CHAPTER XVII
WHY THE AUTO STEERS EASILY
Visions of disaster come to the automobile driver at the thought of running without the steering tie rod. One thinks of wheels trying to go in divergent directions and of ultimate destination in the ditch. As a matter of fact, it is possible to run along quite a while at moderate speed with only one wheel connected to the steering apparatus, and experiments have demonstrated that this can be done at quite a lively gait, on a speed track, whether straightaway or oval. There are several other peculiarities about the front wheels of an automobile which are not fully understood by automobile men, unless they have had technical training.
Most people think the front wheels of an automobile should be perfectly true in every way; that is that they should run parallel, but as a matter of fact they do not run parallel from any viewpoint. The front wheels actually toe in to a slight degrees—that is, the distance between the front part of the wheels should be one-quarter to three-eighths of an inch smaller than the distance measured between the back parts. The tire manufacturer will tell one that the wheels must be in perfect alignment if full service is to be gotten out of the tires. This is true in a degree, but auto manufacturers understand that there must be a slight foregather.
Theoretically there would be a wear on the tires if they were at all out of true, but when it comes to the practical consideration of an automobile there are certain variations of this which are necessary to make the operation of the car safe and a matter of ease.
Of course there cannot be too great a difference in the alignment, but a very slight difference is necessary to enable the steering to be a matter of certainty. There will be, of course, an intangible amount of wear, but it does not cut any particular figure in the life of the tire and it gives a stability in the steering which cannot be neglected.
Perhaps it can best be illustrated by referring to two types of boats which are familiar to most persons. One is the scow with the square nose, which is very hard to steer either with or against the tide; it is pushing flat against the water and there is no lateral pressure to keep it steady. The pointed-bow boat, the ordinary type, has a pressure upon each bow so that there is a constant resistance against any extraordinary pressure from either direction. This makes it necessary in steering to exert a certain pressure to turn the boat about. In actual practice it keeps the boat from wobbling. The same is true when applied to the front wheels. If they are perfectly aligned there will be no tendency to give one way more than another and very little pressure would tend to turn the wheels aside and make the car wobble. Experience teaches that this actually occurs.
At the automobile school, of course, there are many novices learning to steer a car, and they are taken out by experts. Where the wheels are actually aligned perfectly the wobbling effect, especially where the mechanism is a little worn, is very disconcerting to the student. Now, if the wheels toe in there is exactly the same pressure effect working from opposite directions. Each seeks to go slightly out of true and each offsets that tendency in the other. This naturally keeps the car steady and the wheels pass over slight obstructions without turning them in the slightest degree; only a rut or some large obstruction would cause them to turn and this would be the case under all circumstances.
In case there is wear due to neglected lubrication or otherwise, the wobbling tendency is especially noticeable. With the wheels toeing in slightly this is overcome, because pressure is exerted on the knuckle pins, keeping them firmly in place, even though there be considerable wear. The pressure being from both sides, it naturally takes up the play in both knuckles the same as with a chain when it is drawn tight.
Nor do the front wheels run in an absolutely vertical position, for they are set to undergather. This is done to save the tremendous strain upon the knuckles and make it possible to have these much lighter than would otherwise be the case. The fact of the undergather, of course, takes the strain off the steering knuckle pins. Were the wheels absolutely vertical they would set two or perhaps three inches outside the knuckle. The weight line being directly through the knuckle to the ground, there would be a leverage of two or three inches which would put a tremendous strain directly upon the knuckle; where the wheels are undergathered, the line of weight intersects both the knuckle and the tread contact, removing the item of leverage completely, and the wheel necessarily bears its full proportion of the load without undue strain upon the knuckle or pin.
Sometimes the pins are inclined fore and aft, that is, the bottom is farther forward than the top. This is to help in keeping the wheels pointed straight ahead by making the wheels into sort of a caster, so that they trail easily. The knuckles are directly over the center of the wheels and the line of weight would be directly downward, but the pins point forward. In this way, as engineers readily understand, the line of weight is moved forward and the weight is carried ahead of the contact point of the tire with the ground. Anyone who has ever ridden a bicycle understands the principle of this. In the old velocipedes where the wheel fork was vertical, it was very difficult to steer. Bicycle manufacturers discovered that by projecting the fork forward and transferring the weight line in advance of the wheel, they were able to make a machine which would naturally go straight ahead. The principle is exactly that of the caster. The roller trails along after the weight and so moves freely, even over obstructions, where it would be hard going if the roller were directly under the weight.