The more usual troubles which the repair man will encounter are sagging in the middle; fracture in the middle at some heavily loaded point or at some unusually large hole or series of holes; twisting or other distortion due to accidents; bending, or fracture of a sub-frame or cross member; bending or fracture at a point where the frame is turned sharply inward, outward, upward, or downward.
Fig. 87. Final Drive on Light Truck.
A frame sags in the middle for one of two reasons: either the original frame was not strong enough to sustain the load, or the frame was strong enough normally, but an abnormal load was carried which broke it down. Sometimes a frame which was large enough originally, and which has not been overloaded, will fail through crystallization, or in more common terms, fatigue of the steel. This occurs so seldom, and then only on very old frames, that it cannot be classed as a "usual" trouble; moreover, it cannot be fixed.
When a frame sags in the middle, the amount of the sag determines the method of repair. For a moderate sag, say ¼ to ½ inch, a good plan is to add truss rods, one on either side. These should be stout bars, well anchored near the ends of the frame and at points where the frame has not been weakened by excessive drilling. They should be given a flattened U-shape, with a couple (or more) uprights down from the frame between them. The material for them should be stiff enough and strong enough to withstand bending, and should be firmly fastened to the underside of the frame. The truss rods should be made in two parts with a turnbuckle to unite them, the ends being threaded right and left to receive the turnbuckle. When truss rods are put on a sagged frame, it should be turned over and loaded on the under side; then the turnbuckles should be pulled up so as to force the middle or sagged part upward a fraction of an inch - say 1/8 to ¼ inch - and then the frame turned back, the other parts added, and the whole returned to use. A job of this kind which takes out the sag so that it does not recur is a job to be proud of.
Many a frame breaks because too much metal was drilled out at one place. Fig. 88 shows a case of this kind. The two holes were drilled one above the other for the attachment of some part, and were made too large. They were so large that at this particular point there was not enough metal left to carry the load, and the frame broke, as indicated, between the two holes and also above and below. A break of this kind can be repaired in two good ways. The first and simplest - as well as the least expensive - is to take a piece of frame 10 to 12 inches long of sufficiently small section to fit tightly inside this one. Drive it into the inside of the main frame at the break, rivet it in place firmly throughout its length, and then drill the desired holes through both thicknesses of metal.
This is not so good as welding. A break of this kind can be taken to a good autogenous welder who will widen out and clean the crack, fill it full of new metal, fuse that into intimate contact with the surrounding metal, and do so neat and clean a piece of work that one would never know it had been broken. When a welding job is done on a break like this, and no metal added besides that needed to fill the crack, subsequent drilling should be at an angle to avoid a repetition of the overloading condition. In the figure, the dotted lines suggest the drilling. By staggering the holes in this way, there is a greater amount of metal to resist breakage than would be the case with one hole above the other - a method which might preferably have been used in the first place.
So much welding is done now, and so many people know of its advantages, that every repair shop of any size should have a welding outfit. A frame job is essentially an inside bench job, but a large number of cases of welding could be done directly on the car outside the building, particularly in summer when the outside air and cooling breezes are desirable. So, it is well to construct a small truck on which to keep the oxygen tank, acetylene cylinder, nozzle for working, and a fire extinguisher. One form of a truck is shown in Fig. 89, this being a simple rectangular platform with casters, a handle, and a rack to hold the tanks. It saves many a step and is particularly convenient in summer months. This outfit is essentially a home-made affair, but the gas-welding and electric-welding manufacturing companies have designed small outfits especially for automobile repair work, which would be preferable to Fig. 89, especially where the amount of repair work warrants a reasonable expenditure for a welding outfit.
Fig. 88. Reboring Cracked Steel Channel.