The rigid frame, too, has advantages, whether it is of pressed steel or rolled stock. It permits the body to be secured rigidly to it and as it does not give to the inequalities of the road, there is no racking of the body. An advantage of rolled stock is its cheapness, except of course in the lighter models of the assembled type for which frames can be purchased at low figures. Another advantage of rolled stock is the ease with which the wheelbase can be altered. The maker using this type of frame may with little additional expense give customers a shorter or longer frame than standard.
It is possible to alter the pressed-steel frame in length by cutting off from the maximum length, although this disturbs the nice proportioning of the frame for stresses, one of the important advantages of this type.
The effect of frame construction upon the design and duty of the springs must also be considered. This feature is not generally understood, but has an important bearing upon the life of the vehicle. A rigid frame relies upon the springs to allow for all axle displacement. If the front and rear wheels on opposite sides be raised several inches simultaneously, the frame is subjected to a torsional stress. If the frame is rigid, springs of considerable camber must be employed in order to absorb the shock without being bent past the limit of safety and sufficiently flexible to absorb all of the shock without any tendency to lift the other wheels from the ground. For this reason a different type of spring is used on a rigid chassis from that used on flexible ones.
The flexible frame when diagonally opposite wheels are raised does not impose all of the duty on the springs, but warps and absorbs a part of the stress. For this reason springs on flexible chassis are usually flat or nearly so, with a reduced amount of play. Flexible construction also permits the frame to be carried equally as low as with the underslung spring, and yet the spring is perched above the axle, where it is more nearly in line with the center of gravity, thus reducing sidesway.
Details of construction such as spring hangers, etc., vary considerably, as can be noted from the descriptions of the various types, which follow:
Fig. 184. Vim Delivery Car Frame.
The Vim 1/2-ton frame (Fig. 184) illustrates a construction of pressed steel, with straight side rails and cast spring hangers which are riveted to the former. Owing to its shortness on account of the small size of the vehicle this frame is unusually strong.
The Reo 3/4-ton frame (Fig. 185) also employs straight-side rails; however, these are tapered and bent at the front end to receive the spring hanger. This illustration shows in detail all parts which are riveted to the frame and also a rigid sub-frame construction, which is set at an angle to provide as near as possible a straight-line drive to the rear axle. All cross members are provided with integral gussets, while pressed-steel parts such as step hanger, body brackets, etc., are used to keep the weight within reasonable limits for this size of vehicle. One-ton frames are built along similar lines.
Fig. 186 depicts the Fremont Mais 1-1/2- to 2-ton frame, with straight-side members, bent at the front end to receive the spring brackets. In this construction, the scheme is to eliminate unnecessary cross members, so that the frame forms a flexible construction; however, it presents an excellent method of providing strength at the point where the drive, which is taken by the springs, is transmitted to the frame. This is accomplished by placing two cross members together to form an I-beam structure.
Fig. 187 shows the flexible frame construction, which is characteristic of all Pierce worm-drive trucks. The side members are pressed steel and taper at both ends, the front being bent to form the spring hanger, while the bumper is also attached at this point. But two cross members are used, as such parts as the spring brackets and torsion rod support are used for this purpose, while the rear member is of tubular section. A brace of cross shape serves to form a flexible support at the point of drive. In this construction the drive is taken through radius rods attached to a bracket which also forms the spring bracket and carries the tubular member from which the torque arm is supported.
Fig. 185. Reo. Pressed Steel Frame for 1-Ton Truck.
Fig. 186. Fremont Mais Pressed Steel Frame.
The Locomobile trucks are also also worm-driven and are equipped with radius rods and torque arm to take the torque and driving thrust; however, the frame (Fig. 188) is made of rigid construction. It is made of pressed steel with side members tapered at both ends and cast spring brackets riveted to the side rails. The forward cross member forms a bumper, while the remaining members support the transmission and service brake and form braces at the points of spring anchorage to the side rails. The extreme rigidity of this construction can be noted by the numbers of cross members and the method of reinforcing the rail with an etxra channel insert.
Fig. 189 depicts the De Kalb 4-ton frame, which is also of pressed steel with tapered side members. This frame is of conventional design with the exception of the side rails, which are inswept along side the rear springs much the same as the conventional side member is narrowed from the dash forward to reduce the turning radius. Through this feature lower body carriage is obtained than would be possible otherwise. It also provides ample clearance for the radius rods, chains and springs. The flange width is increased at the point of offset to provide proper strength.
Fig. 187. Pierce-Arrow Pressed Steel Frame Flexible Construction.
Fig. 188. Locomobile Reinforced Pressed Stell Frame.
Fig. 189. DeKalb Pressed Steel Frame Inswept at Rear.
Fig. 190. U. S. Structural Channel Frame.