Lesson 3/Learning Event 2
Any hard-driving practices by the operator, such as moving through turns too fast and
stopping and starting too quickly, will cause uneven wear on the tires. Also, it is the
responsibility of the vehicle operator to ensure that the tires have the correct tire pressure.
If the driver fails in this job, the tires wear unevenly or fail too early.
When a tire is operated with too little air pressure, the center of the tire will buckle
inward and the outer edges will carry the load. This causes the outer edges to wear very
fast. Also, a tire without enough air pressure will flex more than it should and generate
too much heat. If operated with too much tire pressure, the vehicle will ride on the crown
or center of the over-expanded tire and, thus, wear the center of the tread too fast.
The driver and the mechanic should make every effort to keep all the tires on a vehicle
wearing at an even rate. One of the checks a mechanic will make is to measure the
circumference or distance around the outside of the tires. This is done with the tape
measure in the Organizational Maintenance Common Tool Set Number 2 (less power).
The allowable tolerances for matching dual tires is determined by the outside diameter.
Differences in the sizes of tires can cause increased wear on the tires and drive trains.
Tires of different size on either side of the front of the vehicle can make steering and
braking difficult. On any of the driving axles, unequal-size tires on opposite sides of the
vehicle will force the differential to work more than necessary. When two unequal-size
tires are mounted side by side on rear dual wheels, the tire with the most tread or larger
diameter will be forced to carry more of the load. With one tire carrying the load of two,
the working tire is much more likely to fail. On vehicles that have more than one driving
axle, each axle will be driven at exactly the same speed. If the tires on the different axles
have different diameters, some of the tires will be forced to slip to keep the wheel speeds
matched, because the larger tire covers more ground during each revolution.
Often the vehicle driver will be careless with the valve caps because they are not
absolutely necessary to keep the vehicle running. The mechanic should check to see if
the caps are present each time the tires are inspected, because they can prevent failure of
the tires due to leaking valves. When the caps are not present, mud, dirt, or other foreign
matter can enter the tire valve openings. This can prevent proper closing
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