SPRING CARRIAGE. The progress of a wheel-carriage, even upon the best of roads, is impeded by the wheels coming in contact with, and being compelled to rise over, undulations of surface which check their rolling motion. In many old rough pavements, owing to the openness of the joints, and the wear of the stones, the road consists of a series of blunt ridges, in passing over which the motion of the wheel can be no other than a succession of jolts. The surface of a well-made road of broken stone, when in perfect order, presents few important asperities ; but when metal or broken stone has been recently laid on, it is exceedingly rough. If a rigid carriage be drawn over any of these surfaces, the irregularities which affect the path of the wheels will be communicated through them to the body, to which they will impart a jolting or vibratory motion. In a four-wheeled carriage the movement of the body will be influenced by the discor dant motions of ,the fore and hind wheels; and it will be continually liable, owing to the imperfections of the road, to have the whole of its weight thrown upon three wheels, whereby the framework will be exposed to injurious strains. To enable it to sustain such strains, the framework must be made very strong and heavy ; and the destructive and painful effect of increasing the velocity of inelastic carriages would alone have been sufficient to limit the speed of vehicles.
One of the simplest means of alleviating concussion is that often adopted in light carts, of suspending the seat from the sides of the body by leather straps. Next to this is the use of straps to suspend the body itself, an expedient which seems to have been occasionally resorted to from an early period. With very few exceptions, it appears that slung or suspended carriages were not used until the 17th century. In the early carriages of this kind the straps were usually attached to a framework of wood at each end of the vehicle, rising to a considerable height above the axles. The antique four-whecled carriages of Europe used for state purposes are mostly constructed on this plan ; and their great weight and slow movement prevent any violent concussion. A serious disadvantage of this construction is the great length of carriage that it renders necessary, and the cumbrous character of the wooden framework which supports the braces. The carriage must also be heavily loaded, in order to make the motion tolerably comfortable, especially when the straps are suspended from points not much higher than the bottom of the body. To remedy the defects of the primitive slung carriage, it was desirable to render the pillars from which the straps were suspended somewhat elastic. This
could not be readily effected with wood, because the pillars were necessarily short, and therefore stiff. Hence arose the use of elastic steel supports, which have gradually assumed the form now well known as C-springs. • Straight springs of steel probably owe their origin to the straight wooden springs occasionally used in light vehicles in this and other countries. Used either singly or in combination, they afford sufficient elasticity for ninny purposes, without raising the body to an incon venient degree, or interfering with its form ; since they may be placed entirely beneath it, and require but little room for their play. Ellip tical springs have, in some degree, the same advantages; but they require rather more depth than straight springs.
Carriage-springs are usually formed of several thin plates of steel, of various lengths, so laid and fastened together that the spring shall be thick iu the centre, or at the end by which it is fixed, and thin, or consisting of only a single plate, at the cud or ends where the greatest play is required. Tho steel used is of coarse quality, and has little carbon in its composition. It is fashioned by rolling-machinery to the transverse dimensions required, which vary from one inch and a half to three inches in width, by one-eighth of an inch to half an inch in thickness. The plate forming the back of the spring is usually thicker than the rest, on account of its being the longest, and having its ends formed into bolt-eyes, to receive the bolts by which the body is connected with it. With this exception, it is not usual to make any difference in thickness in the several plates of a spring, notwithstanding their different lengths. After the plates have been wrought into the form required, they are hardened by heating in a hollow fire, and then plunging into water. They are subsequently tempered by drawing them again through the fire, until they become so hot that a stick rubbed over the surface will be kindled to a blaze. Any accidental warping acquired in these processes is removed by hammering, the plates being slightly warm during the operation, to avoid the risk of breakage. They are then finished by filing all the parts that will be exposed to view when the spring is complete; and are finally put together and secured by a square hoop of iron, which is shrunk on hot, and by a rivet passing through the hoop and through all the plates.