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Fig. 190.—Construction of Conventional Automobile Frames. A—Pleasure Type. B-Frame Structure

for Motor Trucks. LESSON TWENTY-TWO


Q. What is the function of the frame?

A. The frame of the automobile forms the basis of the entire mechanism. It not only supports the power plant, change speed gearing, body and controlling devices but also serves as a tie member between the two axles and carries the springs by which these members are joined to the frame.

Q. What factors determine size of frame to be used?

A. The size and construction of the frame depend entirely upon the class of vehicle to which it is fitted. Some motor cars require frames of great strength and light weight while others can use heavy frames.

Q. What materials are used in frame manufacture?

A. Automobile frames may be made of wood or metal, or combinations of these materials. When metal is used the frame members may be of tubular or channel section steel, of iron angles or channels, or it may be a composite structure using tubing and pressed steel parts in combination. The two typical forms of metal frames with the various running board irons, spring supports, etc., in place are outlined at Fig. 190. That at A is used in pleasure car service while the form at B is a heavier construction intended for commercial car use.

Q. What type of car uses frames made of structural steel?

A. Structural steel channels in standard sizes are commonly used in heavy commercial vehicles.

Q. Describe various methods of using wood.

A. The frame may be composed of wooden beams or of composite members made from various laminæ glued and screwed together or of lighter wood sections reinforced with a strip of iron or steel extending the full length of the piece.

Q. On what type of vehicles are armored wood frames used?

A. Composite frames of wood and metal are seldom used except on heavy vehicles such as motor trucks. The feature of this method of construction is that a strong and yet light supporting member is employed, the strength being derived from the steel reinforcing plate. The wood frame does not transmit vibration as do metal frames and the machinery is better protected from the vibration incidental to vehicle operation on poor highway surfaces than when the shock transmitting all metal frame is employed.

Q. What are the features of laminated wood frames?

A. The laminated wood frame structure is one that possesses great strength without being heavy, which enables this type of frame to be employed in pleasure car use. A laminated frame is not liable .to warp or check and the arrangement of the various pieces comprising the frame is such that the arrangement of the wood fibers makes for great strength. When wood is employed as frame material ash is generally used because of its superior strength and lightness. A laminated wood frame is said to be stronger than a pressed steel frame of equal weight, and as the wood absorbs instead of transmitting vibration it is contended by those favoring wood frame construction that the machinery is better protected from road shock than when all metal frames are used.

Q. Why is pressed steel used so widely in automobile frame construction?.

A. Pressed steel has received general application in automobile work because the frame side and cross members may be quickly and economically formed into shapes that would not be possible with any other material. Side members may be cambered or upswept which would not be possible if wood is used and a frame of superior lightness to that made of standard structural shapes may be evolved by the designer. Pressed steel frames may utilize members best adapted to support the various parts, which enables the designer to adapt the frame to the components to be supported rather than design all parts to fit a frame. Pressed steel frames are light, cheap and strong if properly designed and are especially adapted to quantity production.

Q. What are the advantages of upswept side members?

A. When a frame is upswept at the rear, as shown at Fig. 190, it is possible to use types of springs that have great flexibility of movement without raising the center of gravity of the car to a point where side sways might become dangerous. A frame may be upswept at both front and rear ends and the body carried closer to the ground than if straight frame members were used without sacrificing ease of riding and ample axle and spring movement.


Fig. 191.–Usual Method of Frame Support in Which Weight is Car

ried by Springs Placed Over the Axle. Q. What are the reasons for using inswept or cambered members?

A. Many automobile frame side members are cambered or swept in at a point approximately below the dash so the front portion of the frame is narrower than the other parts. The object of narrowing a frame at the front end in this manner is to provide a greater degree of front wheel movement and permit a car to turn in smaller circles than would be the case if the front wheels were moved only to the angular limits permitted by a wide frame. Another advantage of the cambered member is that it permits the motor to be attached directly to the frame, and thus eliminates a subframe to carry the power plant.

Q. Is the frame always carried above the axles?

A. Automobile frames are not always carried above the axles as shown at Fig. 191 as in some cases the frame may be underslung or carried below the axles, as shown at Fig. 192.

Q. What are the advantages of the underslung frame?

A. Where the frame is carried beneath the axles, the center of gravity is brought lower and it is possible to turn corners at higher speeds than with cars having the frame carried over the axles. An advantage of having a smooth underframe, which can be thoroughly encased with a dust pan and present a smooth outline, is that less dust is created and that the vital parts, such as the motor flywheel, gearcase, etc., are protected against injury should the wheels sink




Fig. 192.-Underslung Frame Showing Method of Supporting From

the Axle.

into a mud hole or in a deep rut. It is contended also that the underslung construction provides greater accessibility to the working units of the power plant and transmission. Another superiority of the underslung frame is said to be easy riding, because side sway is largely eliminated by suspending the frame from the springs instead of balancing it upon the springs. As the center of gravity is carried lower the car must be tipped to a greater angle in order to overturn it. This can be clearly shown by drawings at Fig. 193, which shows a test made to obtain the relative stability of the overslung and underslung types. A block and tackle equipment was employed at one side of the car, so that two wheels could be raised to

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