CHAPTER III.
On Wheels in General and Bicycles in Particular.
The form of the wheel is very ancient, its construction modern, even recent. Its evolution has been gradual. First came the round stick or roller, placed beneath a weight; then a roller with its central portion shaped and thinned to lessen friction; then two disk-shaped sections of a log, connected by a bar upon which they revolved, replaced the clumsy stick.
Each wheel or disk then began to receive separate attention. There was the wear on the edge or rim to be considered, and it was found that if its surface were protected, the disk would last indefinitely longer. Then it was noticed that the hole in the centre of the disk wore unevenly, and it was reinforced, and the hub began to take form. When the rim was strong and the central portion of the wheel remained intact, the disk was found to be heavier and stronger than it need to be to support the outer portion of the wheel. Some of the useless heavy part was removed, and the disk pierced with holes to make it lighter; then these holes were shaped between the remaining portions, which took the form of pillars or spokes. A pillar would break, and be replaced by a rounded stick; and thus, perhaps, the rude idea took form of constructing a wheel out of several pieces, for the sake of securing economy, durability, and lightness.
A wheel, then, was well constructed, with a large, heavy piece in the centre to stand friction and bear weight, and with the rim made of several pieces, each piece supported on a spoke, and all held together by a band called a tire. In the course of time the hub became heavier, the spokes thinner, the rim stronger and lighter, and the tire narrower. The bar connecting two wheels was made very strong, with smooth ends for the wheels to revolve easily upon. Pins were driven into holes in the projecting ends of the axle, or bar, and later the pins replaced by knobs, or nuts. Then the wheels were brought closer together, and found to run more easily; and the tire, cutting too deep into soft surfaces, was widened. Attention, moreover, was paid to the roadway, very bad places being filled and smoothed.
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.
The axle of a modern bicycle wheel is compound, and although there are two ends to the axle, there is but one rim to the wheel. The rim carries all the weight distributed from many points at once; the weights resist each other, and give strength and stiffness. The axle really carries double, two wheels with but one rim; and each end of the axle is supported at so many points that it possesses great weight-carrying power in proportion to the weight of material used in its construction. The weight of the frame is supported on the axles of the rear and front wheels. Of its construction it is sufficient to say that the weight is taken up on the thrust principle and that wherever a point of support for the thrust is located, the frame is strengthened to support and resist the thrust.
By a mechanical application of power, the power of the pressure of the foot on the pedal is multiplied, one revolution of the pedal crank causing the rear wheel to revolve a number of times. In the chain gear the mechanical means is a large wheel on the axle to which the pedal cranks are attached, and a smaller wheel on the axle of the rear wheel. There are teeth on both these wheels, the large wheel having the greater number. The band or chain passing over the large sprocket-wheel has links which engage each tooth of the wheel as the chain passes over it, and as that wheel revolves, it pulls the chain over, link by link.