The connection from the transmission to the rear axle in pleasure cars is usually by shaft, called the driving shaft. On the majority of motor trucks, however, it is by means of double side chains, which will not be discussed here. This shaft is generally inclosed in a hollow torque tube, with suitable connection at the front end to a frame cross member, and at the rear to the axle housing. Its construction is generally such that it contains a bearing for the driving shaft at both front and rear ends. In addition, the majority of final drives contain at least one universal joint, and many of them contain two. As its name indicates, this will work universally, that is at any angle, its particular function in the driving shaft of an automobile being to transmit power from a horizontal shaft - that of the engine clutch and transmission - to an inclined one - the driving shaft - with as little loss as is possible.
The driving shaft drives the rear axle through some form of gear, either bevel, worm, or other variety, and is usually a two-part shaft. The reason for cutting the rear axle, is that each wheel must be driven separately in rounding a curve, for one travels a greater distance than the other. This seemingly complicated act is produced by a simple set of gearing called the differential, which is located within the driven gear in the rear axle. Each half of this is fixed to one part of the axle shaft. All these gears and shafts must have bearings, lubrication, means for adjustment, etc. On the outer ends of the axle shafts are mounted the rear wheels, which carry some form of tires to make riding more easy. The brakes are generally in a hollow drum attached to the wheels. All this goes to make up the driving system.
The front wheels perform a different function. These are so hung on the steering pivots, that they can be turned as desired to the right or the left; in order to have the wheels work together, a rod, called the cross-connecting rod, joins them; while the motion is imparted to them by means of another rod, called the steering link, which joins the steering lever or arm with the right-hand (or left-hand as the case may be) steering pivot. The last-named lever projects downward for this purpose from the steering-gear case, being itself moved forward and back by the rotation of the steering wheel in the driver's hands.
The transformation of the rotation or turning motion of the hand wheel into a pushing (or pulling) or longitudinal movement is accomplished within the steering-gear case by means of a worm and gear; a worm and partial gear; or, in some cases, a pair of bevel gears. All these parts need more or less adjustment, lubrication, fastening means, etc., the complete group being designated as the steering group.
In addition, the steering wheel and post carry the spark and throttle levers, with the rods, etc., for connecting them to the igniting apparatus (magneto, timer, etc.), and the carbureter, respectively. The purpose of the spark lever is to allow the driver to vary the power and speed of his engine by an earlier or later spark, according to his driving needs. Similarly, the throttle lever is for the purpose of opening or closing the throttle in the intake manifold of the carburetion system, allowing in this way more or less gas to pass to the engine, and thus increase or decrease its power output or speed. Actually, these are parts of the ignition and carburetion systems, respectively, but they are usually grouped with the steering, because located on the steering wheel and post.
Little need be said about the frame. The side members generally carry at their front and rear ends the springs, which are connected to the axles, and thus support the car. The front cross member usually supports the radiator, and sometimes the front end of the engine, too. The rear cross member usually supports the gasoline tank, when a rear tank is used. The other cross members may support engine, transmission, shifting levers, or other parts according to their location. In general, the number and character of frame cross members are slowly changing, the modern tendency being toward their elimination. By narrowing the frame at the front, the engine can be supported directly on the side members. With the units grouped, the same is true of the other important units.
Formerly, practically all motors and transmissions were supported on a subframe, but it has been found that the same results can be obtained and this extra weight and work eliminated. Consequently, although the drawing, Fig. 1, shows a subframe, these are not as widely used as was the case formerly.
When the shifting levers are placed on the outside, these are fastened to the frame; the steering gear is always attached to it; the headlights generally are supported from the frame; all step, fender, and, body parts are attached to it; the under-pan for protecting the mechanism from road dirt is attached directly to it; the body, of course, is fastened to it - in fact is constructed with this idea in view - six bolts being used, generally; the muffler previously mentioned is usually hung from a rear-frame member; when electric lighting and starting are used, the battery is most often hung in a cradle, supported by the frame, while the hood or bonnet is supported equally by the side members of the frame (usually covered with wood) and a rod running from radiator to dash.
In Fig. 1. it will be noted that the engine group (1) and the clutch group (2) are together, really forming one unit. Back of this the transmission (3) and the rear axle or final drive group (5) form two separate additional units. When the transmission is united with the motor (forming a unit power plant), or with the rear axle, one unit is practically eliminated. The different functions of the components are not changed, but the grouping previously pointed out becomes less apparent, for units (1), (2), and (3), or (3) and (5) become one, as the case may be.