Put the hammer down

Trucking escaped entanglement in the U.S. TREAD Act, but many in the industry predict future legislation covering tire-pressure monitoring. Such laws may not offer any real safety or efficiency gains, but until tire failure is a thing of the past and road gators are driven to extinction, bureaucrats will scribble new regs.

Regardless of what legislation comes down the pike, tires will always be a big cost for truckers, and improper inflation is the No. 1 reason why. Underinflated tires wear out faster. They flex more, which generates heat–the rule of thumb is that for every 2 psi a tire is underinflated, the internal temperature will rise 5 F at highway speed. If a tire with a recommended pressure of 100 psi is bobbing along at 70 psi, the temperature inside is 275 F hotter than it should be. For something made of steel and rubber, that’s just not good.

Unfortunately, reading the pressure of each and every tire, day in and day out, is time consuming. And thumping a tire with a club to check inflation pressure is sort of like kicking the fuel tank to see how much diesel you have.

Arming drivers with properly calibrated tire gauges would be a clear step up from the old hammer, there are other ways to monitor psi.

Simple, inexpensive, visual pressure gauges have been on the market for a long time. An example is Cat’s Eye from Link Manufacturing, a quick indicator of the inflation pressure in a set of duals. The device attaches to the dual’s hub or lug bolt and creates a conduit for air to move from one tire to the other. The Cat’s Eye takes its name from the visual pressure indicator, which really does look like the bright yellow eye of a cat. If the pressure falls 15 per cent below the recommended level, the “eye” opens up like a dilated pupil. As it does, it trips a check valve that prevents the good tire in the dual from going flat in the case of a tire blowout or a broken pressure hose.

More sophisticated systems use tire sensors attached to the rim of each tire. SmarTire of Richmond, B.C., offers such a system-it monitors air pressure and temperature and sends a radio-frequency signal to a computer that can display the information in the cab. When the vehicle is moving, data is transmitted every four to six minutes unless pressure changes; then data is transmitted within seconds. When the vehicle is parked, SmarTire’s system goes to “sleep,” taking pressure readings every 15 minutes. Data is transmitted to the receiver/display if the pressure drops below the factory or programmed level.

Pressure monitors are ideal for operations where the trucks return nightly. But if you’re a long-haul operator, or you want to bleed off air to broaden your tires’ footprint on the ground, central tire inflation (CTI) may be a better fit.

Some systems focus on pressure equalization, simply distributing air from one tire to another to strike a balance. Others plumb all tires on the vehicle to a controlled air supply that automatically adjusts the pressure to a constant and proper level, even while the vehicle is moving. Toronto-based Vigia is one of the leaders in the field, especially among heavy-haulers and on/off-roaders.

One way to deliver air to tires is through the axle, so it makes sense that axle suppliers now offer CTI. Meritor markets a product made by Pressure Systems International that uses compressed air from the trailer’s air supply to inflate any tire that falls below a pre-set pressure. As air pressure decreases from the tire manufacturer’s recommended level, the system automatically routes air to a control box, into the axle, and out to each tire. A rotary union assembly at the axle’s spindle end distributes air to the tires as needed.

Among the obvious questions about a component that borrows air from the vehicle’s braking system: Is it possible for tire-inflation activity to rob air needed by the truck’s brakes? And will an ordinary-spec compressor be enough for the job?

Fortunately, the answers are no and yes.

For example, Dana Spicer’s CTI system for trailers has a fail-safe mechanism that constantly monitors brake readiness and air demand. Based on a minimum-pressure point, tires won’t get air if the brakes need it more, which means that compressor capacity need not change. A remote, wireless battery-powered transmitter replaces the wheel’s standard valve stem. The valve sensors relay tire-pressure data via radio frequency to a receiver programmed to identify individual tires. The comparison is used to trigger solenoid valves to transfer air from the vehicle’s air supply into the under-inflated tire through rotating axle seals. The receiver can be easily set up and calibrated for a wide range of pressures up to a “cold-tire” 110 psi.

Usefully, the receiver stores inflation events in memory for later retrieval and tire repair. It uses standard SAE J1587/J1708 communication protocols for programming, diagnostics (via blink codes and/or PC), and data retrieval. If tire pressure falls to less than 90 per cent of the desired level, the warning lamp illuminates.

It would happen if, for example, a leak were causing air to escape faster than the system could re-inflate the damaged tire. The light would simply stay on if pressure stayed at less than 90 per cent, signifying a substantial leak.

Generally, however, the light should come on less frequently with CTI. It should act on its own, automatically, to correct a problem.