Lesson 3/Learning Event 2
of the valve and allow dirt and moisture to be forced into the tire tube as it is inflated
with air. Most valve caps contain a seal or gasket that provides a safeguard against a
leaky valve. To function properly and to prevent small air leaks, the valve caps must be
snug when installed.
As the tire and tube are assembled, the mechanic must take care that the tire valve is
properly installed. When inspecting a vehicle with dual rear wheels, note the position of
the valves. To be correct, the two valves must be 180 apart (one valve directly opposite
the other). This aids in the location of the valve stems under all conditions. The valves
of the inner wheel will face the outside of the vehicle. If valves are not installed this way
on the inner tire of the rear dual wheels, it will be very difficult to check air pressure or
to inflate the tires.
The lug nuts that retain the wheels may work loose during the operation of the vehicle.
While inspecting, the mechanic should check all lug nuts by trying to tighten them with a
wrench. When loose lug nuts are found, the nuts should be tightened to the
specifications in the technical manual.
Loose front-wheel bearings, which can cause excess tire wear, can be located by jacking
the vehicle up and rocking the wheels back and forth. The free play can be felt at the
wheel. This check may also reveal free play in kingpins, steering knuckles, or control
arms. The mechanic should first determine exactly where the free play is located. If
loose wheel bearings are found, they should be adjusted in accordance with the vehicle
technical manual. If any other looseness is found, refer to the maintenance allocation
chart (MAC) to determine who is responsible for the repair.
Cracks in the outside rubber of the tire should be carefully inspected. Cracks usually
spread out from the center of the tire and may extend all the way around the sidewall of
the tire. The cracks are caused by the cord plies below the rubber becoming weak and
spreading apart. This forces the rubber to crack open. Failure of the fabric of the tire is
usually due to age and/or dry rot. As cracks open in the rubber, water will enter, and the
complete failure of the fabric is speeded up. Once the fabric of the tire becomes weak,
there is danger of a blowout any time a load is applied. When dry rot is suspected, the
tire should be replaced. In all cases where there is doubt as to the service-ability of a tire,
the mechanic should seek help from the maintenance supervisor.