Bottom Line
Modern Trucking Techniques
Tire monitoring and inflation systems
When pressure is good for you

by Paul Abelson, technical editor

Before we start examining tire monitoring and inflation systems, it might be worthwhile to review the economics of tires and tire management. Here are some interesting facts and figures:

  • Last time I looked, a new 24.5 truck tire was selling for between $350 and $415. A set of new tires for an 18-wheeler could cost as little as $6,300, or as much as $7,470.
  • Recaps can run $200 or more. A set of recaps for a trailer can run $1,600 to $2,000.
  • Used tire casings are worth between $75 and $125 depending on the shape they’re in. That’s $600 to $1,000 off your bill if your tires haven’t been abused.
  • Road service calls for tire repairs average $300 to $350, plus the cost of a tire. Rarely can a tire be repaired along the side of the road.

Tires wear out. With proper care, we can slow the rate of wear. Tire maintenance involves balance, alignment and, most important of all, air pressure. Everyone knows air pressure is important to safety. The Ford-Firestone affair last year reminded us of that fact. But you may not know how important it is to tire life and your bottom line.

Air is a structural part of a tire. It provides stiffness as well as cushioning when it compresses. When air is lost, tires lose some of their stiffness, allowing them to flex more than their design intended. When a tire rolls, flexing always takes place.

Let’s follow a single point on the tire’s tread as you roll down the road. For most of its travel, it follows the normal contour of the tire until it contacts the ground. Then it is forced inward, back toward the center of rotation of the tire. Then the tire flattens slightly to form the contact patch. That patch, about 45 square inches, supports each tire in a 34,000-pound tandem (at 100-psi inflation pressure). For a steer tire, the patch is about 60 square inches.

If there were no flex at all, the tire tread would have a narrow line as its contact patch and not much traction or cushioning. Our point on the tread is now traveling in a straight line along the ground, getting first closer to the tire’s center of rotation, then further away as it passes the midpoint. Then it gets to the end of the contact patch and is pulled back up toward its original rotation. Its inertia, however, wants it to continue its motion away from the center of rotation, so it lags a bit before being pulled up off the ground. Like a spring, it gets to its location, but because of its inertia, it springs back further to the center. Air pressure resists that motion and forces the point back to its proper position. Engineers refer to this as a “standing wave” since the motion appears to be standing still relative to the tire’s center of rotation.

If there is not enough air in the tire, the height and depth of the wave will increase. The tread will flex more than it was designed to do. This flexing causes heat, as the friction of steel cord and rubber molecules rubbing against each other generates heat. To better understand this, take a paper clip, open it up and start bending it back and forth. The more you bend it, the hotter it will get. This same thing happens inside a tire.

Eventually, enough heat can be generated at the cords to liquefy the rubber bonding them together. When enough of the structure has melted, the tire will fail.

The proper amount of air will control flexing. Without enough air, flexing will intensify, leading to tire failure.

Tires are the second most expensive item in trucking (behind fuel), accounting for at least 20 percent of maintenance dollars. More than 90 percent of tire failures are due to under-inflation. That’s why maintaining air pressure is so important.

It also takes energy to flex those under-inflated tires, and that comes from fuel. Here is the relationship of tire pressure, fuel consumption and tire life.

Example for a 100 psi tire
PercentĀ 
Under-inflated
Air
pressure
Loss of
mpg
Loss of
tire life
0
100
10
90
up to 2.5%
15%
20
80
3.0 to 3.5%
30%
30
70
5.0% +
55%

DOT and tire makers say the best thing you can do to save fuel and tire cost is to gauge your tires every day. Even your carriers tell you to do this, especially when you’re pulling their trailer. As my son, the flatbedder says, “Get real! You know and I know no one out in the real world does that.”

I’ll bet many of you don’t even thump them every day. Thumping is better than nothing, but in tests conducted by the Technology and Maintenance Council (TMC) years ago, even the most experienced drivers couldn’t tell when a tire in a tandem was down to as low as 60 psi, a candidate for the scrap heap. I’m willing to bet you can’t either. In fact, I’ll wager $50 that if you’ll let me lower the pressure to 75 psi in just one of your tires, you can’t tell me which one just by thumping. Anyone can take me up on this at Walcott this July. But I suggest you try it out yourself first. It might just save you some money.

Not only does low pressure shorten tire life, but according to TMC and the Tire Retreaders Information Bureau (TRIB), low pressure damages the casings so the reputable retreaders will not recap them. (Caution: there are shops that recap casings without thoroughly inspecting them.)

Making your job easier
If you’re not going to invest your time to save your tires, at least invest some of your money. The most basic device is an equalizer. This connects both tires of a set of duals to a common inflation valve. You can save time by gauging and inflating two tires at once, giving you only 10 valve caps to remove instead of 18. The better equalizers have safety valves so if one tire suffers a catastrophic failure, the other will still hold air. Equalizers may have a role, but they still don’t offer enough incentive to get me to check even 10 valves daily. Some drivers check by axle, doing two pairs or both steer tires each day. They’ll start at the trailer and work to the steers. Some go the other way. That way, it’s not too great a burden and every tire gets checked at least once a week. It’s still a compromise between what should be done and what is often not done at all.

There are devices that let you check air pressure with just a quick glance. The Cat’s Eye from Link Manufacturing has a mounting bracket, a filler valve, two air hoses (one for each tire) and its special feature, a visual indicator that quickly lets you know if your tires are properly inflated, and if not, about how much air they need. The 12-ounce Cat’s Eye allows air to transfer between tires, keeping both at the same pressure for even wear. An internal check valve prevents both tires from going flat in the event of a blowout. The eye, which features two yellow quarter-sphere sections inside a plastic dome, are closed together when air is up to the pre-set pressure. As pressure drops, the yellow sections spread apart, exposing a high contrast black surface, looking like a cat’s eye. The more black that is visible, the lower the tires’ pressure. Models are available from 60 psi to 125 psi.

The latest version of the Crossfire from Dual Dynamics also equalizes air between dual tires and identifies low-pressure problems. It also identifies excessively high pressure, which also shortens tire life, although it improves fuel mileage by a percent or two. The Crossfire allows air to flow freely between tires, and has a central inflation valve. The face of the Crossfire is yellow. It has a bar-shaped window in the face with a colored indicator in it. When pressure is normal, the indicator is yellow, too. Nothing shows. If the tires are underinflated, a black bar shows in the window. For overinflation, the window is red. If a blowout occurs, a safety valve isolates the good tire to prevent air loss. To protect from slow leaks, both tires are isolated when pressure falls by about 10 psi.

These devices make it easy to spot low tires during a quick walk-around, but they don’t help you pump up the tires. Several products do.

The simplest is a long air hose with a tire chuck. Hooked to your glad-hands or to a fitting, the hose lets you air up your tires anywhere, anytime. Be sure you have a rugged, accurate, heavy-duty tire gauge to check for proper pressures. If you drop the gauge or it is a year old, it’s a good idea to check its calibration at a tire shop.

Dana Spicer’s Tire Pressure Control System (TPCS) evolved from the old Eaton (now Dana Spicer) Central Tire Inflation System, developed for the military and in use on more than 35,000 of Uncle Sam’s trucks and HumVees. TPCS lets a driver adjust inflation for terrain and weather, lowering pressure for low-speed operation on mud, gravel or snow, and raising it again for highway use. TPCS monitors tire pressures and, using air from the truck’s air compressor. Depending on how badly a tire is damaged, it may let you maintain air in a punctured tire until you can get to a tire shop. That can save you a road call and the tire, too. TCPS works at all positions: steer, drive and trailer.

The Meritor Tire Inflation System by PSI mounts on any trailer with hollow axle-ends. It draws air from the trailer’s air tank and pressurizes the axle tubes. If a tire is damaged or loses air, the Meritor PSI system sends air to the tire to keep it inflated. If a tire goes flat after the engine is off, the unit can air the tire to operating pressure in just a few minutes more than it would take to fill an empty air tank. When the system activates, a warning light comes on.

A similar unit, the PressureGuard Automatic Tire Maintenance System from Innovative Transportation Products, operates almost the same way, but without pressurizing the axle. PressureGuard runs air hoses inside the axles to their proprietary hubcaps. Each hubcap has a rotary union with graphite seal that automatically adjusts for wear. Tires are equalized for slow leaks, and protected from blowouts or air loss from the reservoir. The PressureGuard hubcaps have plugs to allow oiling.

Both the Meritor PSI and PressureGuard units are “active” but work only on trailers. The Cycloid mounts on any hubcap, so it can operate at any wheel position. It has a small pump, powered by the rotary motion of the vehicle. While it won’t inflate a flat tire, the Cycloid keeps tires at their proper pressure much like a trickle charger keeps batteries topped off. It pressurizes each tire independently so a bad tire won’t bleed air from a good one. The Cycloid pump comes calibrated for 90, 100, 110 and 120 psi tires.

There are differences in functionality between all these units, and also significant differences in price. Which offers you the greatest value? This is truly a situation where you get what you pay for, and the more you pay, the more you get. You can go from adjusting tire pressures up or down from the comfort of your cab, to spotting problems with the blink of an “eye.”

You have enough information to do some serious shopping, but whatever you decide on, use it regularly. With tire costs what they are, you can’t afford not to. And remember the effect tires have on safety and the Ford-Firestone debacle. People died when tires failed. Out on the highway, people still die because of tire failures and almost all of those can be traced to low pressure. Let’s do something about it.

March/April
Digital Edition