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 be determined, 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. The rider 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, and 3, the arrows showing
the direction of progression.
[Illustration: GROUP I.
Fig. 1.
Fig. 2.
Fig. 3.]
[Illustration: GROUP II.
Fig. 4.
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. 4 and 5. 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. 4 and 5 does not allow of
any such possibility; the rider must have momentum enough to throw the
cranks over the dead centre or he is lost. In Fig. 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. 6 illustrates this as follows:
[Illustration: Fig. 6.]
In the descent of the lever from _b_ to _c_ the power will only be
transmitted through the arc between _d_ and _e_; taking an equal arc
from _f_ to _g_ for the power given on the other side, we have the two
small arcs _f d_ and _g 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 in Fig. 5, we have
an advantage, since the arcs _f d_ and _g e_ would 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 no dead 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 arc _d e_ raises and _d f g e_
lowers the pedal, it will then raise quickly and lower slowly; whereas,
if _d e_ lowers and _d f g e_ raises the pedal, it will raise slowly
and lower quickly.
The study of wheels in the market made with front-driving mechanism, on
the plan of Fig. 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. 4 and 5, 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 of Fig. 4 of 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.