Perfect pressure

| February 28, 2007

The TIREMAXX wheelend, hoses and rotary seal.

One of the oldest trucker tricks in the book is beating on a rig’s tires with a club to look for a flat. A tire that’s leaked to almost empty will show up this way. But most leaks are slow, and the pressure won’t change enough for you to detect anything different until long after both the tire and your fuel economy have suffered. In fact, many tire company experts have challenged truckers to detect tires 10 or 20 psi below correct inflation pressure this way and won the challenge.

You need a tire gauge. Or an automatic tire inflation or low pressure warning system.

Pressure warning systems are cheaper and could be effective if you’re religious about maintenance on your trailer. But a tire inflation system is the hot setup, according to Al Cohn, director of new market development for Pressure Systems International, and Jim Beverly, chief engineer of chassis control systems at Dana Corp. P.S.I. makes the Meritor Tire Inflation System (MTIS), and Dana makes the Spicer Tire Inflation and Monitor System (TIMS).

The big advantage of a tire inflation system is that it keeps a tire that has a typical leak at the proper pressure, minimizing damage, while allowing you to continue rolling down the road to the next service stop.

The importance of proper inflation
Running at 90-110 psi, a tire squirms very little, which means it rolls freely, gives good fuel economy and stays cool. Running at 10 or more psi below what it’s designed for actually deprives the tire of the structural strength it needs to support the load. The result is that the rubber flexes violently as you drive, creating friction, overheating the tire and causing serious damage.

Tires are so sensitive to heat because they’re put together at relatively low temperatures. As heat is added, the various rubber parts are “vulcanized” together with powerful adhesives derived from rubber. The process produces an unusually powerful bond, like heat-treating metal engine parts. Tires are vulcanized at temperatures in the range of about 260-300 degrees Fahrenheit.

When a tire overheats, it approaches the temperature at which the vulcanization occurred, and the adhesives soften again and lose their grip. Those road alligators you see are the result of tires getting so hot the bond between tread and casing has been devulcanized.

Justifying the cost
A system costing up to $700 or more before aftermarket installation may sound too expensive to bother with. But inflation system manufacturers argue that you need to better understand the real costs of under-inflation in your operation. True, if you gauge your tires every few days so they remain within a few pounds of their ideal inflation pressure, such a system might be little but a luxury. But not every driver does it, and drivers rarely feel as responsible for the equipment as owners. If you run a small fleet and have a hired driver or two, a system like this could pay for itself in short order. The many financial factors at stake when it comes to tire inflation add up to a lot more money than most realize.

What normally happens when a tire leaks down? Most tires are in sets of duals, and leaky duals are not obvious to the driver the way slightly under-inflated steer tires or wide singles are. A dual tire that’s gotten a large puncture and has been neglected for a few days can leak down to the point at which the other tire is carrying the load alone. Both tires are likely to be damaged, because the inflated tire will carry almost twice its recommended load.

Meanwhile, the leaky tire’s tread scrubs along the road, creating even more heat and direct physical damage. It’s no wonder tires are any trucking operation’s second largest expenditure.

One of the biggest, and least obvious, consequences of low pressure, say both Cohn and Beverly, is tire casing damage. Even if a tire doesn’t get hot enough to cause tread separation, heat, especially over a long period of time, will weaken the casing. The damage may not show up before careful inspection prior to retreading. Every damaged casing reduces the cost savings available through retreading. Even if you don’t retread, used tires with strong casings are worth money.

Another factor, says Cohn, is fuel economy, which drops by 1 percent if pressure is 10 percent low, 2 percent if 20 percent low, and more than 4 percent if 30 percent low. Additional factors include irregular tread wear, which drastically shortens tread life and often creates annoying handling problems.

The biggest costs associated with tire maintenance are those related to roadside service. Reducing the number and cost of service calls alone has enabled many fleets to figure the investment in tire inflation systems has paid for itself in under two years’ time. Add to that the potential costs not only of new tires (because of casing damage) but also unfamiliar vendors’ overcharging and potential DOT fines, and the numbers reach significant totals.

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