Lesson 3/Learning Event 2
Solid. Solid tires generally are not used in transportation vehicles because of their harsh
riding characteristics. They usually do not roll as freely as pneumatic tires and require
more power to drive them.
Road construction equipment and materials handling equipment often are equipped with
tires with special-purpose treads.
Tractors may have a rib-type tread on the front tires and a traction tread on the rear. The
traction tread is the directional type. This means the lugs of the tread are on the tire at an
angle so that mud that collects in the grooves will be forced off to the side as the wheel
drives. For the directional tire to operate correctly, the tire must be mounted so that the
point of the V-shaped tread drives into the road surface. If the tire is not mounted this
way, the self-cleaning action is lost.
Tires used on earth-moving equipment are often very large, with a diameter of 6 feet or
more. Different types of tread designs for regular, rock, or mud and snow operations may
be found on earth movers.
To ensure that the correct size tire is on the vehicle, the mechanic should compare the
size markings on the side of the tire with those listed in the technical manual.
The tire size is molded on the sidewall of each tire at the time of manufacture. The first
number of the size is the width of the tire in inches when properly inflated upon the
wheel and without a load. The second number is the inside diameter of the bead of the
tire. For example, an 8.25-20 marking means the tire is approximately 8 1/4 inches wide
when inflated and mounts upon a wheel with a 20-inch diameter rim.
Other markings on the side of the tire that are of interest to the mechanic are the number
of plies, the balance point, and "type" markings. The ply number indicates the number of
layers of rayon or nylon cord plies used in the construction of the tire. The greater the
number of plies, the greater the strength of the tire. A sedan-will normally have a tire
with two to four plies, while a heavy truck may have a tire with eight or more plies.
Additional plies make the tire stronger, but they also make it more rigid.
Tires of the same size will not necessarily have the same type or number of plies.