Losing air ONLY when rolling?

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.

Royal Tire of Salina, Kansas found these distorted valve stems with diagonal marks across the sealing surface on several EMS vehicles.

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.

When the fleet replaced the rubber, snap-in stems with steel, clamp-in 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.

And, we recommend you use the valve stem extension hoses with stainless steel reinforcing braid and sheathing that helps prevent cuts from the sharp edges of wheel covers.

Finally, be sure that the valve stem extension hose ends are securely fastened to either the wheel cover, using riveted-on metal tabs, or to the lug nuts, using special brackets.

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.

How much air should be in there?

What’s the right inflation pressure?

It's not the number you see on the tire sidewall. That's the minimum inflation for the maximum load. Below maximum load, that pressure could cause hard ride, rapid or irregular tire wear, and might even reduce dry traction.

The best place to start is with the vehicle manufacturer's “placard”, which you’ll usually find on the driver's side doorjamb. (If it’s not there, ask the vehicle dealer where it’s located.)

Bear in mind that these numbers may not be correct either – if the vehicle has been modified without re-evaluating the load and its distribution, or if you have added a lot of equipment the manufacturer didn't know about.

The absolute best way, if you're uncertain about the correct inflation pressure, is to weigh the vehicle, axle by axle, with a full complement of equipment, supplies and people (Don't forget to have someone stand in for the average patient!). Do this at a truck stop or similar facility, then use tire manufacturers’ load and inflation tables to determine the pressure for each tire.

Running tires underinflated can cause rapid or irregular tire wear, loss of wet traction, poor handling, and in some cases, catastrophic tire failure.