MINIMIZING SHOCKS.--It is obvious, therefore, that if part of this shock can be taken up by the tire, the difference due to the smaller diameter of the wheel, will not be so apparent. The thickness, or widths of the tires also minimizes the impact and distribute the jars while running, so that with these advantages a small wheel has been found to be more practical than a large one. RESILIENCY.--Most wheels are now made with wooden spokes, secured by means of a pair of metal-flanged hub plates, bolted together so as to clamp the radiating spokes, but wire wheels are now coming more into favor, whereas cast or pressed solid steel wheels are used on some heavy trucks. CHAPTER III TIRES, TUBES, AND RIMS TIRES.--Three kinds of tires are now used, namely: Solid, cushion, and pneumatic. These forms all use rubber, or some compound with the qualities and characteristics of rubber, so as to afford a good tractive surface, as well as resiliency. [Illustration: Fig. 11. Solid Tire.] _The solid_ tires are used on heavy trucks, where weight and not speed must be provided for. _Cushion_ tires are sometimes employed on cars and trucks of medium weight. _Pneumatic_ tires, in which air is used, are universally used in automobiles for all other purposes. [Illustration: Fig. 12. Single Tube.] The air is confined in two ways: First, by what is known as the “single tube.” (Fig. 12.) Second, by the “double,” or inner tube system. (Fig. 13.) The single tube is well adapted for light vehicles, or where great speed or weight are not considered, and this type is now confined to bicycles. But it has certain disadvantages, namely: That of creeping, due to the impossibility of properly securing it to the rim of the wheel. Sand and grit are also liable to creep in between the tire and rim, and wear the material, thereby ruining it. The outer casing, or shoe, is split on its inner side, and usually provided with an annular flange on each side of the split, which rests against the rim of the wheel, and is adapted to receive a rim which securely fastens the annular flange of the shoe, to the rim of the wheel. [Illustration: Fig. 13. Double Tube.] Various ways are provided for holding the shoe to the rim of the wheel; but in the different types shown by the illustrations, Figs. 13 and 14, the shoe has a flange which is held within channels on the rim, or by some form of fastening device. _The inner_ tube is usually of thin elastic rubber, so made that when properly inflated it will fit the outer tube or casing. The outer part, which can be made of a different rubber compound, and is better adapted to stand wear, whereas, the inner tube, which is made of the best, and more costly material is protected. ADVANTAGE OF DOUBLE TUBE.--The great advantage of the double tube is due to the positive means of fastening it to the rim of the wheel, so as to prevent creeping.