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the turning action of the axle when the wheels are turning to drive the car or when the brakes are applied. A typical rear end of an automobile frame is shown at Fig. 20.
Rear Axles and Brakes.—Rear axles have been made in many forms, but the live axle is a type generally used on modern automobiles. This is called a live axle because the non-rotatable axle housing contains moving parts, as will be seen by reference to Fig. 17. This axle is termed a one bearing full floating con
struction because only one bearing is used in each rear wheel. The power of the engine goes through a clutch and from there to change speed gearing as previously explained. From the gear box the drive is by a propeller shaft, which in the axle illustrated is housed in a torque tube and carries at its lower end a beveled pinion which meshes with a large bevel or ring .gear attached to the differential mechanism. From the differential, axle shafts extend to each rear wheel and it is through these members that the engine power is transmitted to the wheel hubs. In order to eliminate friction as much as possible, all rotating shafts are carried by ball bearings because these members carry heavy loads with very little loss of power.
The differential mechanism which is illustrated at Fig. 18 is a very important part of a live rear axle. Its function is to permit one wheel to turn faster or slower than the other when rounding curves. It is evident that as the car turns a corner
Fig. 20.-Rear Portion of Typical Automobile Chassis Frame, Show
ing Location of Rear Axle and Cantilever Spring Suspension.
the inner wheel is describing a smaller circle than the outer one and therefore should not turn as fast. If the two wheels were joined rigidly together by a solid or one-piece axle member they would have to turn at the same speed. Whenever a corner was turned, the inner wheel would have to slip considerably which would cause tire wear. The differential mechanism is an assembly of bevel gears on most cars and is placed at the center of the axle and between the two rear wheels. Independent wheel driving shafts are used, each one going to an independent differ
ential gear, though these are meshed with small bevel pinions which cause them to turn in the proper relation. When the vehicle is on a straight path and the resistance to the movement of each wheel is approximately the same, the differential mechanism revolves as a unit, no relative movement of the internal gearing takes place and both wheels are driven at the same speed. As soon as the resistance to wheel rotation becomes unequal the wheel that has the greatest resistance tends to remain stationary, the wheel that has the least resistance revolves.
Another important part of the car control system that is usually incorporated in the rear construction are the brakes. These act on drums attached to the rear wheels and usually two distinct braking effects are used on each brake drum. The brake assembly shown at the top of Fig. 18 is typical of standard practice in this regard. The external brake is a steel band lined with heat resisting asbestos fabric. It closes around the brake drum to retard its motion. The internal brake consists of a band also faced with frictional material which is expanded or spread out by toggle leverage so that it exerts a retarding influence against the inside of the drum. The external brakes are usually operated by a foot pedal placed close to the clutch pedal, while the internal brakes are actuated by rods connected to the hand lever. Adjustment means are provided to compensate for wear as the brakes are used.
How Automobiles Are Steered.—The problem of steering the motor car is a somewhat different one than that of directing a horse-drawn vehicle, because in the animal-drawn conveyance the shafts which are attached to the front axle are used to turn the vehicle as well as to pull it along. The front axle is usually pivoted at a central point and turns on a fifth wheel arrangement. When it is desired to turn in either direction the animal is guided by the reins and the axle is turned at an angle to the body sufficient to allow the vehicle to describe a curve. In most motor vehicles the propulsive force is applied to the rear wheels and the structure is pushed from behind instead of being pulled, as is the case with a horse-drawn conveyance. Obviously, it would not be practical to turn the entire axle under the car, because if it described a too acute angle when the car was driven at high speed it would be extremely difficult to control the vehicle.
This was very ingeniously overcome by an engineer named Ackerman, who devised the pivoted axle which is commonly accepted as the proper method of steering automobiles. This consists of a fixed axle member, as shown at Fig. 19, attached to a frame by suitable springs or other means in such a way that it can move only in a vertical direction under the influence of road irregularities. The wheels are mounted on spindles carried in a yoke at each end of the axle, and when it is desired to turn an automobile only the wheels are turned instead of moving the entire axle assembly as is the case in a hôrse-drawn vehicle. In order to actuate the steering knuckles, suitable mechanism that will be easily operated must be placed convenient to the driver. The earlier forms of automobiles were provided with forms of tillers very similar to those employed in controlling boats, but while these simple levers gave a certain degree of satisfaction on light cars operated at slow speeds, the development of the higher speed vehicles made necessary more easily handled and positive forms of steering gears. The disadvantages of the tiller are that it may be whipped out of the operator's hands by road irregularities, and it was very tiresome to hold because of the continual vibration.
With the modern forms of wheel-steering devices the hands are always in an easy position, the wheels may be readily operated, and because of the elimination of vibration by the feature of irreversibility provided by most steering gears of conventional construction, no road shock can loosen the grip of the driver, nor is he fatigued by continued movement of the wheel. Steering gears are made in a variety of forms and all types have their adherents. The accepted construction is clearly illustrated. In this the steering wheel is attached to a rod which carries a worm at its lower end. This worm meshes with a worm gear to which a steering arm is attached, and a rotary movement of the hand wheel will produce a reciprocating movement of the