The front axle with its steering gear, knuckle and arms is largely depended upon for the safe control of the vehicle, while it must also carry the forward portion of the vehicle and load. It must be so arranged as to permit steering the car and in order to accomplish this, the front wheel spindles are pivoted in the axle end and are held in proper relation to each other by a tie rod, which connects levers extending from each pivot. Another lever extends from either right or left-hand pivot (depending upon left or right-side drive) which is connected by a drag link with the steering gear.
This pivot is termed the steering knuckle and has the wheel spindle formed integral, while the levers may either be formed integral, or attached to the knuckle.
There are three general types of steering knuckles, known as the Elliot, Reversed Elliot and Lemoine types. In American practice the Elliot type is most extensively used and the Lemoine least. In the Elliot type the ends of the axle proper are forked and the steering knuckle is T-shaped, while in the reversed Elliot the knuckle is forked and the axle end forms a T. In the Lemoine type both axle end and knuckle form L's. In practice each of these types differ somewhat, depending upon the type of bearings and the method of mounting the knuckle in the axle end.
For some time it was the general impression that when the plane of the front wheel was in line with the plane of the knuckle pivot, the effect of road inequalities would not be transmitted to the steering gear. This contention led to the introduction of a type of knuckle in which the wheel center lies very close to the pivot center.
The knuckle in this type, instead of having a T-shape, includes a sort of a yoke extending outside of the wheel hub to points close to the spokes and the forked axle ends are pivoted to the yokes at these points. However, this is of minor importance, since the speed of commercial cars is comparatively low and with a semi-reversible gear the road shocks are not transmitted to the steering wheel. One prominent maker employed this type of knuckle for a number of years, but has discarded it and is now using the Elliot type.
The levers to which are attached the tie rod used for connecting the two knuckles and the one for connection with the steering gear are known as knuckle arms. These may either be formed integral with the knuckles, or attached by means of a taper and keyway, retained by a castellated nut. One prominent maker forms the spindle and levers separately so that they may be dovetailed together and retained by the pivot pin. The general practice is to attach them to the knuckles since this simplifies manufacturing and replacement.
Owing to their importance, the knuckles and arms are always forged from a good quality of steel and heat treated. The tie rod may either be placed in front or in the rear of the axle, while the steering connection may either be arranged for cross or fore and aft steering. The arrangement of tie rod and steering connections depend upon the general construction of the vehicle and the location of the steering gear.
The axle proper may either be forged from medium carbon steel of solid rectangular section or of I-beam section, approaching a full rectangular section. Cast steel axles are also used, while one maker of a popular priced vehicle uses cast steel ends with a round section center. These axles may also be built up with tubular centers, flat plates riveted together, or from pressed steel of channel section.
In conventional designs the only connection between the axle and the frame is through the front springs, which with few exceptions are of the semi-elliptic type. One maker uses a full elliptic front spring and provides a distance rod to hold the axle in alignment with the frame.
Fig. 158 illustrates a front axle with cast steel center for light delivery cars of 750 to 1,000 lbs. capacity. The center is dropped considerably, that is, the topmost surface of the axle bed is located considerably below the center of the wheel spindle, since it is intended for use with full elliptic front springs and pneumatic tires. The knuckle is of the Elliot type and drop-forged with integral spindle and has a boss at its lower end which is provided with taper and keyway for attaching the knuckle arm. The tie rod is placed to the rear of the axle center and is attached to the knuckle arms by a clevis and bolt. The knuckles are arranged for cross steering and one clevis bolt has an extension which carries a cross to which the drag link is attached. This cross serves as a universal coupling to compensate for the angular positions of the knuckle arms and the variation in the vertical movement between axle and frame. This is necessary as the steering gear is always attached to the frame and the action of the springs tend to vary the distance between the frame and the axle. The hubs are malleable castings with flanges, which hold the spokes of the wheel.
Cup and cone ball bearings are used for mounting the hubs on the spindle. Bearing adjustment is by means of a nut on the spindle and spacing washers.
Fig. 158. Light Truck Axle with Cast Center.
Fig. 159. Vulcan 5-Ton Front Axle.