All good commercial tire dealers know that proper inflation is the single most important maintenance issue for extended treadlife and long-term casing durability.
It is equally well known that tires, even if initially inflated to the optimum pressure, require periodic checks and occasional top-offs to maintain that pressure.
Let’s examine why this is true and what might be done to minimize those checks and top-offs.
Diffusion Reduction
First, there are two basic air-loss mechanisms responsible for pressure loss over time. The first is a concept called ‘diffusion.’ All pneumatic tires lose some high-pressure inflation continuously as air diffuses through the tire casing. This is, however, a much smaller problem today than it had been in older tire designs, especially bias ply and early generation radials.
Tire manufacturers have made great strides in terms of inflation retention. In fact, most quality radial truck tires now employ several layers of diffusion-resistant rubber compounds on the interior surfaces of the casings. Some go so far as to employ multiple components of barrier rubber compounds, together with traditional tubeless innerliners, to inhibit diffusion further.
Two primary advantages are, first, less air loss over time and, therefore, less makeup inflation is required.
Secondly, there is less moisture permeation into the tire casing. Low moisture levels retard breakdown of the bonds between adjacent compounds – there are as many as 12 different rubber types in some radial truck tires – and between rubber/reinforcement material interfaces. This, in turn, extends casing life for long-term durability and retreadability.
Injuries and Sealing Failures
The second type of inflation loss occurs as leakage from punctures and other mechanical sealing failures.
It is imperative that all tire injuries be identified and repaired promptly if tire casing integrity is to be maintained. Inflation loss coupled with moisture entry into the tire structure are detrimental to long-term durability.
Recommended tire inspection procedures and repair techniques are well documented and readily available from individual tire manufacturers, the Technology and Maintenance Council (TMC) of the American Trucking Associations, the Rubber Manufacturers Association and a variety of retread- and repair-material suppliers.
Besides punctures, several other types of sealing failures, which are often overlooked, can also result in slow pressure loss. Dirt, road salt or other foreign material can become lodged in the small, annular sealing area of the valve.
This is a consistent problem for some operations and creates a dilemma. Frequent pressure checks are desirable but accessing the valve so often can increase the opportunity for leak-causing contaminants to foul the valve.
Screw-on sealing valve caps have been a standard industry recommendation for years. While they do keep most external debris out of the critical sealing area, removing and replacing the caps can easily double the time required for routine pressure checks. Additionally, the small caps are easily lost and difficult to handle in cold weather.
Flow-through caps, however, have become popular in recent years. These valve caps feature a separate sealing area and a protective flange that remains closed until depressed by the tire gauge chuck. Though flow-through caps add a small cost to every tire valve assembly, they are generally preferable to traditional screw-on caps – unless a cap replacement program following inflation checks is strictly enforced.
The tire bead/rim seat interface is another area that can be prone to leaks. Corrosion, buildup from mounting-lube residue, rubber transfer from previous mountings, rim material erosion and bead seat and flange damage/distortion are all common troublemakers. Maintenance recommendations from wheel and rim manufacturers are excellent sources of information that can help eliminate inflation loss problems in the bead area.
Cavity Debris
Another less frequent but consistent concern is debris from inside the tire cavity entering the sealing area. Mounting-lube residue, mold release agents from the tire liner, rim-well paint particles, residual dirt from tire interiors and other miscellaneous items can lodge in the sealing area during pressure checks.
Filtered valve cores, simple devices that employ a fine mesh screen on the interior end of a conventional valve core, can eliminate most of these problems. Filtered valve cores were originally developed for use in conjunction with tire additives, such as those containing a powder and media mix that travels within the tire interior for ride improvement. Some fleets are considering using filter cores for all applications in hopes of reducing valve leaks.
Finally, keep in mind that tire pressure monitoring systems (TPMS), depending on system design and location, may or may not eliminate the valve sealing issues discussed here.
Additionally, pressure maintenance systems that provide make-up inflation from on-board air sources may sometimes mask slow pressure losses. These losses, if not detected through periodic tire inspections, can lead to reduced casing longevity. Further, any moisture present in the on-board compressor system will make its way into the tires.
No matter how sophisticated, technology won’t eliminate the need to inflate tires properly in the first place – and keep the inflation contained for as long as possible.
