For decades, two of the top recommendations from truck tire engineers for long tire life and casing durability have been to maintain proper inflation pressures for the loads being carried and to use sealing metal valve caps. Valve caps, however, have been one of those constants that, while highly recommended, have not always been fully embraced by maintenance personnel.

The caps serve several functions. The first, obviously, is to provide a cover and protect from dirt and debris. The vast majority of passenger tire valves are protected by inexpensive plastic material caps that serve essentially as a dirt seal, but do not provide any secondary inflation pressure seal.

For the higher operating inflation pressures typical of truck tires, the secondary seal of a rigid metal or hard plastic cap with a sealing gasket is preferred as backup to a loose or partially obstructed valve core. The dirt/debris protection is, of course, still an issue. Some fleets have found that foreign material inside the tire may also cause slow-leaking valves and use filtered valve cores for added protection. Trucks that are exposed to road salts or ant-icing chemicals, and operate in wet freeze/thaw environments are especially susceptible to leaking valve cores unless protected.

A fairly recent innovation, adopted by many fleets, is the flow-through valve extension. This device is threaded on to the valve stem just like a conventional cap, but it has a center stem extension that, when closed, eliminates the open annular area and keeps foreign material out of the stem sealing area. Its primary advantages are that inflation pressures can be checked quickly and no loose parts (like caps) must be taken off and replaced for maintenance.

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This greatly reduces the time required for pre-trip or en-route inflation checks. Most conclude that the extra cost of these extensions versus sealing caps is quickly overcome by time saved for inflation maintenance.
Another common sense innovation quickly gaining acceptance is automatic tire inflation systems (ATIS) for trailer axles. These systems use hard plumbed inflation lines connecting the secondary air brake reservoirs to individual tires, thereby precluding the need for valve caps or flow through extensions.

These systems have proven performance and are now fitted to approximately 50% of new trailers being manufactured. Similar systems are being developed for truck and tractor power units with the main obstacle being high pressure plumbing through full-floating drive axles.

Front axle fitments, although a bit more challenging than the hollow trailer axles, can be accomplished by drilling a centrally located air passage through spindles. Users are reluctant at this time to purchase power units with ATIS on only steer, tag and pusher axles until the full compliment, including drive axles, is available.

Many vehicles manufacturers do, however, currently offer tire pressure monitoring systems (TPMS) that sense and, in most cases, communicate tire pressure readings, although no make-up inflation is immediately available.

Some of these systems use internal sensors, requiring the use of traditional valve caps, while others use devices that thread onto the valve stem and substitute a secondary inflation seal for the basic valve mechanism.

Fleets need only to look at niche truck applications to see what’s coming down the road. Some military vehicles and other niche market trucks (logging trucks operating in areas controlled by the U.S. Forestry Service, for example), are equipped with tire inflation systems that allow inflation/deflation cycling that varies with off-road versus over-the-highway operation. These systems, however, are expensive, utilize separate air supply systems, electronics packages, and plumbing that add weight and costs unacceptable to our very efficient over-the-road rigs.

It is reasonable to hope and expect, however, that engineering ingenuity will soon overcome the challenge of fitting cost effective

ATIS systems that use already on-board dry air supply with a minimum of additional plumbing to finally allow complete active truck and tractor trailer inflation maintenance. Valve caps for truck tires can then officially be declared obsolete.