A tire which is overspeeded is prematurely destroyed in a manner very similar to that of an overloaded tire. Overspeeding makes every road rough, because it magnifies every irregularity and this increases the effect of all shocks. Hitting a curb, bump or any other obstacle with considerable impetus, even though the truck be empty, is in effect identical with a momenary overload, causing a rapture in the rubber structure in that particular spot and gradual ruin of the tire, for as the wheel revolves at excessive speed the rapidity of compression and expansion of the rubber generates internal friction heat. This is increased considerably by the friction of the road, thus heating the rubber to a higher degree than it can resist. This combined with the increased effect of all shocks is very destructive.
Rough roads have an affect on solid rubber tires similar to that of overloading and overspeeding, as the face of the tire rests successively on irregularities which have the effect of overloading that particular portion of the tire. These irregularities, such as ruts, large stones, crushed stone, loose brick and similar road materials, causa shocks which tax the tires beyond the limit of their power to absorb them and these momentary overloads create a disintegrating effect upon the tread of the tire. Some roads have an extreme heat which also causes disintegration while others produce a similar effect, owing to their composition. Careful driving over rough roads at moderate speed,avoiding rate,stones and loose surface material as much as possible, will greatly increase tire mileage.
Any fault in alignment allowing the wheels to run out if parallel no matter to what small extent prevents their true rolling motion. When two opposite wheels arc not parallel there is a diagonal grind upon these at the point where they come in contact with the road surface. This grinding action quickly wears off the rubber, the wear being very smooth just as though the rubber had been ground off on an emery wheel.
Misalignment of wheels generally is evidenced through ex-cessive wear on one side or by flats, as these when once started rapidly develop. There are a number of things which affect the alignment of front wheels. The cross rod, axle, or steering knuckle may be bent due to violent contact with the curb, another vehicle or obstacle, the cross-rod or knuckle may be improperly adjusted, loose or worn hubs, bearings in the wheels and bushings in the steering linkage may be worn, or the adjustments may have been disturbed through vibration.
All wheels should be frequently tested for alignment and always after a collision or untoward event that is likely to effect the wheel adjustments. A piece of tubing fitted with a sliding rod and a thumb screw, or a stair tread extension rule, form a useful gage for front wheel testing. In testing front wheels it is most important that the dead vertical center is measured, both front and rear. This is necessary because of the tendency of the front wheels to spread under the driving force and it is the practice of commercial vehicle builders to set the front wheels at a toe in from one-fourth to five-eighths in. less in front of the axle than in back of it. This allows one-eighth in. to five-sixteenth in. toe in for each front wheel. This is practically taken out by the action of the vehicle on the road, for under momentum the wheels will be approximately parallel.
Each wheel can also be checked up separately by raising it with a jack and placing a stationary point almost against the wood felloe. Revolving the wheel will determine if the distance between the stationary point and the felloe is the same at all points around the wheel. If the wood felloe rubs at some point around and not at other places it may be due to a slight variation in the felloe, but more usually it is the result of a wheel not running true.
Irregularities of the driving wheels of a motor vehicle do not exist to the extent that is the case with the front wheels, though it occasionally occurs that when the rear axle is displaced to one side or the other it causes the wheels to take up a diagonal position with a consequent grinding action upon the tires. It is essential should it be discovered that these are out of parallel that the trouble be rectified immediately. For testing rear wheels, both comparatively and their relation to the front wheels, the ordinary line and rod method cannot be improved Upon. The measurement between the wood felloes of the rear wheels should be the same both in front and rear wheels of the axle. Whenever an undue amount of play is discovered in the wheels steps should be immediately taken to remedy the defect, otherwise irreparable damage to the tires is inevitable.
Fig. 279. Front Wheels out of Line due to Striking Curb or Obstacle on the Bead.
Fig. 280. Front Wheels out of Line due to Wear or Improper Adjustment.
Fig. 281. Rear Axle out of Alignment due to Poor Chain Adjustment.