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 [Tire]), 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.
| Fig. 7. |
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.
| Fig. 8. |
Roller chains for cycles are made in two pitches, ½ in. and 5⁄8 in., and in widths from 1⁄8 in. to ¼ in. between the side plates of the inside links. The weight of 4 ft. length (96 links) of a ½ in. pitch 1⁄8 in. wide roller chain is about 12¼ 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.
| Fig. 9. |
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.
| 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—
| maximum speed-ratio − minimum speed-ratio | × 100. |
| average speed-ratio |