Page:EB1911 - Volume 03.djvu/938

 The Rudge-Whitworth crank-bracket has outward cups and is cup-adjusting. The cranks are cotterless. Fig. 6 is a sectional view. The left crank and axle are forged in one piece. The fastening of the right crank and chain-wheel is by multiple grooves and teeth, this fastening being better mechanically than the cotter type. Pedals.—The pedal consists of a pedal body, on which the foot of the rider rests, mounted by ball-bearings on a pedal-pin, which is secured to the end of the crank and turns with it. The pedal body is made in many forms, but usually the bearing-cups are contained in a tube from the ends of which project plates, carrying rubber blocks, or serrated plates (rat-trap pedals), on which the foot of the rider rests. Cone adjustment is most used. The fastening of the pedal pin to the crank is best effected by screwing it up against a shoulder, the right and left crank eyes being tapped with right and left hand screws respectively. With this arrangement, if the pedal pin screw is a slack fit in the crank eye, the pressure on the pedal tends to screw it up against the shoulder.

Wheels.—Bicycle and tricycle wheels are made on the “suspension” principle, the spokes being of high-tenacity steel wire, screwed up to a certain initial tension, thus putting a circumferential compression on the rim. In the “artillery” wheel, the wooden spokes are in compression, and the rim is under tension. The rims, which are made to a section suitable for pneumatic tires (see ), may be of sheet steel or aluminium alloy rolled to the required section, either without joint or jointed by brazing or riveting. Wood rims are used on racing bicycles, but in England are not popular for roadster bicycles. Holes are drilled at or near the central plane of the rim for the spoke nipples, which have shoulders resting on the outer surface of the rim and shanks projecting through the rim towards the hub. The spoke ends are screwed to fit the nipples. The shank of the nipple has a square cut on its outside surface by which it can be screwed up. The spoke flanges on the hub are placed far apart and the spread of the spokes gives the wheel lateral stability. Tangential rigidity under driving and braking is obtained by fastening the spokes to the hub tangentially (figs. 1 and 2). The hub fastening of the spoke is simply obtained by forming a hook and head on the spoke end, and passing it through a hole in the hub flange. The best spokes are butted at the ends, i.e. made of larger diameter than at the middle, to allow for screwing at one end and the hook bend at the other.

Chains.—There are two widely used types of chains. The “block” chain (fig. 7) consists of a series of central blocks connected by side plates. The “roller” chain (fig. 8) consists of a series of outside and inside links. The outside link A is made up of two steel side plates P united by two shouldered rivets R. The inside link B consists of two side plates P united by two tubular pieces T, which form bushes for the rivets R and pivots for the rollers L. The rivets, bushes and rollers are case-hardened. Roller chains for cycles are made in two pitches, in. and in., and in widths from  in. to in. between the side plates of the inside links. The weight of 4 ft. length (96 links) of a in. pitch in. wide roller chain is about 12 oz., and its breaking load is about 2000 ℔ In a block chain the ends of the blocks engage with the teeth of the chain-wheels, and the same surfaces continually coming into contact, the wear may become excessive, especially when exposed to mud and grit. In the roller chain the outer surfaces of the rollers engage with the teeth of the chain-wheels, and during the engagement and disengagement may roll slightly on the tubular rivets. The surface of contact of the roller and tubular rivet is not directly exposed to the dust and grit from the road. The rollers therefore serve the double purpose of (1) transferring the relative motion of the parts to a pair of surfaces under better conditions as regards lubrication, and (2) presenting a new part of the outside surface of the roller for the next engagement with the chain-wheel. The durability of roller chains is thus much greater than that of block chains, under the usual conditions of cycling. Chain-wheels.—The pitch line of the chain-wheel is polygonal (fig. 9), a, b, c, d being centres of adjacent joints of the chain when lying in contact with the wheel. The path of the joint a of the chain, relative to the chain-wheel as it enters on to and leaves the chain-wheel, is evidently the curve a3 a2 a a′1 a′2 made up of a series of circular arcs having centres d, c, b, b′, c′, respectively. Similarly for the path of the adjacent joint b. The fullest possible form of the tooth is that between the two parallel curves, of radii less by an amount equal to the radius of the roller, as indicated in fig. 9. But since it is neither necessary nor desirable that the roller should roll along the whole length of the tooth, the radii of curvature of the tooth outline may be less than shown in fig. 9. A good arrangement of tooth form is shown in fig. 10. Owing to the polygonal pitch surfaces of the chain-wheels a chain does not transmit motion with constant speed-ratio of the shafts. The variation of speed-ratio in a chain with links of equal pitch is approximately inversely proportional to the square of the number of teeth in the smaller chain-wheel, as shown in the table annexed, in which the percentage variation is—

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 6 to 7 in. Cranks of 7, 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. 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