Tires are cut by sharp objects, bruised by bad roads and stones, and injured by road shocks in general. To drive with a seriously damaged tire is dangerous, because it may blow out and cause the driver to lose control of the vehicle.
Carefully inspect your vehicle tires during prestart and post operations. Remove glass, nails, stones, and other foreign materials embedded in tires. Tires give longer mileage and safer driving when damages are repaired immediately.
Inflation
Correct air pressure is the basis for reliable tire performance. Tires are designed to operate at specified air pressures for given loads and inflated to the prescribed air pressure for your driving condition. When
Figure 3-10.—Hotchkiss drive.
checking air pressure, use an accurate gauge and check the valve cores for leaks.
NOTE: Reduce the tire pressure when driving in soft sand and over dunes. This increases the amount of tire surface in contact with the sand to provide better flotation (support). However, never reduce the tire pressure so much that the tire slips on the rim. On some equipment, the air pressure for normal conditions and off-road conditions is listed on a data plate on the dashboard or in the operator’s manual. When operating with reduced tire pressure, drive at low speed. Inflate the tires to normal pressure as soon as the situation permits.
PROPERLY INFLATED.— A properly inflated tire, as shown in figure 3-11, view A, shows proper contact with the road.
Figure 3-11.—Proper and improper tire inflation.
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Figure 3-12.—Valve cores.
UNDERINFLATED.— An underinflated tire is shown in figure 3-11, view B. This tire does not contain enough air for its size and the load it must carry. It flexes excessively in all directions and gets hot. In time, the heat weakens the cords in the tire, and it blows out.
Underinflation also causes tread edges to scuff the road that puts uneven wear on the tread and shortens tire life.
Never run a tire flat, or nearly flat, unless the tactical situation in combat requires it. When run flat for even a short distance or almost flat for a long distance, the tire may be ruined beyond repair.
OVERINFLATED.— An overinflated tire is shown in figure 3-11, view C. Too much air pressure also causes tire failure. Excessive pressure prevents the tire from flexing enough and causes it to be constantly subjected to hard jolts. When an overinflated tire hits a stone or rut, the cords may snap and cause a break in the cord body. The center of the tread wears more rapidly and does not permit equal wear across the entire tread.
Hard riding from too much air pressure also increases wear and tear on the vehicle.
Valves
For speed and convenience during inflation, valve stems should be readily accessible. They should be properly centered in the valve holes and slots to prevent scraping against the brake drums. They should be placed so the valves extend through the wheels. Valves on the inside duals should point away from the vehicle, and the valves on the outside duals should point toward the vehicle. On dual wheels, the valve of the outside dual is placed 180 degrees from the inside valve for speed and convenience in checking pressures and inflation. With this arrangement, the locations of the valves are always
known even when you are checking them in the dark.
Spare tires should be mounted so that the valve is accessible for checking and inflating.
VALVE CORES.— The valve core (fig. 3-12) is that part of the valve that is screwed into the valve stem and permits air, under pressure, to enter, but prevents it from escaping. Two types of valve cores and two sizes of each type are in use today. The two types are the visible spring type and the concealed spring type. The two types are interchangeable. Two sizes are provided for the standard bore and the large bore valve stems. The core shell has a rubber washer that provides an airtight seal against the tapered seat inside the stem. Directly below the shell is a cup that contains a rubber seat, which, in the closed position, is forced against the bottom of the shell, forming an airtight seal. The pin on top of the valve core, when pushed down, forces the cup away from the shell, permitting air to flow.
VALVE CAPS.— The valve cap (fig. 3-13) is also a component part of the valve and is screwed onto the end of the stem, providing a second airtight seal. The cap also protects the threads on the end of the stem and
Figure 3-13.—Valve caps.
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Figure 3-14.—Mismatched tires
keeps dirt and moisture out of the valve body. The screwdriver cap has a forked tip that may be used to install or remove the valve core. The plain cap generally is used on rubber-covered valves and has a skirt that contacts the rubber covering on the valve stem. Both caps are interchangeable with each other. Part of your prestart operation is making sure that all valve stems have valve caps.
Mismatching
For longer tire life and more efficient performance, dual tires and tires on all-wheel drive vehicles must be of the same size, tread design, and tread wear.
Improperly matched tires cause rapid uneven wear and can also cause transfer case and differential failures.
Accurate matching of tires is necessary, because tires on axle-drive vehicles rotate at the same speed when all axles are engaged. Dual wheels turn at the same speed, because they are locked together which means that tires on all driving wheels must be of the same circumference and diameter. When one tire of a pair of duals is worn considerably more than the other, the tire cannot carry its proper share of the load and will scrub the road (fig. 3-14). The result is uneven and rapid wear on both tires and/or tire failure.
Tires should be used in sets. Mixing different types (bias ply, fiber glass belted, radial ply) must be avoided.
Snow tires should be of the same size and type of construction as the front tires. Radial-ply tires should always be used in sets.
NOTE: Under no circumstances should radial-ply tires be mixed with bias-ply tires, together or on the same axle.
The problems encountered when mixing tires on a vehicle are loss of steering control, inadequate vehicle handling, and potential mechanical damage. These problems vary depending on the stability of the tires used, differences in dimensions, differences in air pressure, and other operating conditions.
RADIAL-PLY TIRES.— Radial-ply tires (fig. 3-15) are constructed with casing plies perpendicular to the tread direction, with several layers of tread-reinforcing plies (steel or fabric) just under the tread area. This construction permits flexing of the tire with a minimum of tread distortion, better traction, and a softer ride.
Figure 3-15.—Radial-ply tire construction.
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Figure 3-16.-Bias-ply tire construction.
BIAS-PLY TIRES.— Bias-ply tires (fig. 3-16) are constructed of rayon, nylon, or polyester casing plies in a crisscross pattern wrapped around steel bead wires.
These bead wires prevent the tire from opening up and separating from the rim at high speeds. The casing plies give the tire its shape.
Mechanical Irregularities
Uneven tire wear is often caused by mechanical irregularities on your piece of equipment. During your prestart and post operations, inspect your tires for wear, as shown in figure 3-17. These conditions can be caused
Figure 3-17.-Results of mechanical irregularities.
Figure 3-18.—Directional mud and snow tread.
Figure 3-19.-Mounting of directional tread tire.
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