WHEELS OF CARRIAGES. Wheels consist of the nave or stock, in the cen tre, the spokes which are radii, and the ring, which is the periphery of the wheel. When a wheel is to be made, the workman adapts moulds to its exact diameter. Twelve spokes are commonly assigned to the larger wheels of car riages, and to the smaller ones. The working and finishing of the several fellies, to form the periphery of a wheel, consists, after it bee been roughly chop ped to the pattern, in forming its inside edge somewhat rounding, and getting its outside edge perfectly circular, and to form such an acute angle, that, when the wheel is adapted to the axle-tree, it shall stand square and solid under the body of the carnage.
The strength of a wheel depends greatly on the attention paid to the ar rangement and framing of the spokes ; in common wheels they are framed regu larly and equally all round the thickest part of the nave, the tenons of the spokes being so bevelled as to stand, with re ference to the horizontal position of the nave, abort three inches out of the per pendicular : this is done to produce what is called dishing. But, for wheels of strength, such as the wheels of road coaches, the framing of the spokes con sists in getting every other one perpen dicular to the nave. Hence the mortises to receive them in it are not made in a parallel line round it, but stand, in two different parallels, one without the other, by which greater solidity is given to the nave, and an immense addition of strength.
The boxing of a wheel, and adapting the axle-tree is done usually by the coach or tire-smith. The box of a wheel is a hollow conical tube of iron, furnished on its outside with two or three square projections, which have the effect of giv ing it a key when mortised through the nave of the wheel. Patent boxes are of a different construction, and owe their safety to four bolts, which pass Com pletely through the nave of the wheel, having a square shoulder on the back of the nave, with screws and nuts on its front. The box to such a wheel is made, as are the other boxes above described, except being completely closed at its outer end, with a solid and broad cap of iron, of sufficient diameter to inclose completely the end of the nave. The
axle-tree, too, is formed to fill the box, and press up close to this iron cap. High wheels are, in bad roads, much preferable to low ones, except where the shape of any inequality of the road per mits the low wheel to roll, while the larger wheel can bear only on the edges of the hole over which it is to pass. But the increased expense and weight of very large wheels put a limit to their size, be yond which no experienced mechanic would pass. For, although the mechan ical power of a wheel in surmounting a given obstacle constantly increases with the size of the wheel, it does not in crease directly in proportion to its height. It increases but little more than in pro portion to the square roots of the diame ter of the wheel; so that if a wheel pass over an obstacle with a given power, though it may be made to pass over the same obstacle with half that power by increasing the diameter of the wheel, it is not to be expected that this can be done by making the wheel twice as large : for, to effect this purpose, a wheel of four times the former diameter must be em ployed.
Mr. Edgeworth showed that practice agrees with theory. A wheel of seven inches diameter, loaded with twenty pounds, required eight pounds to draw it over an obstacle of one quarter of an inch high, whereas, when a wheel of twenty-eight inches high was employed, four pounds drew the same load over the same obstacle. And when the line of draught was horizontal, the larger wheel required four pounds four ounces, the smaller nine pounds.
It appears, that the higher the wheels the more advantageous is the draught ; but, in fact, the expense, the strength, and the weight of wheels must be taken into account, when they are applied to carriages ; and experience has deter mined, that the best height of wheels is from four feet six inches to five feet, for coaches and carriages that move swiftly ; and that, for heavy carriages, wheels seldom are found useful beyond the diameter of six feet.