Everyone agrees that monitoring tire pressure regularly -whether manually or via some type of automatic system-is a good thing. But it still begs the question: ``Why do tires lose air in the first place?''
Despite the robust appearance of a molded rubber product, the properly maintained pneumatic tire does leak-as much as 1 to 2 psi per month under normal conditions and even more in cold weather, according to a number of industry sources. Even tires just stored lose air, according to a study done by Consumer Reports magazine.
Drivers rarely notice this type of loss because it occurs so slowly. General Motors Corp. estimates 83 percent of tire pressure loss occurs gradually.
So why do they lose air? The simple answer: Rubber is not 100-percent impermeable. Air molecules-or more accurately, the oxygen molecules in air-can and do migrate through the tire's sidewalls and belts/tread area (about 75 percent of air loss), around the bead-wheel interface (about 15 percent) and through the valve stem or the valve mounting spot (about 10 percent), according to Bridgestone/Firestone.
Scientifically speaking, one source said, a tire does not ``lose air''-instead it undergoes ``gaseous solubilization and permeation,'' with the pressurized air inside seeking equilibrium with the normal air outside.
All tires at one time carried inner tubes made originally of natural rubber and later of butyl rubber, which are more impermeable than the rubber used in the tire's construction. With the advent of the tubeless tire in 1947 the tube was replaced by a thin butyl rubber inner liner.
Constant improvement in butyl rubber technology increases impermeability, but none is 100-percent impermeable yet. Most tire makers today use bromobutyl or halobutyl, which are up to three times less permeable than earlier generation cholorobutyl, according to information from ExxonMobil Chemical.
Warm weather causes the air pressure in a tire to increase, 1 to 2 psi for every 10 degrees of temperature change. Likewise, tires lose 1 to 2 psi for every 10 degree drop in temperature, according to a variety of sources.
On the horizon is the use of nanotechnology-manufacturing precision products at the atomic, molecular or macromolecular levels-to produce ever thinner rubber innerliners with enhanced, near 100-percent impermeability.
Inmat Technologies claims an innerliner made with nanocomposites-a category of materials that fuse ingredients like a polymer matrix with inorganic nanoscale particles-would be only microns thick vs. 1 to 3 millimeters now, resulting in cost and weight savings along with environmental benefits.
One potential source of air loss is the valve. Tire makers in general recommend drivers replace their tire valves when they buy new tires, citing the tendency of valves to deteriorate over time either by damage or from prolonged exposure to the elements, including ozone.
Group Michelin also recommends customers use good quality valve caps-at a cost of perhaps 10 cents apiece-for a variety of reasons: they contain the inflation air should the core of the valve fail for any reason; they keep out moisture, which could freeze and in turn depress the valve core, causing loss of air; and they keep out dust and dirt particles, which also could interfere with the proper operation of the valve core and cause loss of air.
At high speeds, a cracked, deteriorated rubber valve stem can bend from centrifugal force and allow air loss, Michelin said in an advisory on its Web site.
And finally, consumers could choose to fill their tires with nitrogen instead of compressed air. Nitrogen does not diffuse as easily because the nitrogen molecules are larger than oxygen molecules, and it is an inert gas that disperses heat more quickly than compressed air, resulting in cooler-running tires.
Branick Industries Inc., the Fargo, N.D.-based company promoting nitrogen in the U.S., said nitrogen is used in various forms of racing tires such as those for Formula 1, Indycar and NASCAR as well as by the U.S. military, the National Aeronautics and Space Administration and other federal government agencies.
The cost to consumers of nitrogen is entirely up to the dealer, according to Gil Schoener, president of Branick. Some dealers already using the N2Nitrogen Tire Inflation System offer it gratis as a marketing tool, while others are charging $2 to $2.50 a tire.
Branick sees considerable potential for nitrogen in the commercial tire industry, where fleets monitor costs down to the fraction of a cent, Mr. Schoener said. Here the dry nature of nitrogen helps reduce oxidation, increasing casing life for retreading.