The spindle upon which the wheel revolves is generally inclined from 1 1/2 to 2 degrees below the horizontal center, while the king bolt about which the wheel pivots is brought as close to the spokes as possible, in order to bring the point of intersection of the center of the wheel with the ground as close to the center of the bolt produced as possible. This distance forms the lever arm at the end of which the resistance to motion of the front wheels acts when the wheels swing around for steering.
To approximate castor steering, some manufacturers also incline the axles and king bolt in a fore and aft plane, as in Fig. 171, the inclination being about four degrees on the average. The reasons for these features are that they make steering easier, in that the construction produces a trailer effect which tends to obviate serious consequences in the event of breakage or disconnection of the steering linkage. The point of wheel contact with the ground is behind the point at which the center of the king bolt produced meets the ground, hence the steering wheels trail and are automatically kept in the straight ahead position by the road resistance. This trailer effect somewhat reduces wobbling of the front wheels and also reduces the shock on the steering gear.
To prevent road shocks from being transmitted to the operator's arms, it is considered best to have the steering gear back-locking, or irreversible to some extent at least. With a perfectly reversible system it is evident road shocks, which are transmitted to the operator's bands, depend on their magnitude and the lever arm through which they act. This system is best adapted to show moving vehicles running over smooth pavements, such as the light electric vehicles in common use.
Between the limits of reversible and irreversible steering gears, is the semi-reversible type, which allows the vehicle wheels to be turned independently of any effort exerted on the steering wheel, yet exerting an even resistance to movement. This feature allows the road wheels to follow the path of least resistance and at the same time indicates to the operator the extent and direction of the movement by more or less swerving of the steering wheel depending upon the gear ratio. The semi-reversible system also relieves the parts of considerable strain which would be present if the vehicle wheels were rigidly held to their position. A disadvantage of the semi-reversible feature is in steering through sandy or muddy roads and in crossing obstructions such as car tracks obliquely.
Heavy service seems to offer a logical field for the irreversible gear as the connection may be made heavy and strong. Considerable manipulation of the steering wheel is usually necessary on these vehicles, which tends to make this type a favorite by relieving the operator of jerks and considerable muscular exertion on the steering wheel. It also permits a very low gear ratio or large hand wheel motion which is quite desirable from the leverage standpoint in operating a heavy vehicle when at a standstill or moving very slowly.
Fig. 171. Trailer Effect when Castor Steering is Approximated by Including King Bolt.
Because a commercial vehicle is heavier and slower than a pleasure car, it necessarily has a different kind of steering gear. Theoretically the layout would be the same for both machines, if they had the same wheelbase, but practically it is necessary to have a greater reduction in the commercial vehicle because it is heavier and naturally takes a greater leverage to turn the wheels; and also, since this vehicle acts at a slower rate of speed, the reduction can again be greater because it is not so necessary to be able to turn the wheels from one side to another quickly.
Owing to the great inertia of a moving loaded commercial vehicle, it is not desirable to make quick turns with the front wheels on account of the tremendous stresses involved by the inertia force and the high center of gravity.
The term, irreversible, in itself, is confusing because it has no exact meaning when applied to a steering gear, beyond the rather indefinite condition, that it means that any ordinary road wheel impact will be insufficient to turn the steering wheel. It is simply a question of reduction between the worm and gear or screw and nut, whichever system is used. The greater the reduction the less reversible the system and likewise the slower the motion of the road wheels in relation to the movement of the steering wheel. Hence, the steering mechanism for a heavy vehicle will be less reversible than the steering mechanism for a lighter vehicle.
The tie rod connects the steering knuckle levers on opposite sides and is usually of tubular section. When placed in front of the axle it ordinarily works under tension, while behind the axle it works under compression. In the forward position the road resistance encountered by the front wheels, acting through the steering knuckles and arms, puts a tension on the rod and in the rear position a compression. The force impressed by the operator in steering the vehicle produces a tension for one direction of motion and a compression for the other, with both constructions.
The ends of the knuckle levers swing in the same plane and the tie rod must be connected with forked connectors. These connectors are generally made adjustable, so that the wheels may be kept properly lined up even if the rod or steering levers become slightly bent. The adjustable connectors must be securely clamped and safeguarded against working loose.