Читать книгу Bicycling for Ladies - Maria E. Ward - Страница 6

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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.

The small wheel is also provided with teeth, and every time the large sprocket-wheel is turned, if only a little way, it pulls the chain link by link, and the chain link by link pulls the rear wheel tooth by tooth. The small sprocket-wheel revolves as the chain pulls it, revolving oftener than the large wheel to keep count with it tooth for tooth. The number of teeth on the sprocket-wheels determines the multiplicity of revolutions of the rear wheel.

The rear wheel revolves very rapidly, in the process becoming virtually a gyroscope; and a gyroscope will maintain the plane in which it revolves unless other forces intervene. The front wheel takes its motion from the friction of the surface over which it is propelled, and after the bicycle is in motion, the forces that are applied to control and direct its movement are friction and resistance. After the cyclist is mounted, there is the added complication of a constantly shifting centre of gravity, caused by change of balance. The steering is effected by changing the direction of the front wheel, the rear wheel being enabled to follow by a slight slipping over the wheeling surface. If the change of direction is too abrupt, the rear wheel will slip enough to lose its hold on the surface, and the weight of the rider will be suddenly shifted from above the point of support (the axle of the rear wheel) to the top of the rim of the wheel, thus becoming a lever with the weight on the end of the long arm, and the bicycle falls over.

As the wheels revolve, there is a constant pull on tire and rim. Just as the chain is pulled over the sprocket-wheels, the tire is pulled by friction over the surface ridden on. If this surface affords the tire no hold, it is impossible for the wheel to advance, as on a muddy surface. The crank may impart a motion to the wheel, but this motion will not enable the wheel to maintain its place; or if, in overcoming the cranks at the dead centre, too much weight is applied to one side of the wheel, the same thing occurs, and the wheel falls over. There are a number of mechanical means for conveying the motion of the foot to the wheel of the bicycle to cause the wheels to revolve.

There are many ways of constructing a frame, and different designs and patterns of fittings for different parts; but the main idea of the bicycle does not change—a fixed wheel to which motion is imparted, and a movable or guiding wheel, independent of the power wheel, and revolving only because the machine is pushed or pulled forward. This second wheel gives stability, and supports the wheel at a movable point.

We have, therefore, a wheel which supports a frame and the weight it carries. The frame is supported on two wheels, one end of the frame taking the weight, and that end supported on one wheel. The second wheel merely supports one end of the frame. If the frame were attached at one end directly and rigidly to the second wheel, the weight carrying wheel would move in the same plane with it. A child’s two-wheeled cart will illustrate this. While moving forward in a straight line, the child is safe until one or both of the wheels begin to travel in a rut, when the rigid handle or tongue of the cart resists the guiding power, and the child is pulled or thrown over. If the tongue or frame of the wagon is allowed play, as it is called, say by being held easily in the hand, the pole may be guided. The supported end of the frame of the bicycle corresponds to the pole or tongue of the cart.

Now, the wheel is made to steer in this way: We have the rigid forks, and a wheel to support them. The forks hold the wheel in the same plane as themselves, but the top part of each fork, instead of being fastened immovably to the frame, passes up through a bearing-head prepared for it in the frame. The wheel is supported, but it can now maintain a separate plane, and as the post of the forks changes its direction, it pulls the frame with it as it advances; and so the controlling or steering power is transferred.

The weight-bearing wheel is led and directed; part of its power is transferred by thrust or push to the front wheel, and as the steering wheel is pushed over the surface, it revolves. As it revolves, part of its power is diverted by the movable head, and as the head is held and controlled by the rider, any desired direction may be imparted to the entire machine.

A bicycle may have either a diamond frame or a drop frame. The drop frame is made to facilitate mounting and to permit the adjustment of a woman’s dress. The diamond frame possesses great strength, and can be lightened to a wonderful degree without injury to the thrust and strain-bearing quality of its construction.

A form of triangle is made use of to carry the greatest weight and bear the greatest strain. This triangle is supported on the rear wheel, and has part of the frame attached to it to connect it with the steering-wheel. The steering-wheel is provided with handles by which it may be controlled. The weight of the rider is carried over the power wheel, and the propelling power, a lever movement, is imparted by the foot.

From this description an idea may be formed of how and why a bicycle works; but the details of its mechanism are of endless variety of form and pattern, material and workmanship. Each small part, its form, its use, its angles of surface, its every detail indeed, is the product of the work of many minds for many years. And though the bicycle was looked for, and hoped for, and worked for, its general acceptance came suddenly, and came only when it had been built light enough and strong enough and elastic enough to warrant confidence in its universal usage.