Commercial car bodies are mounted upon the chassis frame, the latter being supported on the axles through the intermediary of steel springs. These springs are built up of a number of plates varying in length and are used exclusively to support the body, although coil springs are used as auxiliaries.
The upper leaf of this spring usually has an eye at each end for connection to spring brackets on the frame, or shackles. In some few cases the ends are left flat and fit in brackets so that the frame rests directly on them. The balance of the spring consists of a number of shorter leaves, the lengths of which decrease uniformly, except in cases when they are required to carry very heavy loads, in which the first two or three leaves are of the same length. The various leaves are held together by a center bolt or a band.
The method of frame connection depends upon the type of spring and various other factors, while the axle connection is usually made by box clips and a spring seat on the axle. This seat is sometimes called a perch, and may be formed integral, or attached to the axle.
The simplest type of spring is the semi- or half-elliptic type, while all other types are made up wholly or in part of the former. They may be termed combinations of the semi-elliptic type.
The three-quarter-elliptic type consists of a semi-elliptic lower member and a quarter-elliptic top member. These two members are joined by shackles and bolts at one end. This type of spring is used on pneumatic tired vehicles only at present.
The full-elliptic spring consists of two semi-elliptic members joined at both ends by bolts or shackles and bolts.
The three-quarter platform spring consists of two semi-elliptic side members and one semi-elliptic cross member, the side mem-bers being joined to the cross members by shackles and bolts. This type is ordinarily termed the platform spring, since the true full platform spring consisting of two side and two cross semielliptic members is not adaptable to the ordinary chassis construction.
Auxiliary springs consist of a half-elliptic with plain ends. The cantilever spring carries the weight at its small end and may either be quarter-elliptic; in which the big end is secured to the frame or a semi-elliptic; in which case it has a pivot support on the frame at or near its center, and is connected to the rear axle at its rear end.
There are also various other combinations; however, they are not employed at present, and consequently are not within the scope of this work.
Regardless of capacity, the semi-elliptic suspension is a decided favorite. It is simple, and if the length, width and other dimensions are proportioned correctly, nothing better than the semi-elliptic spring for front and rear suspension could be desired.
The remaining spring suspensions employed at present may be classified as follows: Semi-elliptic front, full-elliptic rear; semi-elliptic front and three-quarter-elliptic rear; full-elliptic front and rear; semi-elliptic front and three-quarter platform rear; full-elliptic front and platform rear.
Another point worthy of note is the substitution of the true sweep spring and the elimination of the double sweep spring. Having a simple curve, the true sweep spring is easy to fit, and spring makers recommend them on this account. The double sweep spring is simple to mount and has a legitimate place on every truck as an auxiliary or overload spring. Comparison of these two types may be made by referring to Figs. 206 and 207, the former being a true sweep and the latter a double sweep spring.
Until recently, very few springs were equipped with bumpers; however, in most cases these are in the form of coil springs, and on some vehicles they are made of a heavy square section.
The general construction of the various types can readily be understood by referring to accompanying illustrations; however, the method of frame suspension and axle mounting warrants discussion.
Fig. 206 illustrates the De Kalb rear spring, which is of the semi-elliptic true sweep type. These are placed outside of the frame to permit carrying the frame low, and the main leaf has an eye at each end which is connected to the frame by means of shackles and bolts through brackets riveted to the frame. All bearing surfaces are provided with removable bushings and grease cups. The leaves are held together by means of a steel outer band which is shrunk over them. The spring is attached to the axle by means of a spring seat which is mounted on the axle spindle and prevented from tinning by a set screw. Box clips of square sections, placed at an angle are used to hold the spring to its seat. Two nuts are used to hold the spring rigid, while the upper ends of the clips ore held in position by a pressure block on the top of the spring which fits snugly over the center band.
The front spring is of similar construction; however, the front end of this is attached directly to the front bracket, while the rear end is shackled to its bracket. The necessity of directly connecting the forward end of a front spring to frame is due to the fact that this is the only connection between the frame and the axle, the spring being utilized to hold the front axle in position. This is also true of the forward end of a rear spring when the torque and driving thrust is taken through the spring. This feature was explained in a previous chapter on the final drive.
Fig. 206. DeKalb Spring Mounted Outside of Frame and Over Axle.
Fig. 207. Mogul Six-Ton Hear Spring with Plain Ends Showing Method of Mounting on Axle.