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at the rear wheels than would be possible if the direct drive lock was depended on for the highest speed ratio.
With a four speed gear box with direct drive on the third, one can do more driving under the conditions of maximum efficiency than would be permissible with the higher gear ratio provided when direct drive is on the fourth speed. In one instance, efficiency is sacrificed to some extent on the highest ratio which is used but 25% of the time to obtain the maximum efficiency on the speed ratio that is used over 50% of the time. With the direct drive fourth speed the third speed gears are often used and considerable power is lost through the gearing which would be conserved if the third speed was a direct drive instead of the fourth.
Q. Where is the gearset usually placed ?
Fig. 168.—Gearset Located at Front End of Torque Tube and Form
ing a Unit With Rear Axle Structure. ent unit immediately back of the clutch which usually forms part of the power plant. This brings the change speed gearing almost directly under the floorboards at the front end of the car.
Q. Name other methods of gearset placing?
A. The gearset may be placed as a unit with the power plant; at the front end of the torque tube of the rear axle, as shown at Fig. 168, as part of a jack shaft assembly when used in motor trucks, as at Fig. 169, or attached directly to the rear axle, as shown at Fig. 170.
Q. What are the advantages of unit gearset and power plant or axle assembly?
Fig. 169.-Four-Speed Motor Truck Gearset With Drive Pinion, Ring Gear and Differential Assembly In
corporated in Same Case and Used in Connection With Counter Shaft Drive.
A. The important advantage of combining the gearset with either the power plant or driving member or the rear axle driven member is that the parts are kept in absolute alinement because there is no possibility of the gearset getting out of line with the
inkshaft if it is housed in a part of the motor case or to lose its nement relative to the driving gearing if it is part of the rear axle or countershaft assembly.
METHODS OF DRIVE TO REAR WHEELS
Q. Describe common methods of power transmission in automobiles.
A. The power of the engine is usually transmitted from the speed changing mechanism to the rear wheels by some method of positive drive mechanism which may be bevel, spur, or worm gearing, or sprocket and chains.
Q. What is the influence of power plant location on method of drive?
A. The arrangement of the power plant relative to the rear axle has material influence on the method of drive employed. If the engine is a simple form with the crankshaft at right angles to the frame side it is customary to drive directly from a simple planetary gearing on the crankshaft to a live axle by a chain. If the engine is located in such a way that the crankshaft is parallel to the frame members, then a shaft is employed to transmit the power from the gearset directly to gearing in the rear axle or from sprockets on a countershaft to similar members on the rear wheels.
Q. What is the influence of gear ratio on final drive?
A. When low gear ratios are desired, as on heavy commercial cars, it is necessary to use either a double reduction drive system or worm gearing, if a single reduction is employed.
Q. What determines the speed reduction needed between driving and driven members?
A. The ratio of rear wheel speed relative to that of the crankshaft of the power plant depends upon the amount of power available, the maximum speed it is desired to attain, and the weight of the vehicle. On high powered pleasure cars, where high speeds