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Fig. 189a.-View of Austin Two-speed Rear Axle, Showing Two Pair of Bevel Drive Gears and Positive Clutches by Which Either Gear Ratio May be Brought In Action

an engine speed of 700 revolutions per minute would mean a car speed of approximately 20 miles per hour, whereas if the high direct drive gears are used the car will travel approximately 30 miles per hour with no increase in engine speed. This increase in car speed without a corresponding increase in crank shaft revolutions means that there is a decrease in fuel consumption for a given mileage. A given quantity of gas is utilized to greater advantage and produces more actual power at the rear wheels than with the engine turning over more rapidly on the low gear ratio. Friction and wear of power plant and transmission gearing is also materially reduced because the parts are operated more slowly. When a low gear ratio is employed a car will vibrate very much at high vehicle speeds on account of the great speed at which the motor is operating, while with the higher ratios there is comparative freedom from vibration because the power plant is not operated at excessive speed.

Fig. 190.-Construction of Conventional Automobile Frames.

A-Pleasure Type. B-Frame Structure

for Motor Trucks,

LESSON TWENTY-TWO

AUTOMOBILE FRAMES AND SPRINGS

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 to

gether 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

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