If the reaction from the ground is in the direction da, the top and bottom tubes are subjected to pure compressive and tensile stresses respectively. When no brake pressure is applied a bending moment due to the overhang ab is superimposed on these tubes. Thus a short socket head with top tube sloping downwards towards the head gives a stronger frame than a horizontal top tube. The steering axis ef is arranged so as to cut the ground at f, a little in front of the point of contact d of the wheel with the ground, giving a slight castor action, and making steering possible without use of the handle-bar. The rake of the steering head (that is the angle between ef and bd) and the set of the fork (that is the displacement of the wheel centre c from the axis ef) may be varied within tolerably large limits without much affecting the easy steering properties of the bicycle. The transverse stresses on the rear frame due to the action of pedalling are more severe than those due to the vertical load. The pedal pressure is applied at a considerable distance from the central plane of the bicycle, and the pedal pin, cranks and crank-axle are subjected to a bending moment which is transmitted by the ball bearings to the frame. The down-tube from the seat lug to the crank-bracket and the bottom tube from the foot of the steering socket tube to the crank-bracket are made fairly stout to resist this bending moment. Further, the pull of the chain causes a transverse bending moment in the plane of the chain-stays, which must be stiff enough under heavy pedal pressure.
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| Fig. 2. |
The tubular portions of the frame are made of weldless cold-drawn steel tube. The junctions or lugs are usually of malleable cast iron, bored to fit the outside of the tube, the final union being effected by brazing. In very light bicycles the tubes are kept thin, 22 or 24 W.G. (·028 in. or ·022 in. thickness) at the middle, and are strengthened at the ends by internal liners. Or butt-ended tubes are employed, the tubes being drawn thicker at the ends than in the middle. The steering post and fork sides especially should be thus strengthened at their junction with the crown. Some of the best makers use sheet steel stampings instead of cast lugs, greater lightness and strength being secured, and in some cases the sheet steel lugs are inside the tubes, so that the joints are all flush on the outside. The front fork blades are best made of sheet steel stamped to shape and with the edges brazed together to form a hollow tube. The sheet steel that can be thus employed has a much higher elastic limit than a weldless steel tube.
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| Fig. 3. |
Bearings.—Ball bearings are universally used. Each row of balls runs between two ball-races of hardened steel, one on the stationary member, the other on the rotating member. The outer is called the “cup,” and the inner the “cone.” One of the four ball-races is adjustable axially so that the bearing may run without any shake. The ball-races are often made of separate pieces of steel, but the crank-axle usually has the cones formed integral with it, the necessary hardness being obtained by case-hardening. According as the two cups face outwards or inwards the bearing is said to have outward or inward cups, and according as the adjustable ball race is the cone or cup, the bearing is said to be cone-adjusting or cup-adjusting. Fig. 3 shows a ball-bearing hub with outward cups. The hub-shell H is turned out of mild steel, and the cups C are forced into the ends of the hub-shell and soldered thereto. A thin washer W is then spun into the end, for the purpose of retaining oil, and a thin internal tube T unites the two cups, and guides the oil fed in at the middle of the hub to the balls. The projecting flanges S are for the attachment of the tangent spokes used to build the hub into the wheel. The spindle A has the two cones screwed on it, one C1 against a shoulder, the other C2 adjustable. The spindle ends are passed through the back-fork ends and are there adjusted in position by the chain-tension adjusters. After adjustment the nuts N clamp the spindle securely between the fork-ends. The chain-wheel or free-wheel clutch is screwed on the end of the hub-shell, with a right-hand thread. The chain being at the right-hand side of the bicycle (as the rider is seated) the driving pull of the chain tends to screw the chain-wheel tight against the shoulder. A locking-ring R with a left-hand thread, screwed tight against the chain-wheel, prevents the latter from being unscrewed by back-pedalling. With a free-wheel clutch screwed on the hub, the locking-ring may be omitted.
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| Fig. 4. |
Fig. 4 shows one end of the cup-adjusting hub, with inward bearings. The cones are formed of one piece with the spindles, and the adjusting cup C is screwed in the end of the hub shell, and locked in position by the screwed locking-ring R. The figure also illustrates a divided spindle for facilitating the removal of the tire for repair when required without disturbing the wheel, bearings, chain or gear-case. The chain side of the hub-spindle, not shown in the figure, is secured to the frame in the usual way; on the left side the spindle S projects very little beyond the adjusting cup. A distance washer W is placed between the end of the spindle S and the fork-end F. A detachable screw-pin, or the footstep, P, passes through the chain-adjusting draw-bolt B, the fork-end F, and the distance washer W, and is screwed into the end of the spindle S, the hexagon head of the detachable pin drawing all the parts securely together. On unscrewing the detachable pin, the distance washer W drops out of place, leaving a clear space for removing the tire without disturbing any other part.
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Fig. 5. |
The inward-cups bearing retains more oil than the other form. The pressure on a ball being normal to the surface of contact with the ball race, and each ball touching two ball races, the two points of contact must be in line with the centre of the ball. All the lines of pressure on the balls of a row meet at a point f on the axis of the spindle. The distance between the two points f (fig. 5) may be called the virtual length of the bearing. Other things being equal, the outward-cups bearing has a greater virtual length than the inward-cups bearing. In hubs and pedals where the actual distance between the two rows of balls is sufficient, this point is of little importance. At the crank-axle bearing, however, where the pedal pressure which produces pressure on the axle bearings is applied at a considerable overhang beyond the ball-races, the greater virtual length of the outward-cups is an advantage.
Fig. 5 shows diagrammatically the usual form of crank-axle bearing which has inward-cups and is cup-adjusting. The end of the bracket is split and the cup after adjustment is clamped in position by the clamping screw S. The usual mode of fastening the cranks to the axle is by round cotters C with a flat surface at a slight angle to the axis, thus forming a wedge, which is driven in tight. The small end of the cotter projects through the crank, and is screwed and held in place by a nut. The chain-wheel at the crank-axle is usually detachably fastened to the right-hand crank.
