside the tire carries the load. To visualize this, imagine the rig is a hover craft riding on a cushion of air. If there is too much weight or not enough air, the craft will not hover, it will touch the ground. The tire simply provides a chamber to contain the air.

If the air in the tire is insufficient, the vehicle will not be supported at the proper height causing the tire's sidewall to flex excessively. The condition is known as over-deflection. The sidewall doesn't really know the difference between over-loading and under-inflation. It only knows that its sidewall is flexing beyond its capabilities generating excessive heat and weakening the integrity of the cords.

To picture the damage, pick up a paper clip and bend it back and forth repeatedly. You'll notice that the paper clip will heat up and eventually break. While the steel cords inside the tire sidewall are more flexible than a paper clip, they behave just like those paper clips when they are repeatedly bent too far.

DON'T THUMP

The problem with thumping a tire with a "Tire Billy" is that the average person can only tell if a tire is completely flat. When I present technical tire seminars, I give each participant a "Tire Billy" and ask them to hit several tires, then tell me which if any need air. When one tire has 100psi and the other has 40psi, almost everyone can tell. Hitting one generates a "ping," the other a "thud." When lining up tires ranging from 60psi to 90psi, surprisingly few people can detect which are under-inflated as the difference between them is too subtle. In service that range might be the difference between safety and catastrophe.

Hitting a tire with a stick to check pressure is about as silly as hitting the engine with a stick to check the oil. You must use a gauge.

You may be thinking that a tire will still reach its destination at 60psi, and in regards to many applications you would be right. Unfortunately, these moderately under-inflated tires may be a greater safety risk than flat tires. Continual use may severely weaken the sidewall cords but not to the point of breakage. Later when the tire is removed from service for repair and/or retread, the peril is concealed. The technician that re-inflates the tire could be in for a terrifying, even life-threatening surprise.

This condition is called a zipper rupture and is difficult to detect before the failure. It's called a zipper because the tips of the metal cords along the failure tend to look like the teeth of an open zipper. I've had some technicians tell me they heard a few pops prior to the blast. Others have witnessed tires that just blew with no warning.