All-steel automobile bodies are lighter, stronger, roomier, and cheaper than composite bodies having wood framing and metal panels. They are free from squeaks, afford better vision of the road and scenery, take a superior finish with less preliminary work, and permit marked economies in quantity production. Steel has 40 times the strength to resist breakage that wood has and, in bending, may be stressed 7 times as much as wood, hence the cross-sectional area of steel members may be only a small fraction of that of wood members having equal strength. This makes for lightness of construction and reduction of the size of frame members, thereby affording more space in the interior of the body for the passengers and reducing the amount of obstruction to vision.
Whereas joints between sills and posts in a wood-frame body are weakened by the cutting away of a large part of the wood and consequently require reinforcing with irons and the use of glue and screws, corresponding joints in the pressed-steel body are strengthened by flanges and riveting and the formation of box section. Such joints are not loosened by shrinkage and vibration and do not become noisy. Steel members lend themselves readily to riveting and welding, which methods of fastening may produce the strength of continuous or integral metal. This is important because space limitations often prevent proper use of screws, braces and stiffeners in wood bodies so that they are sometimes weakest in the planes of greatest stresses.
In steel construction, where the formation is hollow, as in the doors, hardware such as locks has been designed which does not require the removal of any metal at points where the members are in tension or compression. Side panels and the rear panel of closed bodies are formed in one piece from floor to roof, including the formation of the rear window, thus avoiding the expensive and objectionable horizontal joint at the belt line and making it possible to assemble the entire side structure of the body in one piece for shipment as a unit. The panels may be joined together along their vertical lines by in-turned flanges, leaving an open joint, or the panels can be welded so as to avoid the open seam.
A factor of great economy in original manufacture has been the design of removable upholstery, which is also a great advantage in the case of repairs. The development of methods of working sheet metal by which stampings can be produced that are correct as to contour and free from surface defects has made it possible for finishing to be done without filling or rubbing and without the use of many preliminary coats of paint. It is the practice of some car builders to finish the bodies with three coats of japan, without any filling and rubbing, and to bake the japan on at a high temperature. The steel body is also especially adaptable to the use of lacquers for getting hard finish with various colors.