[Ian]

Here are some interesting perspectives and a nice summary of the state of leaf springs in the early 1900s. It is apparent that the materials from which springs were being made was in a transition from plain carbon steel to alloys that had better properties suited to the greater performance demands of automobiles. At the time of publication (1912) the formulation of these new alloys was still being worked out. Although this book does not use the Society of Automobile Engineers' (SAE) terminology, it is clear that the usual steel used for leaf springs had been SAE 10XX stock, whereas some of the newer alloys included, among others, SAE 51XX materials. The advantages of these new alloys is clearly spelled out.

... Steels contain various percentages of carbon, depending on the purposes for which they are intended. When used for springs approximately one per cent of carbon has been found best. In the early eighties [i.e., 1880s] the Pennsylvania Railroad Company made long and costly investigations into the merits of the various carbon steels then on the market. These investigations were conducted by their chief chemist, the late Dr. Charles P. Dudley. It was found that steels having carbon ranging from .95 to 1.10%, or practically one per cent, were the most efficient from all points of view. The spring steel specification then prepared has since become standard for vehicle springs also. It is universally recommended in all branches of the spring industry, having withstood the hardest service. As a simple carbon steel it has so far had no rival, and may be looked upon as the acme in that class of material. It is the only material used in Dragon Brand springs.

The advent of the motor car forced springs into a service more severe and exacting than any they had been called upon during the past. The speed and weight of the new vehicle produced shocks and deflections unknown before. There has consequently arisen in recent years a need for a steel even better than the carbon stock described above.

It has been found that a carbon steel can be greatly improved by the addition of very small percentages of the heretofore less used elements, such as silicon, manganese, chromium, nickel, vanadium, tungsten, etc. Steels containing these elements singly or in various combinations, in addition to carbon, are called "Alloy" steels.

It is not our purpose here to extol the virtues of any one alloy steel. All have their inherent advantages and purposes for which they are admirably suited, while many undoubtedly have bad points. New elements and combinations are constantly appearing. In the course of time, by the natural processes of selection, the best alloy will survive, and, as in the case of the carbon steels, will be looked upon as standard.

It is often supposed that an alloy steel will, in itself, improve the riding qualities of a spring. It is imagined, for instance, that to replace a poor riding carbon spring by an alloy spring of the same dimensions would result in a marked betterment of the riding qualities. This is an error which we most emphatically contradict. The new spring will ride exactly the same as the old one. It will, however, possess one vast advantage in that its "life" will have been remarkably lengthened. The alloy is a hardier material, better able than the plain carbon spring to resist repeated deflection. In everyday language, the spring "will last longer." This is the only superiority which can be claimed for an alloy steel legitimately. The increased cost of the better material is returned in greater endurance and greater resistance to fatigue. …

It is a matter of everyday experience that if we double the load upon a spring its deflection will be doubled. In a simple spring the deflection varies directly as the load. That relation will, however, not hold true indefinitely, for when a load is increased beyond a certain point the steel is injured. To make this clear examine Figure1f again and imagine it to be a spring of one leaf. Suppose we increase the load on the bar by increments of 50 pounds, releasing it to its free height after each increase of load. It will be found that each additional 50 pounds produces practically the same increase in deflection and that each time we release the bar we find it to have resumed its original shape. But as we keep on adding weight continually, we will notice that our differences in deflection are no longer the same and uniform, but that they have suddenly increased, each being larger than that preceding. We will also notice that if we now release the load the bar no longer has its former shape; it has been permanently bent. This point in the experiment, at which the bar is permanently bent and at which the deflections begin to increase in greater proportion than the load is called the elastic limit of the material. That limit can best be measured by stating the stress which exists at the time. Each kind and quality of metal has its own elastic limit. Wrought iron can be stressed to about 25,000 pounds per square inch without injury, structural steel to from 30,000 to 40,000 pounds, carbon-spring steel after treatment to 110,000 pounds.

If we now examine an alloy steel in the same way we note a marked and truly wonderful increase in the elastic limit. This increase in the elastic limit, together with the accompanying ability to resist fatigue, are the essential characteristics of alloy steels. We can point out a certain Silico-Manganese steel, made in the electric furnace, which has an elastic limit of 220,000 pounds per square inch. The vast advantage of such a steel can easily be comprehended. A bar of it held in a vise could be bent just twice as far without injury as a carbon steel bar of the same dimensions. This does not mean that a spring of this alloy will merely last twice as long as a similar carbon spring. The ratio between the two is very much greater than this. For in addition to having a high elastic limit these steels also possess remarkable anti-fatigue properties. Instead of only doubling the life of the spring by employing alloy steel we increase its life many fold. …

From Landau D (ed.) Leaf Springs: Their characteristics and method of specification. Sheldon Axle Company: Wilkes-Barre, PA, 1912.