It possesses also this advantage : that if the axle be slightly bent downwards towards its extremity, so as to bring the spokes of the lower half of the wheel into a nearly vertical position, which will enable them to bear the greatest possible weight, the upper half of the wheel will have such an inclination outwards as to leave more room for the body of the carriage, and to throw particles of dirt, caught up in its revolution, away from it. Very strong wheels are occasionally made in a double-dished form, or with the spokes alternately inclining outwards and inwards from the felloes, so that the centre or nave of the wheel forms the base of a pyramid of which the felloe forms the apex ; but such wheels are very deficient in elasticity, and consequently will not bear much concussion. In ordinary dished wheels the spokes are arranged in two sets, being alternately more and less inclined or dished; and in some eases every alternate spoke is set absolutely straight or square with the nave. The dished form of wheel, together with the bending of the axle, involves some increase of axle friction, and also, if the wheel be wide, the use of a conical tire, which cannot possibly roll in a straight line without a degree of rubbing friction most injurious to the road, and which also increases the draught. Some years ago the use of conical wheels for waggons, combined with the inordinate breadth of tire encouraged by injudicious legislation, was carried to a most absurd extent, and broad-wheeled waggons were used which were far better adapted for grinding stones into dust and mud than for the purpose of locomotion. This extrava gance, however, is now seldom seen.
The ordinary mode of making a coach-wheel is as follows. The piece of elm for the nave is turned in a lathe to the proper size and shape, and is hollowed within to receive the axle. It is then fixed in a groove, and holes are chiselled out for the reception of the ends of the spokes. There is nothing but the practised eye of the workman to guide him in making these holes in the proper position for producing the dishing of the wheels; ;—half of the spokes are near one end of the nave, and half near the other, and the boles have to be regulated accordingly. The pieces of oak for the spokes are shaped by hand : a small cutting tool, called a spoke-share, being the chief instrument employed. One cud of each spoke is formed into a tenon to fit the mortice-hole in the nave. The spokes are fixed into the holes by driv ing with a mallet, and are finally shaped after fixing. The rim being formed of several felines, and each felloe being large enough to receive the ends of two spokes, the pieces of ash to form them require to be wrought into segments of curves; this is done by means of pattern boards and various cutting tools. The felloes are drilled with holes to join them together by means of dowels, with other holes to receive the ends of the spokes.
Few mechanical operations of equal complexity, and requiring an equal amount of precision, have received so little aid from machinery as the manufacture of carriage-wheels ; though wheels made by machinery are said to be superior in truth, firmness, and durability to any others. In ordinary wheels the neatness and strength are increased by the application of a hoop of iron to each end of the nave, to enable it the better to resist the strain of the spokes. The spokes, which are usually formed of oak saplings, are wrought into the proper form after being driven into the nave, and are usually cut to a narrow edge in front to lighten their appearance. Wheels have been made with the periphery in one or two pieces, bent into the required form after being softened by boiling and steaming ; but the plan has not been found successful, because, among other disadvantages, the wood is injured by the long boiling required. In ordinary eoach-wheels, from 4 feet 3 inches to 4 feet ti inches high, there are fourteen spokes; and in fore-wheels, which are about a foot lower, there are commonly twelve spokes; and the usual arrangement is to have half as many felloes as there are spokes. The introduction of solid or hoop tires is a great improvement upon the former system of wheel-making, as it affords the means of binding the whole of the wheel together with irresistible. force. The tire is made very hot, and the 'wheel is made of such a size as only just to receive-it when it is thus expanded ; but so soon as the hoop* is brought into its proper place, water is thrown upon the wheel to coo] the tire, and to prevent the wood-work from catching fire, and the result of the sudden contraction of the hoop is to compress the felloes, and to force each spoke into a slightly curved form, so that when complete the wheel forms a flat dome-shaped figure, admirably adapted, by its combined strength and elasticity, for the purpose for which it is designed. The tire is further secured, after cooling, by a few pins driven through it and the felloes, and riveted inside the latter.
Having found, during his experiments on steam locomotion upon common roads, that wheels of the ordinary construction were not strong enough for his purpose, Mr. Hancock contrived patented a wheel in which the nave is abandoned altogether, and the inner ends of the spokes are formed into wedges which abut against each other, and form a kind of arch surrounding the axle-box. They are firmly secured bi their places by an iron plate on each side of the wheel, and a bolt passing through each spoke. Though too rigid for very rapid motion, this wheel is exceedingly strong, and its simplicity of construc tion forms a great recommendation.