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.
Now, as a matter of fact, wheels which are set in the manner described
may run for a considerable distance without the tie rod, which fastens
the knuckles together. It is usually understood that when the tie rod
breaks there is bound to be a smash at high speed, and under most
circumstances this might be true, but at moderate speeds it often
happens that the car runs a considerable distance, steering perfectly,
after the tie rod has been broken.
The author had an experience of this kind. He was going up Riverside
Drive in New York City, toward the viaduct. After he had gone almost
across the viaduct he was startled to see what looked like the tie rod
protruding out in front of the car, although the car was moving along
with no sign of trouble. The car was stopped and it was found that the
bolt had jumped out of the knuckle on one side and that the tie rod had
actually swung out in front, and not until then had it been discovered
that there was any trouble.
Going back over the route across the viaduct and some distance beyond
did not disclose the missing pin, but the conclusion was reached
that it must have come out on the hill leading to the viaduct. Yet
the wheels remained perfectly straight and the car steered properly,
although there were no very sharp turns, and the car did not go to
smash as it ought to have done in theory. It was the caster effect
which saved the car from an accident. The trailing effect described
kept the wheels running straight even after the tie rod was out of
commission.
Some time later it was learned that the same thing actually had
occurred in one of the big auto races. It so mystified the driver and
his mechanician that they decided to experiment. They fixed up a car
without a tie rod and started to speed up on the track. They attained
quite a high speed without mishap and slowed down and drove to the shed
as though the steering gear were regularly equipped.
However, it is not recommended that any automobile owner neglect to
have his steering apparatus regularly inspected and to see that the
pins are kept tight and that the wheels do not get too much out of
alignment. The pin might drop out when going down a steep hill or when
running at high speed, and tires do wear out when the wheels are too
much out of alignment; but the instances cited show that sometimes
strict theory and actual practice do not always meet.