THE FRAME, AND ITS ACCESSORIES
Under this title should be included the frame, axles, springs, wheels,
steering gear and brakes.
From the beginning it was recognized that the different strains and
stresses set up by the passing of the wheels over uneven ground and by
the motor and driving mechanism, must be taken care of before reaching
the body of the automobile, which otherwise would soon go to pieces.
[Illustration: Fig. 1. Views of Plain Frame.]
THE FRAME.--Therefore, not only springs had to be interposed between
the body and the wheel axles, but also a substructure for the body,
called the frame, which must be rigid enough to prevent any destructive
strains from reaching the body.
In Fig. 1, A shows a top view of a frame made up of channel bars and B
shows a side view to illustrate how the torsion or twist takes place.
It will be understood that the frame thus made is not designed to lend
itself to the entire inequalities of the road, as the springs are
interposed for that purpose.
Experience in the construction and use of tubular frames, as first
employed in bicycles, proved too expensive for assembling, when used in
automobiles. The tubular form of construction was very soon displaced
by frames consisting of metal parts bolted or riveted together. The
main or side members are now usually made of channel steel which gives
great rigidity and strength, compared with its weight.
[Illustration: Fig. 2. Quarter Elliptic.]
HOW THE FRAME IS SUSPENDED.--The important feature is to mount this
frame on the axle. The frame, carrying a body and all the load of the
vehicle, has to permit three distinct movements.
First. That due to the inequalities of the road, which produces a
torsional twist.
Second. A lateral swing, caused by traveling alongside a hill, or due
to centrifugal force when making a turn rapidly.
Third. A fore and aft movement, as when traveling over undulating
surfaces, or in suddenly stopping and starting.
[Illustration: Fig. 2a. Half Elliptic.]
For these reasons springs must be made to compensate for such motions,
and to absorb the jar as much as possible.
[Illustration: Fig. 3. Three-quarter Elliptic.]
THE SPRINGS.--Many forms of spring mountings have been devised, but the
following illustrations show the types which set forth the principles
involved. Outside of coiled springs which are used in some forms of
delivery cars, the standard springs are leaf springs, built up from a
number of steel leaves.
There are four distinct forms of springs used, as follows:
1. The quarter elliptic, used on Ford, and similar cars, as illustrated
in Fig. 2.
2. The half elliptic, Fig. 2a, which is the most widely-used form.
These springs are usually attached with their front end directly to
the frame, and with the rear end by means of a shackle; the center is
fastened by spring clips to the axle.
[Illustration: Fig. 4. Full Elliptic.]
Where a distance rod is used, as on the rear axle, both ends are
attached by shackles.
3. The three quarter elliptic, Fig. 3, always used as a suspension for
the rear axle. This form gives more flexibility than a half elliptic,
and is still stiffer so far as side motion is concerned, than the
following type.
4. The full elliptic, Fig. 4, was formerly used much more than at the
present time.
There are also in use springs comprising a combination of half
elliptic, or three quarter elliptic, on each axle, in which the front
end is shackled to the frame, and the rear ends connected by shackles
to another half elliptic spring, the center of which is fastened to the
frame.
[Illustration: Fig. 4a. Cantilever Spring.]
_Fore and Aft Motion._ Provision must be made, in all cases, for the
fore and aft movement of the car body which takes place in stopping or
starting, and, particularly when the wheels strike an obstruction.
[Illustration: Fig. 5. Fore and Aft Motion.]
_Flues._ Fig. 5 shows a side view of a car, in which the dotted lines
indicate the position of the body, relative to the normal, when the
wheels strike an obstacle.
_Lateral Motion._ In like manner when the car swings around a corner,
or is traveling along a hill-side, the springs must hold the body from
swinging too far. Fig. 6 illustrates, by means of the dotted lines, the
side movement. It is obvious, therefore, that the springs have a duty
to perform in addition to that of merely giving flexibility to the body.
[Illustration: Fig. 6. Lateral Motion.]
CANTILEVER SPRING.--A special form of half elliptic springs, lately
developed, and of increasing use, is the cantilever spring, where the
axle is attached to one end, the center of the spring being pivoted to
the frame, and the other end shackled to or sliding in the frame.
SHOCK ABSORBERS.--Shock absorbers are mechanical means placed between
the frame and the axles for the purpose of dampening the sudden recoil
of the springs after being compressed, when meeting a road obstacle. In
the absence of such a device the recoil is likely to suddenly throw up
the frame, body and passengers, or produce an unpleasant shock.
Originally, simple leather straps were used, reaching from the body to
the axle, which only limited, but did not dampen or gradually absorb
the shock. Now different forms of frictional resisting toggle-levers
are used, which not only absorb the shocks, but also prevent the
bumping of the axle against the frame, and eliminate breaking of
springs.
THE AXLE.--Axles are of two kinds, generally designated as “live,” when
they turn the wheels; and “dead” when they do not turn the wheels, but
simply support the weight of the frame and of the body.
Dead axles are used with double chain drive, as, in that case, the
sprocket wheels are attached directly to the sides of the wheels and
the wheels turn on the studs, or ends of the dead axle.
LIVE AXLES.--1. _Plain live axles_ originally consisted of a shaft
without differential gearing, having one wheel fast on it, the other
turning. Modern construction shows two axle shafts in a housing, the
weight of the car, and the tooth pressure of the differential being
carried by the axle shafts.
2. _Semi-floating axles_ have the weight of the car carried by the axle
shafts, whereas the tooth pressure of the differential is supported by
the housing, and only the turning effect or torsion is transmitted by
the axle shafts.
[Illustration: Fig. 7. Floating Axle.]
[Illustration: Fig. 8. Semi-floating Axle.]
3. _Full floating axles_ carry the full weight of the car, and the
differential bevel gear teeth pressure with the housing, so that the
axle shafts carry no load but only the torsional stress.
Both full and semi-floating constructions are applied to rear axles
only. The front wheels are now universally applied to knuckles, which
swing on vertical pivot pins at the ends of the dead axles.
WHEELS.--Wheels are now in a transition state. The ultimate wheel has
not yet appeared; but whatever its form or construction, certain things
are essential.
FLEXIBILITY.--In the ordinary wagon or carriage wheel, there is but
little, if any, flexibility; but in automobiles, where speed is a
consideration, elasticity, either in the rim, or in some other part of
the wheel, is necessary.
One of the reasons for this is, that on account of tire expense,
motor wheels are smaller than carriage wheels. Making them smaller,
however, produces certain disadvantages. One is that in going over the
inequalities of the road, the axle on the small wheel has a greater
vertical movement than on a large wheel, and the jar on striking an
obstruction is more pronounced, also. These disadvantages, however, are
more than counterbalanced by the elasticity of the invention.
LARGE VS. SMALL WHEELS.--Fig. 9 shows a large wheel A, passing over a
depression B. The large arc of the wheel does not permit the rim to go
to the bottom. On the other hand, the small wheel C goes to the bottom
of the depression, and the vertical distance which the axle of this
wheel must travel, is three times as far as in the case of the wheel A.
In Fig. 10, where the large wheel strikes an obstruction D, the angle
of its upward movement, as designated by the line E, is much less than
the impact force of the small wheel, as shown by the greater slope or
incline of the line F.
[Illustration: Fig. 9. Crossing Depression.]
[Illustration: Fig. 10. Striking Obstruction.]
MINIMIZING SHOCKS.--It is obvious, therefore, that if part of this
shock can be taken up by the tire, the difference due to the smaller
diameter of the wheel, will not be so apparent.
The thickness, or widths of the tires also minimizes the impact and
distribute the jars while running, so that with these advantages a
small wheel has been found to be more practical than a large one.
RESILIENCY.--Most wheels are now made with wooden spokes, secured by
means of a pair of metal-flanged hub plates, bolted together so as to
clamp the radiating spokes, but wire wheels are now coming more into
favor, whereas cast or pressed solid steel wheels are used on some
heavy trucks.