| Number of teeth on hub chain-wheel | 10 | 12 | 14 | 16 | 18 | 20 | 24 | 28 |
| Percentage Variation | 5.1 | 3.5 | 2.7 | 2.1 | 1.6 | 1.3 | 0.9 | 0.7 |
The rollers as they come in contact with the chain-wheel strike it with a speed proportional to the angular speed of the chain-wheel and to the pitch of the chain, causing a certain amount of noise.
Chain Adjustment.—To keep the chain running at correct tension, it is necessary to have some adjustment of the distance between the crank-axle and hub. This is obtained either by an eccentric adjustment at the crank-bracket, an eccentric adjustment at the hub-spindle or by draw-bolts at the fork-ends, the last method being most common.
Gear-case.—The modern roller chain by makers of repute is so durable that the necessity for a gear-case is not so great as when chains were of inferior quality. But if the bicycle is to require the minimum amount of care and attention a gear-case should be fitted. The Sunbeam gear-case is built into the frame and is oil-retaining, and the chain, chain-wheels, free-wheel and two-speed gear are continually lubricated by an oil-bath. A detachable gear-case is not usually oil-retaining, but serves to exclude grit and mud from the chain.
Gear and Crank-length.—The “gear” of a bicycle is given by the formula Dn1/n2 where D is the diameter of the driving wheel in inches, n1 and n2 the numbers of teeth on the crank-axle and hub chain-wheels respectively. At each revolution of the crank-axle, the bicycle is moved forward a distance equal to the circumference of the circle of diameter equal to the gear. Thus with a 28 in. diameter driving-wheel, 18 teeth on the hub chain-wheel, 45 teeth on the crank-axle chain-wheel, the bicycle is geared to 70 in. The usual crank-length is 6½ to 7 in. Cranks of 7½, 8 and 9 in. length can be had, but require a bicycle frame of special design. The gear should be roughly proportional to the crank-length. The gear 10 times the crank-length is a good proportion for an average rider.
| Fig. 11. |
Free-wheels.—A free-wheel clutch transmits the drive in one direction only, allowing the pedals to remain at rest at the will of the rider, while the bicycle runs on. With a free-wheel, chain breakages are reduced or nearly eliminated, as should the chain get accidentally caught the free-wheel comes into play. There are three principal types of free-wheel clutches—roller, ratchet and friction cone. The roller type was the earliest in use, but has fallen into disfavour. A sectional view of a ball-bearing ratchet free-wheel, with outer cover removed, is shown in fig. 11. The ring on which the three pawls and springs are carried is screwed on the end of the hub; the chain-wheel is combined with an inner ratchet wheel and is mounted by two rows of ball bearings on the pawl ring. The friction cone type of free-wheel clutch is usually combined with a brake inside the hub, the whole combination being termed a coaster hub. Fig. 12 shows a sectional view of the Eadie two-speed coaster, in which the free-wheel clutch and brake are combined with a two-speed gear. The free-wheel clutch action is as follows: A forward pressure of the pedals turns the externally threaded driving cone H in the internally threaded cone F, the latter being thus forced to the right into engagement with the cup J which is screwed to the hub-shell, thus forming a friction driving clutch. The pedals being held stationary the driving cone H is stationary, and the hub running on the ball bearings G, the cone F travels towards the left until released from the cup J, when it also remains at rest. In this type of free-wheel clutch it is essential that there be little or no friction between the screwed surfaces of H and F, else on beginning to pedal, the cone F may remain stationary relative to the driving cone H, and no engagement between F and J may take place. If F be prevented from turning faster than the hub-shell, as is sometimes done by a light spring between the two, the engagement of the friction clutch must take place as soon as the pedals tend to move faster than the speed corresponding to that of the hub-shell.
| Fig. 12.—Eadie Two-speed Coaster Hub. |
Brakes of many types are used, differing in the place and mode of application. The tire brake has fallen into disuse, rim brakes and internal hub brakes being usual. The retarding force that can be applied by a brake is limited by the possibility of skidding the wheel. In riding at uniform speed, without acceleration, the greater part of the load is on the rear-wheel; but as soon as the brake is applied to cause retardation the wheel load distribution is altered, more load being thrown on the front wheel. Thus the most powerful brake is one applied to the front wheel. On the other hand, a front-wheel brake often sets up an unpleasant vibration of the front fork. On a greasy road too powerful pressure on the front-wheel brake may cause a side-slip with no chance of recovery; while with the back-wheel brake recovery is possible. The Bowden system of transmission, which is largely used for cycle brake work, consists of a steel stranded cable inside a flexible tube formed by a closely wound spiral of steel wire, the cable being practically inextensible and the spiral tube practically incompressible; if the ends of the latter be fastened it forms a guide tube for the cable, any movement given to one end of the cable being transmitted to the other end. The spiral tube may be led round any corners, but the frictional resistance of the cable inside the spiral tube increases with the total angle of curvature of the guide tube; the laws of friction of a rope passing over a drum apply. In fitting the Bowden system the total curvature should therefore be kept as small as possible. With a back-pedalling rim brake the cycle cannot be wheeled backwards unless a special device is used to throw the operating clutch out of action. A back-pedalling brake is most conveniently applied inside the hub, as in the coaster hub. In the Eadie two-speed coaster (fig. 12) the braking action is obtained by the expansion of the steel band I against a phosphor bronze ring L carried by the rotating hub-shell. The steel band I is mounted on a disk with a projecting arm, the end of which is clipped to the frame tube. The expansion of the steel band is effected by the movement of the lever K fixed to the cone E. On moving the pedals backward the screw drive-ring H forces the cone nut F with which it engages to the left into contact with the cone E. The backward movement of the pedals being continued sets up the required movement of the lever K, and applies the brake.
| Fig. 13: Sunbeam Two-Speed Gear. |