The care of the motor-truck tire, while an important item in the maintenance of a commercial vehicle, is not generally understood by operators of these vehicles. All manufacturers of solid rubber tires issue instruction books or cards on this subject, but these, like most all other instruction books, find their way into the tool-box and remain there until trouble arises. Tire makers, however, are endeavoring to educate operators on this subject, and since very little mention has been made regarding this, the writer will endeavor to cover it in such manner as to enable the layman to become familiar with the attention tires require, to give maximum mileage.
There have been many refinements in the construction of motor-truck tires, and the majority are very dependable; but tires, like the engine, transmission, axles and all other parts of the vehicle, must have a reasonable amount of attention if one expects to obtain the best results. Tire equipment is given careful consideration by the engineer and the tire maker in determining the necessary sizes and types. However, no provision can be made for the usage of the vehicle or the care of the tires. Provision is made for taking up wear and alignment in the mechanical parts of the chassis which affect the life of the tires. However, these are not automatic adjustments, and require frequent inspections. Tires, like all other parts, have a physical limit, and results can only be obtained if their usage is within the limits of their physical strength, which is based upon the compression of the vulcanised rubber.
For maximum tire mileage, three Factors -the tire, the road and the driver most be considered. The tire most be considered because the type of tire used for quick delivery cannot be used for heavy duty. The road naturally effects the mileage of truck tires, because the bumps and ruts of hail roads throw local-used shocks on them. The driver, next to the tire itself, is the most important factor. It depends upon him whether the truck be abused.
In the following an outline of the common injuries to motortruck tires is given, most of which are sustained in running, and may be traced directly to one of the three factors mentioned above. These common injuries may be summed up as follows: Overloading, speeding, rough roads, wheel alignment and irregularities, anti-skid devices, neglected cuts, skidding and application of brakes, application of power, running; in ear tracks, beat, oil and grease and abuse of trailers.
The most premature tire failures are due to overloading, not only by constant overloading. but by the momentary overload-ing as well. Rubber, like any other material, has its limits of resistance, this resistance being its ability to return to its original shape after being compressed. This may be compared with an ordinary rubber hand, which will snap if stretched beyond its limit of elongation, as the rubber in a motor-truck tire will snap at once, even though momentarily loaded beyond its limits of compression. This compression is noticeable by the bulging out of the rubber, both left and right, and even front and rear. If the load is within the capacity of the tire the rubber will with-stand the strain and as the load is released, return to its original shape, the same as a rubber band when stretched and released. However, if the load is beyond the capacity of the tire the rubber will break down as inevitable as when stretched beyond its limit of elongation, if the tire is overloaded momentarily the rupture may not be apparent, as the broken portions may be hidden by others not noticeably affected, yet the strength of the tire is impaired and failure of the whole structure is merely a matter of a short time, as the damage is bound to spread.
The distribution of the load also has an important bearing upon the life of a tire, as trucks are frequently loaded so that the heavy articles are carried near the tail-board, while the forepart of the body carries little. In this case the rear tries usually carry an overload, although the total load may be within the capacity of the vehicle. Loads which overhang the rear of the body, such as lumber, pipes, etc., also produce the same effect. It does not matter whether the overload is a constant one or a momentary one as far as the cause of the damage is concerned, and is only material to the extent of the damage. A momentary overload may have ruptured the tire structure only in a single spot, whereas a constant overload will damage the whole tire, thus hastening it to complete failure; but in both cases the tire is doomed to premature ruin.
Small dual tires are often exposed to momentary overloads, as the camber of the road may be such as to throw the total weight alternately on one of the outer or inner tires, the mates being momentarily relieved of their load.
Fig. 278. Overloaded, Overspeeded and Bad Road Tire Effects.