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Where the Rubber Meets the Runway
by Elliott Cox
tire is at its maximum compression. If you roll the tire 180 degrees where the valve stem is at the 12 o’clock position, the sidewall is at its least compressed. Now imagine that spot making a full revolution 27 times ev- ery second, which is what the main tires on our Falcon 900 are turning when we rotate. If I were better at math, I’d calculate approximately how many revolutions our tires make on a typical takeoff roll. As a placeholder, I’ll just say a bunch.
The tread is where the rubber meets the runway and comprises the sacri- ficial part of the tire. Keeping an eye on the tire tread will give you good insight into the health of your entire landing gear system. If your main tires are wearing evenly, you’re keeping the tire pressures where they need to be, and your camber and toe are as they should be. If your tires are wearing unevenly, you may have a problem somewhere in the landing gear that is starting to manifest. Odd tire wear is a great early indication of problems to come, and prompt attention may save you a few dollars or maybe even a trip off the side of the runway.
Those are all parts of the tire that you can see, but the real magic hap- pens inside the tire in the form of plies. “Radial” and “Bias” are the two ply options, and those terms describe the way in which the reinforcing cords are oriented within the tire. The cords in a radial tire are perpendicular to the direction of the tire tread. Bias tire plies run at about a 40-degree angle to the direction of the tire tread and make a crosshatch with every ply.
I also think it’s important to note that just because a tire is “six-ply” doesn’t mean that it contains six plies. “Six-ply” means that the tire has a ply rating of six, which indicates the load
When performing a pref light inspection, there’s usually little more than a cursory glance given to the tire treads. I think it’s worth your time to give the rest of your tires a closer look.
Tires are designed to take abuse from literally every angle. They must be rigid in one area while simulta- neously being f lexible in others, un- der heavy loads at high speeds, all while enduring dramatic temperature changes over a relatively short time. At first glance, it may look like a tire is nothing more than the product of pouring some rubber into a mold, but a tire’s construction is almost as com- plicated as its mission.
The part of the tire that comes in contact with the wheel is called
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the “bead.” Only a few dozen square inches of tire bead connect your air- plane to the earth, so the rubber is thick and reinforcing wire is used to ensure an airtight seal, as well as to prevent the tire from slipping on the wheel. The last thing you’d want is for your tires to spin on the wheels when you grease that landing. You may be surprised by how much heat and smoke can be generated instantly when you spin a tire.
The part of the tire that takes the most abuse is the sidewall. Any “cush- ion” that comes from the tire is pro- vided solely by the sidewall. The tire bead retains most of its shape as it goes through a rotation, but the sidewall is constantly in flex. Let’s say that the valve stem of a main landing gear tire is at the six o’clock position, where the