Motor-car principles; the gasoline automobile

by Roger B. Whitman

DRIVING-GEAR RATIOS

Even when on the direct drive, the crank shaft makes more revolutions than the rear wheels, in order that the momentum of the moving parts of the engine may be sufficient to keep the car in motion. On shaft-driven cars, the bevel on the driving shaft has fewer teeth than that on the axle, so that it revolves more than once to one revolution of the axle. On chain-driven cars, the driving sprockets are smaller than those driven. This driving-gear ratio, as it is called, varies from one and a half to three and a half, or, in other words, the wheels revolve once while the driving shaft or sprocket makes from one and a half to three and a half revolutions. Other conditions remaining equal, a higher driving gear gives the car lower speed, but greater ability in hill-climbing and the traversing of heavy roads.

CHAPTER X
RUNNING GEAR

The steering of a motor car, or the change in the direction of its movement, is effected by changing the position of its steering wheels, usually those in front, in relation to the rear wheels. In a horse-drawn vehicle, the axles are parallel when it is moving straight, as are also the planes of its front and rear wheels. To turn the vehicle to one side or the other the front axle is swung so that it is out of parallel with the rear axles, the vehicle turning to the side on which the axles would meet if they were extended. This construction requires the wheels to run loose on the axle, and the axle to be permitted to swing on the pivot by which it is attached to the body of the vehicle.

Such a construction would be impracticable for an automobile, because the weight resting on the front axle would require the pivot to be of greater strength and stiffness than could be conveniently obtained, and because of the effort that would be necessary to swing the axle in steering. The front axle of an automobile is stationary, and the steering effect is obtained by pivoting short pieces to its ends, these carrying the wheels. From these pivoted ends, called knuckles, extend short steering arms, which are connected by a drag link, so that moving the drag link moves the pivoted ends of the axle and the wheels to correspond (Fig. 44).

Fig. 44.—Two Arrangements of the Drag Link.

Fig. 45.—Steering Principles.

For a wheel to follow a curved path without slipping, it must be at all times tangent to the path, and will be perpendicular to a radius of the curve at that point. The front axle of a horse-drawn vehicle points toward the center of the circle on which the vehicle may be turning and forms part of the radius, the wheels, of course, being perpendicular to it (Fig. 45). As the main part of the front axle of an automobile is stationary, only its pivoted ends may point to the center of the circle, and this must occur in order that the wheels may be tangent to the curve (Fig. 45). Both axle ends point to the center, but along different radii; if both pointed along the same radius, it would necessitate their being in line with the stationary part of the axle, which then also would be part of the radius. As the axle ends are in line with different radii of the same curve, it follows that the wheels are perpendicular to different radii, and not parallel with each other, a condition impossible in horse-drawn vehicles. The front wheels of an automobile are parallel with each other when the axle ends are in line with the stationary part, but go out of parallel as soon as they are at an angle with it, as is the case when the car takes a curve.

If the steering arms projected from the knuckles at right angles to the axle ends on which the wheels revolve, moving the drag link would move each knuckle through an equal angle, and the wheels would be parallel at all times; this is prevented by so constructing the knuckles that the steering arms incline toward each other, with the result that when the drag link is moved, one of the wheels swings through a greater angle than the other, the difference between the angle of each steering arm and the stationary part of the axle increasing with a greater movement of the drag link.

The control of the drag link is obtained either by a steering lever or wheel. When the steering lever is moved by the driver, the drag link is moved to correspond by a connecting rod, and this type is usual in small cars. It is impracticable for heavy cars because it is reversible; that is, the moving of the steering wheels moves the lever, and a firm grasp is required to prevent the shock of striking a stone or rut from tearing the lever from the hand and changing the course of the car. The irreversible type is used for all but the lightest cars, and while it permits the driver to change the direction of the front wheels it prevents any movement from being transmitted from the wheels to the steering wheel or lever. The screw-and-nut type of irreversible steering mechanism (Fig. 46, B) consists of a heavy screw attached to the lower end of the shaft that revolves when the driver turns the steering wheel. The screw passes through a nut that is held in guides so that it cannot revolve, and is therefore moved up or down when the screw is turned by the steering wheel. From the nut extends an arm that is connected to the drag link, so that its movement is transmitted to the wheels. The turning of the steering wheel thus moves the nut and the front wheels, but a movement of the front wheels from any other cause is prevented, because no pressure on the nut can revolve the screw. Another type is the worm-and-worm wheel, or worm-and-segment (Fig. 46, A), which, while of different construction, depends on the same principle that the movement of the worm or screw can move a worm wheel or nut in mesh with it, but the movement cannot be reversed.

Fig. 46.—Steering Mechanisms. A, Worm and worm-wheel steering gear; B, nut and screw steering gear.

The stationary part of the front axle of an automobile is usually a forging, and must be of considerable strength in order to support the weight imposed upon it by the engine. It is usually bent down in the center, in order that it may be the lowest point of the mechanism, thus to receive possible blows of stones or other high points of the road that would cause serious damage should they strike the crank case or fly wheel.