The first car credited by the author as being equipped with two or more direct drives is the Sizaire-Naudin, in 1905. The transmissions of this car and of one embodying similar principles of gearing, brought out in 1909, are described and illustrated by diagrams. After the Sizaire-Naudin, the next double direct-drive transmission was the Pleukharp transmission axle, made in 1906, although the real ancestor of the present double-drive rear axles is the 1906 Pilain transmission; both are described and illustrated. Other early American and foreign forms are commented upon and diagrammed, including the Austin design, believed by the author to be the first to use a two-speed axle of the simplest and lightest possible type to provide two direct drives in connection with a separate gearset to give additional forward speeds and the reverse.
Modern two-speed axles are reviewed, with critical comment and diagrams, and considerable discussion of gear ratios is included. Following this the author exhibits and describes several so-called practical axles, a light two-speed axle weighing not more than the average ordinary axle in common use, a two-speed and reverse axle with splined differential housing upon which the gears slide and a three-speed forward and reverse axle, the last being shown connected to the chassis and as used in connection with an internal-gear drive for truck purposes. Regarding the relation of gear ratio to horsepower, the author quotes J. G. Vincent and E. T. Birdsall to the effect that an indirect gear ratio below the direct, that is higher than 60 per cent of the direct, will give very little added ability. An assembly for high speed is illustrated and described, followed by diagrams of gear assemblies, a special ball bearing and a carrier bearing.


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