CHAPTER IX
SPRING FRAMES
Like every other road vehicle, there have been attempts made from time to time to spring the road wheels of bicycles. Before the advent of pneumatic tyres these attempts were more frequent, but patents in connection with the springing of bicycles have been exploited as recently as 1902–3.
Omitting the early bow spring used to support the saddle of boneshakers, the first patents in connection with spring frames followed the lines of some of the spring frames now employed for motor bicycles. The rear fork was hinged to the frame near the crank bracket and the spring was placed at the top of the fork between the seat tube and the apex of the triangular fork. One of the first of these was, I believe, the Star, in which a rather large volute spring was used. The Whippet spring frame was an ingenious arrangement of toggle joints and springs by means of which the rider was partly insulated from vibration, but the road wheels were unsprung. This frame was introduced just prior to the pneumatic tyre and had a short life in consequence; had the pneumatic tyre arrived several years later the Whippet frame probably would have met with the success its ingenuity deserved.
Sharp's air spring frame was a device which was exploited about 1904–6, and in this system metal springs were replaced by cushioning devices in which the movement of the road wheels was controlled by air alternately compressed and released by an action that is analagous to the movement of a pump. The invention was clever but it never caught the public favour, partly due to increased weight and the average cyclist's objection to complication.
Spring forks were far more common than spring frames, and it is rather surprising that they are almost obsolete on a modern bicycle. Among the pioneer front spring fork inventions may be mentioned the Dunlop. In this design the wheel was carried in jointed links which were connected to the fork blades by interposed spiral springs.
A Nottingham firm, at a later date, made the fork blades or sides like a small carriage spring of two to three leaves of flat spring steel, the flat part of the leaf facing the direction of travel. This form of spring made the machine very comfortable to ride but was said to detract from the rigidity of the drive when the bicycle was pedalled up steep hills; the blades or leaves were also liable to fracture.
Following the withdrawal of this last type of spring fork the question of springing of bicycles has lain dormant, and shows no signs of revival.
The principal objections to springs from a mechanical view point are that they add weight and complication to the machine; they interfere with the action of brakes and render them far more difficult to fit; the joints wear and adjustment devices have to be designed which are costly to produce, and however well made the joints may be, they are almost sure to rattle.
To sum up, the objections appear to outweigh the advantages of this method of insulating the rider from road shock when the pneumatic tyre absorbs so much of the vibration at the point of contact of the wheel with the road surface.
In addition, the rider, if he be skilful, can act as a natural spring by partly lifting his weight off the saddle and supporting himself to some extent on the pedals. Particularly does this apply when free wheeling, because if the cranks be allowed to assume a horizontal position, one at 3 o'clock and the other at 9 o'clock, the rider can stand on the pedals and allow his knees to form the joint of a spring, the muscles of the calves and thighs being the springs. A skilled rider invariably adopts this position when free wheeling on rough roads and also instinctively falls into it when pedalling forward, with either a fixed or free wheel; with the former he slackens speed and bears his weight on the rising pedals, so raising his weight out of the saddle; with the latter he allows the machine to over run the cranks, by free wheeling momentarily, and at the same time raising his weight on the stationary pedals.
Motor-cycle Springing. The motor-cycle springing mechanism is a far bigger and more complicated proposition than that of a bicycle. Practically every motor bicycle has a spring front fork, although at one time only rigid forks were obtainable. Increase of speed and deterioration of road surfaces made spring forks a necessity, and no motor-cyclist would buy a machine now without one.
Lately there has been a tendency to spring both wheels of a motor bicycle, the generally accepted design being some form of leaf spring, which permits the rise and fall of the wheel against the action of the spring without throwing the wheel spindle out of its correct position in relation to the transmission.
The drawbacks to any form of springing are the rapid wear of the hinges or joints by which the wheel is connected to the springs and the springs to the rigid portion of the frame, and the bouncing effect that is set up by the uncontrolled movement of the springs.
Springs are also fitted to some designs of side-car wheels as well as to the body of the side-car.