A wheel is defined as “a circular frame turning on an axle”; an axle as “a shaft or rod, either solid or hollow, on which a wheel is placed.” The first bicycle wheels were constructed like carriage wheels, the limit of that method of construction arrived at. The rim was supported on the spokes, which rested on the hub. The minimum definite quantity of material was ascertained, but the wheel was still too heavy and bulky. If the weight of material was lessened, however, it would fall to pieces.
The bicycle wheel of to-day is a compound mechanism constructed on reverse principles. The wheel is made on the principle of suspension, an inverted application of weight and thrust. The hub is hung from the rim, and the axle supported in that way. Each bicycle wheel is really two wheels, graceful in form, with but one rim, and with two hubs, one on either end of a short axle, the spokes being drawn to a common rim, and made stiff enough to carry weight, and elastic enough to withstand shock. The rim or frame is elastic and durable. To this rim many wire spokes are fastened, and the hubs for each wheel are centred and hung from them. The hubs and axle are wider than the rim of the wheel, and the spokes are fastened alternately to either end, thus giving a tangent strain which stiffens the wheel and gives it strength. The tire is a separate construction, possessing several individual features. The only office of the old tire was to protect the rim of the wheel from wear; the pneumatic tire protects the rim, presents a good friction surface, and is enabled by its elasticity to take the shock and jar of the entire bicycle.
In order that the wheel may turn, the axle must be lubricated; otherwise the inside of the hub will become hot, and wear the face of the axle a little rough. The surfaces then cannot pass, but remain fixed and immovable, and the wheel cannot turn. The introduction of a third material of a different consistency between the revolving surfaces prevents their wearing against each other, and the lubricant is rubbed and rubbed again; there is so little of the lubricant that it does not accumulate sufficiently to cause resistance, and the moving surfaces slip smoothly over each other.