Lesson 3/Learning Event 2
The drop-center rim is made in one piece and is permanently fastened to the wheel disk.
Its important feature is a deep well. On some wheels, the well is to one side of the rim
center, and the tire must be removed and replaced from that side. Drop-center rims are
generally used on smaller vehicles, such as passenger cars and 1/4-ton, 4x4 trucks. Some
passenger cars and light trucks are equipped with safety-type drop-center rims. Safety
rims have a slight hump at the inside edge of the bead ledges. This holds the tire beads in
place when the tire goes flat.
The semidrop-center rim has a shallow well in the rim. It has tapered bead seats to fit the
shape of the beads on the tire. A demountable rim flange or side ring fits into a groove
(gutter) on the outside edge of the rim to hold the tire in place. You must remove this
side ring to remove and replace the tire.
The flat-base rim has a flat seat for the bead, and the tires must also have flat beads. This
rim is flat with no well. This type rim has a demountable side ring to permit mounting
and demounting the tire. Flaps are required on the flat-base rim to protect that part of the
tube not protected by the tire. Flat-base rims are no longer used on vehicles.
The advanced rim is similar to the flat-base rim except that it has a slight taper (50) on
each bead seat. The taper provides a squeeze fit of the tire beads to prevent slippage
between the rim and tire. Different manufacturers make the side rings and grooves
(gutters) in different shapes, which prevents interchangeability. Military vehicles use
advanced rims with all side rings and rim gutters shaped the same, which permits
interchangeability. This rim is known as the military or the military standard rim.
To operate a vehicle in combat without air pressure in its tires, it is necessary to have
some device that locks the tire beads in place. The tire beads may be locked into place on
the rim by either beadlocks or bead clips. Bead clips are metal clips that fit tight on the
tire bead and the rim. Five or six clips are equally spaced on each bead of the tire. When
bead clips are used, a flap is generally required to protect the tube.
The beadlock is a metal device that fits between the beads of the tire. The beadlock is
slightly wider than the space between the tire beads with the tire mounted on the rim.
Therefore, when the rim side flange is installed and tightened, pressure is applied to the
tire beads. This locks the beads in place so that they will not slip on the rim and will hold
the tire in position.