Lesson 3/Learning Event 2
Standard tubes are made of one layer of rubber and are used for standard-type tires.
Tubes for combat tires are constructed the same as standard tubes, except that they are
smaller than standard tubes with the same size-markings, because combat tires are smaller
on the inside due to their heavier construction. Combat tubes are stamped "combat" and
should be used in combat tires only.
Some tubes are built to seal bullet punctures automatically. These tubes are made of
thick, soft rubber. Bullet-resisting tubes are identified by their extra weight and thickness
and generally have green-painted valve stems.
Air pressure is forced into the inner tube through a valve. The valve stem is threaded on
the inside to allow the installation of a valve core. The valve core fits airtight against a
tapered seat inside the stem. It permits air under pressure to enter but prevents it from
escaping. The valve stem is threaded on the outside at its outer end to permit installation
of the valve cap. The valve cap keeps the valve core clean and serves as the final seal.
CONSTRUCTION OF WHEELS AND RIMS
The tire assembly is mounted on a wheel and rim assembly. Wheels must have enough
strength to carry the weight of the vehicle, transfer driving and braking forces, and
withstand side thrusts. Passenger cars are driven at high speeds, which means that they
must be set close to the ground. Therefore, they use perfectly balanced wheels of small
diameter. Even a slightly unbalanced wheel and tire assembly will cause steering
problems and rapid tire wear at high speeds. Small balance weights can be attached to
the wheel to offset an unbalanced condition.
Passenger car wheels are usually of the steel-disk type. The disk may be solid, or it may
have holes in it to reduce weight and to provide ventilation for cooling. The disk is
welded or riveted to the rim and bolted to the brake drum and axle. Some passenger cars
use wire wheels, which are light and easy on tires. They consist of a pressed steel hub
and rim connected by spokes that are welded at each end. In both types of wheels, the
wheel is dished to bring the point of ground contact under the large wheel bearing.
The purpose of the wheel rim is to complete the enclosure for the tube, hold the tire
beads in place, and connect the tire to the wheel. Several different types of rims are
used. For correct mounting, demounting, and tire fit, you should be familiar with the
difference in rim types.