CHAPTER IITHE 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.
Fig. 1. Views of Plain Frame.
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 madeup 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.
Fig. 2. Quarter Elliptic.
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 travelingalongside 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.
Fig. 2a. Half Elliptic.
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.
Fig. 3. Three-quarter Elliptic.
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 standardsprings 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.
Fig. 4. Full Elliptic.
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.
Fig. 4a. Cantilever Spring.
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.
Fig. 5. Fore and Aft Motion.
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 thebody, 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.
Fig. 6. Lateral Motion.
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 sucha 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 axlesoriginally 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 axleshave the weight of the car carried by the axle shafts, whereas the toothpressure of the differential is supported by the housing, and only the turning effect or torsion is transmitted by the axle shafts.
Fig. 7. Floating Axle.
Fig. 7. Floating Axle.
Fig. 8. Semi-floating Axle.
Fig. 8. Semi-floating Axle.
3.Full floating axlescarry 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 verticaldistance 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.
Fig. 9. Crossing Depression.Fig. 10. Striking Obstruction.
Fig. 9. Crossing Depression.
Fig. 9. Crossing Depression.
Fig. 10. Striking Obstruction.
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 ofmetal-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.