The chassis frame practically forms the foundation of a commercial car, since all the power-transmitting and other units are attached to it. It is often referred to as the backbone of a commercial car. Its construction depends to some extent upon the general scheme of the chassis layout, the construction of power-transmitting units and their mounting, as well as the method of final drive, wheel base, etc.

When the commercial car was first introduced, comparatively little attention was paid to the frame, as other things such as the power plant, axles, etc., were deemed of greater importance, hence the frame received slight consideration. However, after experiencing considerable difficulty, due to accidents and other failures which were traced directly to poor frame construction, commercial car builders discovered that frames could be designed with greater strength and with less weight if the problem was given proper consideration.

Unit Power Plants And Flexible Mounting

The constant trend of obtaining perfect alignment for the engine, clutch and transmission has resulted in the adoption of the unit power plant on some models, while in others, particularly of the heavier type, flexible mounting of the units has been resorted to. In fact, regardless of unit construction, all individual units are generally flexibly mounted to some degree, in order to relieve them of the heavy stresses due to frame weaving when the road wheels mount an obstacle on the road surface. However, since this subject of power plant mountings is of considerable importance, this will be discussed in detail in the succeeding chapter, the author confining this chapter to the construction of the frame. In discussing this subject, it may be necessary in some cases to refer to the general chassis construction in order to clearly depict each type.

The most prominent types of truck frames may be divided into three classes, according to their popularity: (1) The pressed steel frame, (2) the structural channel frame, (3) the structural I-beam frame. These may again be divided into various classes, depending upon the general construction and material as well as the distribution of the main units.

The Pressed-Steel Frame

The pressed-steel frame is quite popular on all types of vehicles and is now universally used on vehicles up to and including those of 5-ton capacity. Structural channel and I-beam frames are still used by a number of makers; however, the pressed steel frame is rapidly gaining favor and from present indications will eventually replace the other types.

In discussing the advantages and disadvantages of the various types, the pressed steel frame may be mentioned as being lightest in weight for equal strength of the structural or rolled channel and I-beam section, while its cost is somewhat higher, due to the use of heat treated material to obtain maximum strength. The cost varies with the section, material and the nature and extent of bending.

The straight side rail is of course the cheapest construction; however, when conditions permit, this is usually tapered at the front and the rear and the forward end is sometimes formed to receive the spring hanger. When the seat is placed above the engine this taper is usually very short, permitting the paying load to be carried well to the front. Bumpers are sometimes provided to protect the chassis, these may either be formed integral with the frame or attached to it by castings.

When the side members are inswept to permit a short turning radius, it is necessary to make the flanges of the side rail of considerable width at this point, tapering gradually toward the rear, to provide the proper strength at the point of offset.

Cross Members

Cross members are usually made of the same material as the side rails and when pressed have integral gussets, this, of course, is not possible with the rolled sections so that the separate gusset plates must be used, thus placing the strain on the rivets, instead of on the cross member.

These frames of either type are used in both flexible and rigid constructions. While both kinds of material are subject to heat treatment, it is generally conceded that pressed steel is a higher-grade metal than rolled stock. Owing to its temper it will stand a certain amount of bending which would give rolled stock a permanent set or crack it. It is alleged that pressed steel is more sensible to vibration, in that it will crystallize sooner than rolled steel under similar conditions. Instead of being built up rigid, as are rolled-steel frames, the pressed-steel frame may be built up flexible, so that instead of taking the vibration dead, as well as sudden shocks, it gives to them and transmits them to other parts, so that the individual vibrations in any part are reduced by distribution. The pressed steed flexible frame may also be made lighter for its strength because of its flexibility.