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.
BRAKES
The brakes used in controlling the speed of an automobile may be as many as four in number, and there should be at least two, for on them depends the safety of the car. Brakes are of two types, expanding and contracting, and usually operate through the friction between a drum and a band that surrounds it, or blocks that press against its inner surface. The band or contracting brake may be either single- or double-acting, the latter being by far the better. In a single-acting band brake (Fig. 47) a flexible steel band surrounds the drum, one end being made fast to the frame of the car or some other stationary part, and pressure applied by drawing the free end. The friction caused by the binding of the leather or fiber lining of the band on the drum restrains the movement if the drum is revolving in the opposite direction to the pull, but there is little effect if the revolution is in the same direction as the pull. In the double-acting type, both ends of the band are pulled, and the drum is prevented from revolving in either direction. The single-acting type would be satisfactory when the car moves forward, but will not hold it from running downhill backward, while the double-acting brake holds it in either direction. The expanding brake usually consists of two bronze shoes, of such shape that they fit the interior surface of the drum. The shoes are pivoted together at one end, and so arranged that the pull of the brake pedal or lever expands them, binding them against the drum (Fig. 47). When pressure is not being exerted, a coil spring, not shown in the diagram, holds them together and out of contact with the drum.
Fig. 47.—Three Varieties of Brakes.
Brake drums are usually attached to the spokes of the rear wheels, and one drum often serves for both an expanding and a contracting brake. Brake drums are also applied to the jack shaft, or to an extension of the countershaft of the change-speed mechanism. It is usual to have one set of brakes controlled by a foot pedal, and another, called the emergency brake, by a lever at the side of the car. The foot brake, or running brake, is always connected to the clutch, so that applying it throws out the clutch. The emergency brake is also connected in the same manner in some makes of automobiles, but this is not recommended, for if it is necessary to stop the car when going uphill, the brakes must be released before the clutch can be thrown in, and the possibility of the car starting downhill backward before power can be applied, the chance of stalling the engine through this, and the danger of the combination to any but an experienced driver, make it advisable to have the emergency brake separate from any connection with the clutch.
Band brakes are usually lined with leather, to increase the friction between the band and the drum, and this often gives rise to troubles in the burning of the leather when the brake is applied for a considerable period, as in the descent of a long hill. The emergency brake has advantages in that it operates through the friction of metal against metal, but excessive heat from continued application may be enough to melt the metal and fuse together the shoe and drum. For long descents, it is well to use the motor as a brake, for it is logical to consider that the means of propulsion may also be the means of retarding, as the wind that urges a sailboat forward may also bring it to a stop. It is obviously impossible to reverse the motor or gears, but by switching off the ignition circuit and throttling down, the forward movement of the car is caused to operate the motor, and the work necessary in driving the motor as an air compressor is sufficient to check the speed. The effect is so great that if the low-speed gears are engaged, the car will be brought to a stop even on a steep hill. Another advantage of this course is that it gives the motor an opportunity to cool, which is often necessary after a long ascent.