CHAPTER V.
THE CONNECTING LINK BETWEEN THE LEGS OF NATURE AND THE WHEEL OF MECHANICS.
We now proceed to compare the different modes which have been devised to transmit power from the rider to revolve the wheel; of these there are two principal classes, the simple crank and the lever and clutch. These devices or connecting links relate to the motion of the legs as well as to the power transmitted through them. It is not necessary to treat of the horizontal motion of the limbs, as it is of little consequence provided the rider remains substantially over the work. Power is applied mainly through the vertical resultant, and the consequent fatigue is the effect of the amount of energy given out in a vertical direction. Crank riders acquire a horizontal power, or resultant force, by what we call ankle-motion, which has, to quite an appreciable extent, overcome the most serious inherent defect of the crank device; without this force the dead centre appertaining to the crank, in which the vertical resultant has no power to turn the wheel, would have made it a prey to the champions of other contrivances.
The above remarks in regard to horizontal motion and resultant force apply equally well if the rider is not over the work, except in that the phraseology would be different. A man in straightening out his leg can apply power in a certain direction or in a certain line; now, if he is not over the work, this will not be a vertical line; hence the term horizontal motion would have to be called motion at right angles to the line of transmission of power.
The importance of the dead centre is too great to be passed over without some further discussion. It would be a source of great satisfaction if a general conclusion could be reached in this crank versus lever and clutch controversy, but aside from the difficulty of drawing our conclusion there is a lack of a specific hypothesis in regard to an important element of the problem,—to wit, that as to the nature of the road and other resistance and consequent speed attainable or usually desirable. There is little doubt but that, so far as present developments show, the crank machine has excelled upon a smooth road and at high speed; yet this very fact leads us to suspect that perhaps for rough roads and at slow speed it might be objectionable, for it is easy to see that all questions of dead centres would eliminate themselves at high speed. Taking a steam-engine, of the crank and pitman type, for example, there is no trouble so long as speed is kept up, but it is well known that a certain velocity must be maintained or the crank will stop at the dead centre, even when provided with a heavy fly-wheel. Now, in a bicycle there is practically no fly-wheel at all, and, to pursue the comparison still further, we know that if the fly-wheel of an engine were removed great trouble would ensue; still it might be possible to keep running if the speed were great enough. It is evident, from common observation, that for intermitting slow and high speeds an engine, or any other machine, constructed without a fly-wheel must be provided with some means for continuing the power or carrying it over what would otherwise be dead centres. Multiple cylinders and rotary engines are made to serve this purpose.
The commonly accepted idea that a cycle for racing purposes upon a smooth road is a certain guide as to the requirements under other conditions is therefore hardly justifiable. For best results the form of mechanism used as the connecting link between the legs of nature and the wheel of mechanics must bedetermined, or at least be modified, by the conditions under which we intend to work. This problem is not at all confined to the art of cycling, it appears in many departments of mechanics. The same question has been mooted in respect to sewing-machines, and non-dead-centre attachments have been made and used upon them, but naturally the demand was not urgent, as this machine comes within the realm of high-speed devices with fly-wheel and evenly-running resistance. In scroll sawing by foot-power and in portable forges, non-dead-centre clutches are used with great effect. Hence our general mechanical experience makes it safe to say that such modes of continuous application of power have valuable uses applicable to this problem. It is not attempted to set up a definite unequivocal comparison or dictum in this matter as applied to cycles, for it is the desire of the writer and his right to make conclusions comparable only to the proofs recognized in practice, which in this case, in the cycle art, appear to be in favor of the crank machine. However, the writer’s opinion, based upon his theory and individual experience, is that we have more to fear from the weight, complication, and friction of parts in the lever and clutch than from the inherent principle of transmitting power upon which it works, and that some non-dead-centre device will finally prevail in the best all-around road cycles, if it can be relieved of purely mechanical objections which somehow seem to be naturally coupled with it. If the writer’s conclusion in this respect is tenable, the induction would follow that such a system, or connecting link, forms the most economical mode of applying power. The body can stand a steady, even pull upon its energy better than uneven intermitting jerks, and I feel sure new riders who have not acquired the ankle-action on the crank cycle will agree in this. This theory will apply to hill-climbing, in which lever and clutch machines have made so enviable a reputation. Therider has in clutch machines an even, steady resistance during the entire downward thrust, and he does not have to get all his power doubled up into a few inches of motion.
The two principal classes of connecting links, the crank and ordinary form of lever and clutch, need no explanation or discussion beyond their fundamental characteristics, but there are several combinations of lever and crank which are of interest and properly come under the head of modifications of the crank. These modifications are numerous in the market, and there exists cardinal distinctions between them. We annex diagrams of five distinct types which fall into two groups, the first group being a combination of lever and crank, in which the foot has an oval motion, as shown by Figs.1,2, and3, the arrows showing the direction of progression.
GROUP I.
Fig. 1.
Fig. 1.
Fig. 1.
Fig. 2.
Fig. 2.
Fig. 2.
Fig. 3.
Fig. 3.
GROUP II.
Fig. 4.
Fig. 4.
Fig. 4.
Fig. 5.
Fig. 5.
Fig. 5.
The second distinctive arrangement of lever and crank is where the lever is pivoted so as to return over the same track in which it descends, as shown in Figs.4and5. The first group, with its oval motion, has a decided advantage in regard to dead centre or continuous power; since by an ankle-motion the rider can transmit some power in a circular direction to the crank; that is to say, he can actually push to some extent in a forward horizontal direction. But it will be seen that the pivotal connection shown in Figs.4and5does not allow of any such possibility; the rider must have momentum enough to throw the cranks over the dead centre or he is lost. InFig. 4, which represents a form of pivoted treadle used on a reputable make of front-driving machine, it will be noticed that the rider has less than one-half of the revolution of the crank in which any power can be transmitted at all, which becomes apparent in observing a pedal in such devices while in motion, from the fact that it descends more rapidly than it ascends, thus giving the rider less than half the time in which he can transmit any power. We are now speaking of one side only of the machine; taking both sides together, there are two short arcs of a circle in which there can be no propulsive power transmitted to the wheel on either side.Fig. 6illustrates this as follows:
Fig. 6.
Fig. 6.
In the descent of the lever frombtocthe power will only be transmitted through the arc betweendande; taking an equal arc fromftogfor the power given on the other side, we have the two small arcsf dandg e, all of whose points are dead points, and we might say we have a dead line. Upon the other hand, if the machine happens to be driven in the opposite direction from that of which we have been speaking, or, in other words, if the pedal is in advance of instead of in the rear of the driving-axle, as seen inFig. 5, we have an advantage, since the arcsf dandg ewould represent arcs in which the rider has power on both treadles instead of on neither, and it might be said that, instead of having an arc of dead centre or no power, we have considerably less than nodead centre at all. The lever and crank,Fig. 5, is a device used on some rear-driving machines,—the pedal descends slowly and ascends rapidly; certainly a desirable arrangement. That is to say, if the arcd eraises andd f g elowers the pedal, it will then raise quickly and lower slowly; whereas, ifd elowers andd f g eraises the pedal, it will raise slowly and lower quickly.
The study of wheels in the market made with front-driving mechanism, on the plan ofFig. 4, suggests an incontrovertible argument in favor of getting over the work, in spite of the difficulty noticed in respect to dead centres; such machines actually have a creditable reputation as powerful hill-climbers and rough-road machines, which can only be explained on the theory that the vertical application of power more than makes up the deficiency caused by the arc of no power at all.
In speaking of the second group, Figs.4and5, it must be understood that the matter of driving from either the front or the rear wheel has nothing to do with the principle, except in so far as it regulates the arrangement of the pedal and the direction of translation appertaining thereto. The difference in principle depends on whether the driving or the returning arc of the crank is towards or farther from the pedal. It strikes me that the style of lever and crank of the first group is a kind of cross between the direct crank and the pivoted lever and crank of Group II., and especially ofFig. 4of that group, since it possesses some of the advantages and some of the objections found in both.
I find from observations, which will be spoken of later, that the ankle-power in the direct crank is very considerable, and that it is diminished in the oval-motion lever, Group I., and that it disappears absolutely in the pivoted lever, Group II. These facts are really evident, but as they came within the domain of other experiment, I merely state the result.