Recently, an emergency medical service fleet complained of loss of inflation pressure, but only when they made runs – not while their trucks sat parked.

What could cause tires to lose air pressure only when going down the road?

The fleet had been using extension hoses on their valve stems to make inflation maintenance simpler. The valves in the hoses weren’t leaking, nor were the connections to the valve stems.

But when they removed the snap-in, rubber valve stems, the dealer, Royal Tire of Salina, Kansas, found an odd, sharply defined, diagonal mark running across the part of the stem that passes through the wheel.

It looked as though the stems were being pulled to one side, possibly by centrifugal force acting on the extension hoses. This distorted the seal against the wheel and left behind the diagonal mark – with a shiny surface where the stem was in contact with the wheel – and a dull surface where the rubber was exposed to the outside elements.

Engineers believe rubber, snap-in valve stems were being distorted, probably by centrifugal force acting on valve stem extension hoses. Clamp-in metal valve stems eliminated the problem.

When the fleet replaced the rubber, snap-in stems with steel, clamp-in valve stems with grommets, the problem was solved.

So, if you’re running smaller vehicles with dual tires and valve stem extension hoses – and having unexplained inflation pressure losses – check those valve stems. You may have the same problem our EMS fleet had.