Some interesting problems pertaining to the design, construction and operation of the modern commercial car are found in the various steering mechanisms employed. Like almost every other important mechanical features, the steering devices now in general use have resulted from a careful study of the conditions to be fulfilled, supplemented by extensive experiments with different types. These researches have resulted in a general steering system which is applied in different forms to all standard commercial vehicles. This, in brief, consists of a hand-wheel, connected through some form of linkage and gearing leverage to the front wheels of the vehicle, these wheels being carried on pivoted ends of the front axle. The design and construction of some of the most important features of this general arrangement afford interesting subjects for discussion.
Commercial vehicles must necessarily be operated within a limited space such as a narrow street, and it is of importance that the extreme throw, from side to side of the front wheels be settled upon, as the amount of this throw, together with the wheelbase and tread determines the turning radius. In practice the latter two items are established by the load or body requirements, and with these fixed, the throw of the front wheels is usually limited by the width of the body, frame or springs and the permissible distance between the pivot centers about which the wheels swing. The angles which the connecting linkage make bcome too acute or obtuse according to their location, if the maximum throw of the wheels is made more than 35 degrees. If this throw is exceeded, steerage is difficult and unsafe.
The theoretical center about which the vehicle turns is somewhat in the center line of the rear axle prolonged, the exact location being determined by the intersection of this line by a line drawn normal to the inside front wheel. Another line drawn from this intersection to the center of the outside front wheel should be normal to this wheel if it is at the correct angle to prevent excessive side slip, which is very destructive to the vehicle tire. The four wheels will describe concentric circles about this theoretical center. This is clearly illustrated in Fig. 169 and it will be noticed that the minimum turning radius is the radius of the arc described by the outer front wheel when these wheels are in the position of maximum throw. The differential device allows the proper relation of speed between the rear wheels.
The outer front wheel pivot is turned a smaller angle than the inner, and they are connected, by a cross rod, rigidly attached to them. There must be some compensating device interposed to give the same angular relation when the wheels are turned in the opposite direction. This is accomplished by making use of the principle of varying ratios of sines of the angles at different points in the arcs through which the knuckle levers turn. This necessitates locating the pivot or knuckle levers so that their centers diverge if they project ahead of the axle, and converge if they project back of it. As the wheels swing, the knuckle lever mounted on the spindle of the inner wheel travels away from the dead center arc. thus traveling a smaller angular distance. This is illustrated by heavy lines. Fig. 170, which are shown for both front and rear location of the cross-rod.
Fig. 169. Relation of Front and Rear Wheel in Turning.
Fig. 170. Arrangement of Knuckle Livers and Tie Rod for Front and Rear Positions.
There are various methods of theoretically determining the proper angles of these levers, either mathematically or graphically: however, this is largely a matter of compromise between what is theoretically correct and what is attainable practically. Some makers follow the practice of laying out these steering connections so that the center line of the knuckle lever extended will intersect the center of the rear axle. Others endeavor to obtain a knuckle lever length and angle, that while fitting into the general layout of the vehicle, gives the largest possible range of steering angle without undue error. This is generally determined more by experience than by any figures. Another point which must be considered in the general layout, is the various lengths of wheel bases and this suggests making the angle of the knuckle levers as small as possible, consistent with reasonable clearance between the ends of the cross tube and the spokes of the wheels.
Both front and rear positions of these two levers are used and each position possesses its advantages and disadvantages. Some makers prefer the forward location for the reason that it enables large steering angles to be used safely, thus diminishing the turning radius of the vehicle. With the rear position the tie-rod, connecting these two levers, is better protected from injury, providing the proper clearance can be obtained under the engine base. The length of this tie-rod is so adjusted that with the front wheels in the central position the distance apart of the wheel felloes at the height of the spindle is from 3/16 in. to 5/8 in. less in front of the axle than back of it. The amount of toe-in depends upon the diameter of the wheel, the lower figures being used for wheels 34 ins. or less. This toeing-in is intended to allow for slight play in the joints of the tie-rod and the flexure of its members. It is also intended to correct the tendency of the wheels to toe outward when the vehicle is moving.