Anti-skid devices, especially those which are stationary upon the wheel, contribute it great deal toward causing solid rubber tires to give unsatisfactory service. The loose chain is by far the least injurious as it will work its way around the tire and equally distributes the wear and strain, However, with a stationary chain this is constantly confined at the point of bearing.
These anti-skid devices are mostly applied to the driving or rear wheels of a vehicle and these are quite apt to spin in slippery places, causing sharp blows at the points of contact. With a sationary device the shock received by the tires in striking the ground is concentrated in a few points around its circumference, causing heavy strains at these points. In order to avoid this, de-vices having numerous cross-pieces should be used for as the distance between these la decreased the force of the blow decreases as the wheel gains momentum. Every type of anti-skid device is more or less injurious to the tire and they should only be used temporarily to pass over soft slippery places.
Cuts are of common occurrence and are generally caused by road conditions. These cuts, no matter how small, afford an entering place for sand and fine gravel, causing the cut to increase in length and depth. Sand, gravel and other gritty substances are enemies of rubber and once they effect an entry into the tries it is pretty hard to combat them. Cuts near and at the edges are most injurious, and if attended to in time are easily remedies. These should be trimmed off smoothly with a sharp knife as soon as they occur, and if they are not trimmed the revolving wheel causes the loosened edges to catch on every obstruction so that the tear constantly increases. When one unit of a dual tire is permitted to weaken in this manner it causes an overload on its mate.
Skidding is generally caused by sudden application of the brakes, turning corners too rapidly and turning corners so small that the crown of the road may cause the rear wheels to skid. The effect of skidding or locking the wheels is quite serious, as this causes flats on the tread of the tires, in addition to placing the tires under side strains, which tears them loose from the base. This same condition will also exist if the brakes do not grip evenly, causing one wheel to roll while the other drags. Turning corners too rapidly increases strain and wear on the tires with a similar effect. Some tire makers recommend truing up tires if flats develop, by turning down the tread, otherwise these will develop rapidly and cause a great loss of mileage.
The sudden application of power by quick engagement of the friction clutch produces the same effect as locking the wheels. The power applied at the hub starts the rim first and the resistance of the road prevents the tire from starting at the same instant. This brief delay slightly stretches, displaces or strains the rubber, just as the life is taken out of a rubier band by continual stretch. This danger is greatly augmented as wear takes place in the driving members, such as the hubs, universal joints, driving chains, etc.
Injuries resulting from running in car tracks are serious and readily apparent. Under this condition, the outside edge of the tire rests upon the raised edge of the car track, so that the distribution of the load is on but a small portion of the tire. That is, the weight of the vehicle is being supported by that small portion of the tire which is riding on the raised part of the track. Throwing the load upon half of the tire causes it to wear rapidly, while the rest of it is in apparently good condition. With dual tires this effect is still more pronounced because that part of the unit riding on the car track must sustain the load intended for both tires.
Heat causes disintegration of the rubl)er. In winter the large garages are generally heated by steam, the headers of which are fastened close to the ceiling of the floor below. This heats the floor above to quite an extent, and if a heavy truck is parked for two or three days directly over a big hot spot in the floor a condition is developed that results in a flat being formed in the tire shortly afterwards. This is due to the action of the heat soften-in-: the rubber, while the weight of the truck is resting upon it, causing a flow that never fully returns.
Gasoline, oil. grease and other fatty substances are solvents of rubber. If garage floors are not kept clean and the tires stand in a pool of oil, a similar action to that of the effect of beat will take place. Grease and oil can easily be removed by a rag saturated with gasoline. Gasoline, although a solvent, evaporates quickly and if applied in small quantities will not cause any injury if used as a cleansing agent. Another difficulty occurs in the abuse of trailers which is caused by turning too sharp a corner while loaded. The effects of this are similar to those caused by skidding, as it tends to wrench the inside wheel, twist-ing the tire, which is firmly held to the road by the weight of the load. The results are a loose tire in a very short time. The above gives the general abuse of commercial vehicle tires, and while there are numerous others of minor importance, it has been proven by maintenance engineers that many tire miles can be saved by avoiding the above sources of trouble. In order to obtain maximum tire mileage, commercial vehicle owners should instruct operators to avoid all overloads, momentary ones as well as constant ones, speeding, curbs, ruts, car tracks and reckless backing up against curbs, and also to properly distribute the load and to select the best roads and smoothest pavements.
Truck operators can save a considerable amount by using judgment in the operation of a commercial vehicle.
Fig. 282. Tires Showing Undue Wear.