This off-the-truck communication enables dispatch, fleet or maintenance personnel "to react and assist the driver in developing a game plan on how to best address the tire issue before catastrophic tire failure and costly downtime occur," Nau says.
There are two ways to monitor whether a tire has lost pressure: indirect and direct. Indirect monitoring systems uses the vehicle's antilock brake computer to measure the circumference of each tire and then compare the dimensions to find a tire that may be low or have lesser circumference. On a tire that is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other tires.
The direct tire pressure monitoring method uses a wireless pressure sensor, attached either inside or on the outside of the tire, to send the pressure data to a receiver in the vehicle. These systems measure, identify and warn the driver of low pressure. With a sensor in each wheel, they generate accurate warnings and can send a real-time alert if the pressure in any one tire falls below a predetermined level due.
"The direct method has somewhat become the standard over recent years because many trucks have dual tires and limit the effectiveness of measuring the tire circumference," observes Nau.
There are also automatic tire inflation systems (ATIS) that monitor and continually adjust the level of pressurized air to tires. Unlike a tire pressure equalization system that keeps inflation pressure and load distribution equal by distributing air from one tire to another, ATIS maintains proper tire pressure automatically, with no action required by the driver.
One type of system uses the vehicle's existing air-brake compressor to supply all of the tires from a central source. Another type of ATIS uses self-contained compressors mounted on each hub that are powered by the rolling motion of the wheels. Both systems keep tires properly inflated even while moving.
The concept of automatic inflation works best on trailers because the hollow trailer axle and hub allows for an exit point for the air supply to get to the tire valve stem, says Doran Manufacturing's Nau. This technology is not conducive to a truck application because of the solid steer spindles and drive axles.
Also, an active inflation system requires moving parts which have tendency to require additional maintenance and because the tire is automatically inflated serious tire issues can easily be over looked.
The first company to develop automatic tire inflation system technology for commercial trailer was PSI. The Meritor Tire Inflation System (MTIS) by PSI uses compressed air from the trailer's air system to inflate any tire that falls below a preset pressure whenever the vehicle is in operation. As air pressure drops below the tire manufacturer's recommended level, MTIS automatically routes air through a control box, then into each axle. The axles carry the air to a rotary union at the spindle that distributes the correct amount of air to each tire as needed.
Other automatic tire inflation systems for trailers are offered by Airgo Systems, Dana Spicer, Hendrickson and Pressure Guard.
"With all of the increased efficiencies possible with automatic tire pressure monitoring, and considering the more miles driven, the faster the return on the investment," says Van Dyke of Fleet Specialties' Tire Sentry Div. "An average of six to 10 months may be quite realistic for a tire pressure monitoring system to pay for itself.
"Often, just by saving one tire and a road service call you can will pay for the system on that vehicle."
Along with improved safety, truck owners and operators will also get a return on investment "realized through better tire wear, longer tire life, less number of on-highway catastrophic tire failures, a lower loss of retreadable tire casings and finally an increase in fuel economy," adds Nau.
The Transportation Recall Enforcement Enhancement, Accountability and Documentation (TREAD) Act was passed in the fall of 2000 as a result of the Ford/Firestone crisis. That situation involved a high incidence of tire failures on Ford Explorer vehicles equipped with Firestone tires. The law came about because Congress wanted to make tires safer for the motoring public.
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