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bevel pinion and ring gear used in the ordinary construction, there are two bevel pinions and two ring gears attached to the differential casing. A typical assembly of this nature is shown at Fig. 189-a. The drive pinion B is integral with the drive shaft and always revolves with it while the larger drive pinion A only revolves with the drive shaft when the sliding clutch A is engaged with the corresponding clutch member attached to the 'pinion. When the sliding clutch A is shifted into position, the actuating bell crank shifts another sliding clutch attached to the differential gear case which releases ring gear B and allows the engine to drive the differential casing and the gearing it contains through the medium of drive pinion A and ring gear A.

When the other ratio is desired the positive clutch shifting lever is moved so that it brings sliding clutch A out of engagement which causes a simultaneous movement of sliding clutch B so that ring gear B is clutched to the differential casing. It will be apparent that the drive is through only one set of gears at a time, the set that is not in use revolving idly on suitable bearings. This construction affords two gear ratios, each driving direct from the engine to the wheels without any intermediate gearing other than the regular driving gears ordinarily employed in a one speed axle.

It is contended that no one single gear ratio can be just right for all vehicle speeds and all road conditions. The usual gear ratio of a single direct drive axle ranges from 3.5 to 1 to 4 to 1 according to the weight and power of the car. If a car is geared low, high speeds can only be obtained by racing the engine but its hill climbing abilities on the high are excellent. At the other hand, if the gear ratio is such that the car can be operated at high speed without racing the engine it will be a poor hill climber and the low gears will have to be resorted to on any gradient of magnitude. In the axle shown there are two gear ratios, one of which is low, and which is especially adapted for city driving where starting, stopping and slowing down are frequent and where cautious operation is necessary. When conditions are more favorable, the higher direct drive gearing may be brought in action and higher vehicle speeds are possible without a corresponding increase of engine revolutions.

For example, with the lower direct driving gears in operation,

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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.

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Fig. 190.-Construction of Conventional Automobile Frames. for Motor Trucks.

A-Pleasure Type. B-Frame Structure

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

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