Part 27 (2/2)

Two of the most important improvements recently added to the cycle are--(1) The free wheel; (2) the change-speed gear

THE FREE WHEEL

is a device for enabling the driving-wheel to overrun the pedals when the rider ceases pedalling; it renders the driving-wheel ”free” of the driving gear It is a ratchet specially suited for this kind of work

Fro the many patterns now222), which is extremely simple The _ratchet-wheel_ R is attached to the hub of the driving-wheel The s,” as it is often called) turns outside this, on a nuroove chased in the neck of the ratchet Between these two parts are the _pawls_, of half--wheel is assumed to be on the further side of the ratchet

To propel the cycle the chain-ring is turned in a clockwise direction

Three out of the six pawls at once engage with notches in the ratchet, and are held tightly in place by the pressure of the chain-ring on their rear ends The other three are in a midway position

[Illustration: FIG 222]

When the rider ceases to pedal, the chain-ring becomes stationary, but the ratchet continues to revolve The pawls offer no resistance to the ratchet teeth, which push the Each one rises as it passes over a tooth It is obvious that driving power cannot be transain to the road wheel until the chain-wheel is turned fast enough to overtake the ratchet

THE CHANGE-SPEED GEAR

A gain in speed -up a cycle we are able to -wheel revolve faster than the pedals, but at the expense of control over the driving-wheel A high-geared cycle is fast on the level, but a bad hill-clie on the flat, but is a good hill-cli-wheels, the goods engine s-wheels, to perform their special functions properly

In order to travel fast over level country, and yet be able to mount hills without undue exertion, we ear Two-speed and three-speed gears are now very commonly fitted to cycles They all work on the sa-wheels, theso devised that the hub turns more slowly than, at the same speed as, or faster than the s to the wish of the rider

We do not propose to do more here than explain the principle of the epicyclic train, whichround) a wheel”

Lay a footrule on the table and roll a cylinder along it by the aid of a second rule, parallel to the first, but resting on the cylinder It will be found that, while the cylinder advances six inches, the upper rule advances twice that distance In the absence of friction the work done by the agentthe force which opposes the forward h which the cylinder advances is only half that through which the upper rule advances, it follows that the _force_ which reat as that overco the cylinder The carter makes use of this principle when he puts his hand to the top of a wheel to help his cart over an obstacle

[Illustration: FIG 223]

[Illustration: FIG 224]

[Illustration: FIG 225]

Now see how this principle is applied to the change-speed gear The lower rule is replaced by a cog-wheel, C (Fig 223); the cylinder by a cog, B, running round it; and the upper rule by a ring, A, with internal teeth Weattached to the fixed axle It is evident that B will not et ahead of B can be calculated easily We begin with the wheels in the position shown in Fig 223 A point, I, on A is exactly over the topmost point of C For the sake of convenience ill first assu round C, B is revolved on its axis for one complete revolution in a clockwise direction, and that A and C224 If B has 10 teeth, C 30, and A 40, A will have been moved 10/40 = 1/4 of a revolution in a clockwise direction, and C 10/30 = 1/3 of a revolution in an anti-clockwise direction

Now, co back to what actually does happen, we shall be able to understand how far A rotates round C relatively to the225) B advances 1/3 of distance round C; A advances 1/3 + 1/4 = 7/12 of distance round B The fractions, if reduced to a common denominator, are as 4:7, and this is equivalent to 40 (number of teeth on A): 40 + 30 (teeth on A + teeth on C)

To leave the reader with a very clear idea ill summarize the matter thus:--If T = number of teeth on A, _t_ = number of teeth on C, then movement of A: movement of B:: T + _t_: T

Here is a two-speed hub Let us count the teeth The chain-ring (= A) has 64 internal teeth, and the central cog (= C) on the axle has 16 teeth There are four cogs (= B) equally spaced, running on pins projecting from the hub-shell between A and C How much faster than B does A run round C? Apply the formula:--Motion of A: motion of B:: 64 + 16: 64 That is, while A revolves once, B and the hub and the driving-wheel will revolve only 64/80 = 4/5 of a turn To use scientific language, B revolves 20 per cent slower than A

This is the gearing we use for hill-cli-wheel to turn as fast as, or faster than, the chain-ring To ether In one well-known gear this is effected by sliding C along the spindle of the wheel till it disengages itself from the spindle, and one end locks with the plate which carries A Since B is now being pulled round at the bottom as well as the top, it cannot rotate on its own axis any longer, and the whole train revolves _solidly_--that is, while A turns through a circle B does the sa, ed that the drive is transmitted from the chain-wheel to B, and from A to the hub

While B describes a circle, A and the driving-wheel turn through a circle and a part of a circle--that is, the driving-wheel revolves faster than the hub Given the same number of teeth as before, the proportional rates will be A = 80, B = 64, so that the gear _rises_ 25 per cent

By means of proper ear either (1) froear; or (2) from chain-wheel to A and C siear; or (3) froear In two-speed gears either 1 or 3 is omitted