RUNNING GEAR
The steering of a motor car, or the change in the direction of its
movement, is effected by changing the position of its steering
wheels, usually those in front, in relation to the rear wheels. In
a horse-drawn vehicle, the axles are parallel when it is moving
straight, as are also the planes of its front and rear wheels. To
turn the vehicle to one side or the other the front axle is swung so
that it is out of parallel with the rear axles, the vehicle turning
to the side on which the axles would meet if they were extended. This
construction requires the wheels to run loose on the axle, and the
axle to be permitted to swing on the pivot by which it is attached to
the body of the vehicle.
Such a construction would be impracticable for an automobile,
because the weight resting on the front axle would require the pivot
to be of greater strength and stiffness than could be conveniently
obtained, and because of the effort that would be necessary to swing
the axle in steering. The front axle of an automobile is stationary,
and the steering effect is obtained by pivoting short pieces to its
ends, these carrying the wheels. From these pivoted ends, called
=knuckles=, extend short =steering arms=, which are connected by a
=drag link=, so that moving the drag link moves the pivoted ends of
the axle and the wheels to correspond (Fig. 44).
[Illustration: FIG. 44.—TWO ARRANGEMENTS OF THE DRAG LINK.]
[Illustration: FIG. 45.—STEERING PRINCIPLES.]
For a wheel to follow a curved path without slipping, it must be at
all times tangent to the path, and will be perpendicular to a radius
of the curve at that point. The front axle of a horse-drawn vehicle
points toward the center of the circle on which the vehicle may be
turning and forms part of the radius, the wheels, of course, being
perpendicular to it (Fig. 45). As the main part of the front axle of
an automobile is stationary, only its pivoted ends may point to the
center of the circle, and this must occur in order that the wheels
may be tangent to the curve (Fig. 45). Both axle ends point to the
center, but along different radii; if both pointed along the same
radius, it would necessitate their being in line with the stationary
part of the axle, which then also would be part of the radius. As
the axle ends are in line with different radii of the same curve, it
follows that the wheels are perpendicular to different radii, and
not parallel with each other, a condition impossible in horse-drawn
vehicles. The front wheels of an automobile are parallel with each
other when the axle ends are in line with the stationary part, but go
out of parallel as soon as they are at an angle with it, as is the
case when the car takes a curve.
If the steering arms projected from the knuckles at right angles
to the axle ends on which the wheels revolve, moving the drag link
would move each knuckle through an equal angle, and the wheels would
be parallel at all times; this is prevented by so constructing the
knuckles that the steering arms incline toward each other, with the
result that when the drag link is moved, one of the wheels swings
through a greater angle than the other, the difference between the
angle of each steering arm and the stationary part of the axle
increasing with a greater movement of the drag link.
The control of the drag link is obtained either by a =steering lever=
or =wheel=. When the steering lever is moved by the driver, the drag
link is moved to correspond by a connecting rod, and this type is
usual in small cars. It is impracticable for heavy cars because it
is =reversible=; that is, the moving of the steering wheels moves
the lever, and a firm grasp is required to prevent the shock of
striking a stone or rut from tearing the lever from the hand and
changing the course of the car. The _irreversible_ type is used for
all but the lightest cars, and while it permits the driver to change
the direction of the front wheels it prevents any movement from
being transmitted from the wheels to the steering wheel or lever.
The =screw-and-nut type= of irreversible steering mechanism (Fig.
46, _B_) consists of a heavy screw attached to the lower end of the
shaft that revolves when the driver turns the steering wheel. The
screw passes through a nut that is held in guides so that it cannot
revolve, and is therefore moved up or down when the screw is turned
by the steering wheel. From the nut extends an arm that is connected
to the drag link, so that its movement is transmitted to the wheels.
The turning of the steering wheel thus moves the nut and the front
wheels, but a movement of the front wheels from any other cause is
prevented, because no pressure on the nut can revolve the screw.
Another type is the =worm-and-worm wheel=, or =worm-and-segment=
(Fig. 46, _A_), which, while of different construction, depends on
the same principle that the movement of the worm or screw can move
a worm wheel or nut in mesh with it, but the movement cannot be
reversed.
[Illustration: _A_ _B_ FIG. 46.—STEERING MECHANISMS. _A_, Worm and
worm-wheel steering gear; _B_, nut and screw steering gear.]
The stationary part of the front axle of an automobile is usually a
forging, and must be of considerable strength in order to support the
weight imposed upon it by the engine. It is usually bent down in the
center, in order that it may be the lowest point of the mechanism,
thus to receive possible blows of stones or other high points of the
road that would cause serious damage should they strike the crank
case or fly wheel.
BRAKES
The brakes used in controlling the speed of an automobile may be as
many as four in number, and there should be at least two, for on them
depends the safety of the car. Brakes are of two types, =expanding=
and =contracting=, and usually operate through the friction between
a drum and a band that surrounds it, or blocks that press against
its inner surface. The =band= or =contracting= brake may be either
=single-= or =double-acting=, the latter being by far the better. In
a single-acting band brake (Fig. 47) a flexible steel band surrounds
the drum, one end being made fast to the frame of the car or some
other stationary part, and pressure applied by drawing the free end.
The friction caused by the binding of the leather or fiber lining of
the band on the drum restrains the movement if the drum is revolving
in the opposite direction to the pull, but there is little effect
if the revolution is in the same direction as the pull. In the
double-acting type, both ends of the band are pulled, and the drum
is prevented from revolving in either direction. The single-acting
type would be satisfactory when the car moves forward, but will not
hold it from running downhill backward, while the double-acting brake
holds it in either direction. The expanding brake usually consists
of two bronze =shoes=, of such shape that they fit the interior
surface of the drum. The shoes are pivoted together at one end, and
so arranged that the pull of the brake pedal or lever expands them,
binding them against the drum (Fig. 47). When pressure is not being
exerted, a coil spring, not shown in the diagram, holds them together
and out of contact with the drum.
[Illustration: _SINGLE ACTING BAND BRAKE_ _DOUBLE ACTING BAND BRAKE_
_INTERNAL EXPANDING BRAKE_ FIG. 47.—THREE VARIETIES OF BRAKES.]
Brake drums are usually attached to the spokes of the rear wheels,
and one drum often serves for both an expanding and a contracting
brake. Brake drums are also applied to the jack shaft, or to an
extension of the countershaft of the change-speed mechanism. It is
usual to have one set of brakes controlled by a foot pedal, and
another, called the =emergency brake=, by a lever at the side of
the car. The foot brake, or =running brake=, is always connected
to the clutch, so that applying it throws out the clutch. The
emergency brake is also connected in the same manner in some makes of
automobiles, but this is not recommended, for if it is necessary to
stop the car when going uphill, the brakes must be released before
the clutch can be thrown in, and the possibility of the car starting
downhill backward before power can be applied, the chance of
stalling the engine through this, and the danger of the combination
to any but an experienced driver, make it advisable to have the
emergency brake separate from any connection with the clutch.
Band brakes are usually lined with leather, to increase the friction
between the band and the drum, and this often gives rise to troubles
in the burning of the leather when the brake is applied for a
considerable period, as in the descent of a long hill. The emergency
brake has advantages in that it operates through the friction of
metal against metal, but excessive heat from continued application
may be enough to melt the metal and fuse together the shoe and drum.
For long descents, it is well to use the motor as a brake, for it
is logical to consider that the means of propulsion may also be
the means of retarding, as the wind that urges a sailboat forward
may also bring it to a stop. It is obviously impossible to reverse
the motor or gears, but by switching off the ignition circuit and
throttling down, the forward movement of the car is caused to operate
the motor, and the work necessary in driving the motor as an air
compressor is sufficient to check the speed. The effect is so great
that if the low-speed gears are engaged, the car will be brought to a
stop even on a steep hill. Another advantage of this course is that
it gives the motor an opportunity to cool, which is often necessary
after a long ascent.
In case of the failure of the brakes to operate, which may result
from poor adjustment or worn bands and shoes, the speed may be
checked by throwing out the clutch, switching off the ignition,
engaging the intermediate speed gears, and letting in the clutch very
slowly. Great care must be taken that the clutch is not permitted to
bind suddenly, for that would probably result in the stripping of the
gears. If the low-speed gears are engaged, the checking would be so
sudden, no matter how slowly the clutch might be engaged, that the
shock would probably throw the passengers from their seats.
The failure of the brakes when descending a hill produces a condition
that requires skill and coolness, and danger can only be averted by a
steady hand and a clear head.
Brakes applied to the rear wheels must have an equal grip on each,
for if one binds more tightly than the other, the car will have a
tendency to skid, or slide sideways. In the best cars this is taken
care of by an =equalizer=, in which the pull of the lever or pedal
is not applied directly to the brakes, but to the center of a bar,
each end of which is connected to one of the bands or shoes. The
lever action of this bar distributes the pull equally between the two
brakes, and unless there is a great difference in the grip on the two
drums, as might be the case if one were oily and the other dry, the
effect will be the same on both sides.
TIRES
For low speeds, solid tires give good results in traction and the
absorption of jolts from small obstacles, but for anything above six
or eight miles an hour, pneumatic tires are a necessity in preventing
the rapid shaking to pieces of the mechanism. A hard tire touches
the ground at but one point, and its grip on the road will be much
less than that of a pneumatic tire which, being slightly flattened
by the weight it bears, presents an oval or elliptical surface to
the road. While a pebble will force a solid tire to roll over it,
it will sink into a pneumatic tire, and the jolt that it might cause
will be entirely absorbed. Pneumatic tires are formed of alternate
layers of heavy canvaslike fabric and soft rubber, and the processes
through which they are put in manufacture are supposed to effect
their perfect combination; but as it is not the nature of rubber to
be absorbed by the fabric, the layers are only bound together by its
tenacity. The bending of the sides of the tire under the weight of
the car tends to separate these layers, and water or dirt entering
between them through cuts quickly brings ruin; it is obvious that
the less the sides bend, the smaller will be the opportunity for
the layers to work apart. A pneumatic tire should always be pumped
as hard as possible, so that it stands up practically round under
a loaded car. While the car will ride a little harder under these
conditions than when the tires are soft, there will be greater
resistance to punctures, and the life of the tires will be increased.
The normal wear to a tire should give it a smooth surface, but if it
is noticed that the tread is rough and uneven, it may be taken for
granted that the wheels do not run true. Rear wheels will be thrown
out of true by the springing or bending of the axle, and front wheels
also from this cause, but more probably from faulty adjustment of the
steering mechanism or the bending of the drag link or steering arms.
The grip of the tire on the road is much affected by the nature of
the surface, the traction on dry macadam being much greater than on
wet asphalt. When the pull of the engine on the wheel exceeds the
grip of the tire on the road, there will be a slip, and the wheel
will revolve without moving the car. This will wear the tread of
the tire far more rapidly than will ordinary running. The better
the traction of the tires on the road surface, the less will be the
tendency of the car to skid or slide sideways, and less power will
be lost through the slipping of the wheels. To reduce the chance of
slipping, because of wet asphalt or muddy roads, various devices are
in use, all of which encircle the tread of the tire, and present a
rough surface. The form in most general use consists of chains that
fit across the tread, these being detachable and used only in case of
necessity. While it is often done, it is nevertheless bad practice
to apply chains or other antiskid devices to only one of the rear
wheels instead of to both, for it increases the diameter of the wheel
and makes a difference in the resistance against the wheel, causing
the differential to operate at all times. The differential is not
constructed to operate steadily, and will wear rapidly if forced to
do so.
SPRINGS
[Illustration: FIG. 48.—_A_, Full elliptic spring; _B_, half elliptic
spring.]
In addition to tires, an automobile is fitted with springs, which are
necessary to absorb the shocks and jolts that are too great to be
taken up by the tires. These are usually =full= or =half elliptic=
(Fig. 48), and made of flat plates, or =leaves=, of different
lengths, the small being placed on the large, and all bound together
at the center. The combined action of the springs and tires permits
the frame and body of the car to move in a nearly straight line,
while the wheels and axles follow the inequalities of the road. When
springs break, as is frequently the case, it is from the rebound of
the body that results when the wheels drop into a deep hole, the
upward movement separating the leaves, and the entire strain coming
on the long leaves alone. To prevent this, =shock absorbers= are
recommended, which permit the springs to have a certain amount of
action, but check them if they tend to expand or compress to too
great an extent. They act either by the friction between metal plates
and washers, or by air or oil in a cylinder that permits a piston to
move freely to a certain degree, but presents resistance to a greater
motion. Shock absorbers are placed between the axles and frame, and
there should be four, two to each axle.
DISTANCE RODS
As the springs are placed between the axles and body and are
flexible, it is necessary to provide some method of preventing an
obstruction in the road from twisting the axle, as might result if
one wheel struck heavy sand or a stone while its mate was on good
surface. A twist of this sort would throw the axle out of line with
the drive and bind the chain or driving shaft.
[Illustration: FIG. 49.—DISTANCE OR RADIUS RODS.]
To prevent this, =radius= or =distance rods= are attached to the
axle, one on each side, extending to a point well forward on the
frame (Fig. 49). These rods are pivoted to the frame, and have a
loose joint on the axle, so that the latter is free to move up and
down, but prevented from moving forward or back. Distance rods are
adjustable, and on chain-driven cars serve to adjust the chains,
which are tightened by lengthening the rods and slackened by
shortening them.