The drag link may be placed either in a fore-and-aft position or crosswise of the vehicle. The fore-and-aft position is more generally used when the engine is under a hood, while with the seat over the engine it is difficult to place the gear in such a position as to permit placing the drag link parallel with the frame. The gear usually is so close to the front axle that the drag link must be placed crosswise.
Cushion springs are usually put in the joints of this member to assist in absorbing abrupt shocks which might be transmitted in either direction. These members sometimes are made adjustable for length, while adjustments are also provided to take up the tension on the springs.
An important point in the steering gear layout is a desirability of a proper geometrical layout for the drag link to avoid front wheel wobble under front spring deflection when the vehicle is in motion. In other words, it is very important to have the drag link so arranged as to produce the least tendency to rotate the steering gear arm as a result of the action of the front springs. When this member is placed crosswise it should be in nearly the same plane as the tie rod under normal load, but with the fore-and-aft position conditions are entirely different. With this arrangement the drag link may be either forward of or behind the front axle. In the latter position, which is the more popular, this link should be so placed that when the truck is loaded, a straight line drawn through the eye of the front spring will approximately intersect both front and rear ball-joint centers of the drag link, as in Fig. 172. A slight deviation from this intersection will not materially affect the results, depending upon the characteristics of the front spring, but the centers must fall approximately on this line to obtain the proper front wheel action under spring deflections, particularly when such spring deflection is at all excessive. To give as nearly as possible true steering under extreme conditions, it is well to make the front spring as flat as possible, to prevent any great extent of forward or backward movement of the axle.
On the Mack model "A.C." trucks (Fig. 173) the steering gear and drag link are ahead of the front axle. This member is slightly out of parallel with the frame, when the mechanism is in midpo-sition, but swings into position more nearly parallel when the road wheels are turned. The front axle, in its movements, due to road inequalities, swings through an approximate arc about a point P. The ball on the steering knuckle-arm is as close to this point as conditions permit. The drag link, extending backward, swings about a center L. Thus, both the axle itself and the ball on the steering knuckle-arm swing through approximately concentric arcs and no backward or forward motion is imparted to the steering gear arm due to spring deflection.
On the Manly trucks a similar arrangement is used; however, this is just the reverse to that of the Mack trucks, in that, the front end of the spring is shackled and the rear end rigidly attached to the frame. By this method the end of the drag link, which is pivoted on the steering gear arm, is brought very nearly in line with the pivoted end of the spring so that the axle and the forward end of the drag link connected with the axle is allowed to travel in practically the same curved path.
Fig. 172. Steering Gear Back of Axle and Drag Link Parallel with Frame.
Fig. 173. Mack Model "AC" Steering Gear Arrangement.
Fig. 174. Manly Front Spring Mounting.
Horse-drawn vehicles are ordinarily steered by means of a fifth-wheel attached to the forward unit of the vehicle gear, which pivots on what is known as the king bolt. However, the divided axle is universally employed on commercial cars. This was described in the preceding chapter on "Front Axles," and the arrangement of pivoting the wheels is known in the country as the Ackerman Steering Gear.
Technically, this has been revised and at present it is based upon the principle that if the vehicle is to turn a corner without lateral slip of any of the wheels, the steering linkage must be so arranged that the axles of all wheels produced always intersect a common vertical line, this vertical line forming a momentary axis of rotation. This accounts for the use of inclined knuckle arms instead of parallel arms. When they extend toward the rear they must incline toward each other and away from each other when they extend forward of the axle. The inclination is such that the center lines of the arms produced meet at the point near the center of the rear axle.