CRANKS AND LEVERS AND TANGENT SPOKES.
The subject of cranks and levers has been touched upon from a
philosophical stand-point, but an ingenious squib in a maker’s
catalogue on the subject has suggested the propriety of treating its
mechanical features more fully. The squib referred to runs as follows:
“CRANKS VS. LEVERS.
“The question of a motive power for cycles is as old as the
first idea of wheel riding. Inventors, after having persistently
tried and abandoned every other known motor,—steam, electricity,
etc.,—have made every effort to discover the best way of applying
leg-power.
“While nine-tenths of the cycles have always been driven by cranks,
in a few cases the attempt has been made to show that power could
absolutely be created by the use of levers, and that if the power
could be applied on one side only of the axle, avoiding the return
stroke of a crank, the result would equal a man’s lifting himself
over a fence by his boot-straps.
“In their eager pursuit of this one idea its advocates have lost
sight of the fact that the question is of the economical use of
the power we have, and that it is as impossible to _create_ a
power as to overcome the laws of gravitation. For hundreds of
years the machinery of the world, practically speaking, has been
driven by cranks. In this fact we have the testimony of the highest
mechanical genius the world has known.
“Engineers agree that the crank is the only economical method of
applying power—that it transmits to the driving shaft ninety-nine
per cent. of the power applied. In no class of machinery except
cycles is the attempt made to use levers where cranks could be used.
“Careful experiments have shown that the use of a lever is
misleading, in that, while power can be converted into speed and
speed into power, the development of either is at the expense of
the other. It is at once evident that with levers we have more
friction, more weight, and more complication than with cranks, and
that absolutely more power is required, as the springs which are
used to return the levers must be forced down at the expenditure
of power which should be applied to the propulsion of the machine.
Several years ago lever-power was tried in England on bicycles and
tricycles and extensively introduced, but has been so generally
abandoned that there is to-day no machine of importance so driven.
The worst feature of the lever action, however, is that the
movement of the foot does not become automatic, as is the case
in the use of the crank. There is absence of regularity, and a
consequent loss of momentum. A rotary motion is more natural to
the feet, being more like walking, while a lever motion is like
treading water while swimming, or like constantly climbing up
stairs. Not only does the mechanical use of the legs require a
regular movement, but it is better to use always the same length of
crank, never varying the throw.
“A special set of muscles can be trained to such work as the use
of the lever action; but such development is abnormal and at the
expense of other parts of the body.”
There is little doubt in the minds of reasonable people that a good
machine can be made either with cranks or levers; and this possibility
makes it an interesting point in cycle discussion. It is hardly fair,
however, to hold a maker responsible for matter written for the
purpose of advertising his wares, nor do I wish to do so. The article
above quoted puts, in unique form, the opinions of a large class of
observers, and for that reason it is given here. I take up the lever
side of the question simply because there is more to talk about on that
side, and also perhaps for the reason that I have had large experience
at considerable cost in experimenting on different forms of levers.
Some of the remarks about “creating power” are true, but might be
applied equally well to some of our crank theorists.
To say that the machinery of the world is driven by cranks, is hardly
tenable; even though the engine generally has a crank. But now, since
we must reduce our comparisons down to the human motor, in combination
with the crank of a bicycle, let us say the pitman rod represents the
man’s leg. This rod has to push and pull, which a man cannot do with
one leg; but for this you say he has two legs; admitting, then, that
two legs represent the pitman, we are still out a fly-wheel and an
evenly-running resistance. (See chapter on “Connecting Link.”)
A great deal of the power of machinery is transmitted through pulleys
and belts; now I take it that this is much more similar to some of the
drum and lever machines than to a simple crank. There is, however, a
form of lever and crank combined, of which I have spoken elsewhere,
that is really worse than any simple form of either, but we have just
as much right to say the crank ought to make it good as to say the
lever makes it bad; if the crank is such a great cure for all evils,
as the maker quoted seems to imply, it ought not be so bad in any
combination.
There is no loss of power in pushing down a spring if it is only just
strong enough to lift the leg, since the leg would otherwise have to
be lifted by the expenditure of muscular energy. In using a spring
we press down with a little more weight than is required to run the
machine, so that a storage of power is the result which is given out
in lifting the leg. In fact this is done to some extent in the crank
machines; the rider not only puts enough power on each crank to turn
the wheel, but also enough to lift the other leg; this is true at least
when the rider is quite tired. Examples are known wherein a racer on
long distances could no longer lift his legs, even with the aid of a
spring, though at the same time, he still had strength enough left to
propel the machine. In fine, this difference between the crank and
spring lever is that in the former, a little extra power is exerted to
lift the _other_ leg, while, in the latter, energy is stored to be used
in raising the _same_ leg.
In a perfectly fresh man I have found, by the registers of the
cyclograph, that the rider lifts all weight from a returning crank, but
this does not happen when he becomes tired. Evidently, if the spring is
strong enough to more than lift the leg, a loss of power will result,
since the rider would have to hold it in check even in coming up in
order to keep it from stopping with a bang, as is sometimes noticed
when he jumps from a treadle machine. The winding and unwinding of the
spring involves no loss of power except in heat incident to motion and
imperfect elasticity, which is quite small. This loss from heat within
the molecular structure, I am constrained to think, is not what is
popularly meant by loss of power in springs.
Coming back to our quotation, true, in England levers have been tried
and expunged. A prominent American, I believe, assisted some little in
enlightening our too susceptible English brethren on the subject, yet
some attempts have been made with them in this country which no fair
person can call unsuccessful.
A little printer’s ink will answer the last sentence of our quotation.
Simply change the words “lever-action,” and substitute “cranks,” and
you will have the following: “A special set of muscles can be trained
to such work as the use of cranks, but such development is abnormal and
at the expense of other parts of the body.” So the reader can see how
a little slip in the type would have changed the whole argument. This
discussion could be continued with great interest to both sides if we
could only find in some maker’s catalogue of lever machines an attempt
to “down” the crank machine on general principles. As it is, it must
close for lack of antagonism in so far as broad principle goes.
As to the construction of crank machines, the subject is so familiar
to every one, and the device is so simple, that it is impossible to
write much of an essay on it. With regard to levers, however, the
subject is inexhaustible. The most salient features claimed for the
clutch machines now in the market are, first, non-dead centre,—that
is, even, continuous power; second, entire rest of the legs when
power is not required. The objections are chiefly, first, insecurity
and entire dependence on the brake found in the absence of all back
pedalling; second, non-support of the legs, springs being insufficient
to sustain their weight. To the above objections appertaining to the
lever and clutch machine, a third may be added,—viz., the complexity
of parts, liability to breakage, and danger of accidents therefrom. At
one time the advantage of safety was found in the clutch machine almost
exclusively, but at the present time we have complete safety elements
in certain forms of crank-wheels.
Much difficulty has been experienced by makers of lever cycles in
finding a suitable clutching device, a difficulty with which most
of the experience the writer has had is concerned. In conducting
experiments in this line I have found that the rattle of the old
ratchet was annoying, and it was quite a problem in my mind why makers
used them; but any one who undertakes to make a bicycle clutch will
soon discover the reason, though at what cost “deponent sayeth not.”
A neophyte in the bicycle experimenting ranks might justly suppose
that the matter of clutches is a well-developed art in mechanics; to a
certain extent it is, but not in the direction he will need. Clutches
may be divided into three classes,—first, the common ratchet and pawl,
either spring or gravity; second, the ratchet and friction pawl; third,
surface-friction clutches proper. The first two grip on corrugated
surfaces, the last on a perfectly plain or smooth surface. The first
class rattles according to the pressure on the pawl or the weight of
the same, and also to the amount of drop. The second class rattles
only under certain conditions; that is, when both ratchet and pawl are
in motion in the same direction, one moving a little faster than the
other. The third class is entirely noiseless. Let us pass over the
first class, as being familiar to everybody. The second class is not so
well known and has never been used in any of the arts in this country
so far as I know, except as recently applied to bicycles. This clutch
is very similar in appearance to a regular ratchet, the difference
being that in the former the pawl is held out of contact by friction
against some of the moving parts, and when the motion is reversed the
friction in a certain direction throws the pawl into action. A good
mechanic would have hardly conceded such an arrangement as practicable
in any machine, much less in a bicycle, for the reason that when the
motion is reversed the pawl plunges into the teeth with so much force
that damage would be supposed to result. Several patents are registered
in England upon the noiseless ratchet; they are all alike in general
principle, but it is due to the energy of an American maker that it has
been made a success in cycle construction, and I am inclined to think
it is the first time such a ratchet has ever been used to any extent in
any kind of machinery.
[Illustration: Noiseless ratchets.]
As to the third class of clutches, much of interest can be said for
the benefit of those particularly concerned. “A friction clutch” to
mechanics is a familiar term, since the name is applied to all pulley
clutches, that grip on a smooth surface. Many of these clutches are a
success for the purpose for which they are intended. The most common
form used on machines where the requirements are similar to those of a
cycle, is the “Roller.” The cycle experimenter nearly always strikes
upon this clutch first, and with sufficiently good reason. It has
been adopted in many arts, and is used in England upon tricycles in
combination with cranks, with moderate success, but just here allow
me to call attention to a cardinal difference in the requirements
of a clutch as used on crank tricycles and successfully in the arts
heretofore. In the crank-clutch cycles the clutch is used for the
purpose of detaching the cranks from the spindle when the machine
requires no driving, as in running down grade, but when once the clutch
is gripped, it remains so till further power ceases to be required.
Now, this is also just the action of all belt-pulley clutches, and
between such action and that required in a lever-clutch cycle the
difference is exceedingly conspicuous. In the crank-clutch cycle, as
in other uses, the immediate solid grip is a matter of little concern;
if a half turn of the parts takes place before clutching, it does
little harm, since it is so small a fraction of the entire number of
revolutions to be made before the grip is released. But if a grip is to
be taken at every down stroke of the foot, as in a lever-clutch cycle,
the least slip or lost motion is fatal.
This incessant clutching action, together with the great weight the
parts have to sustain, and the repeated concussion of one piece upon
another under this weight, makes up a combination of disturbing
elements which will cause mischief against which it is almost
impossible to provide.
In a form of roller-clutch I have tried, the inner frame or carrier is
made loose upon a spindle.
In the drawing herewith annexed we have first a spindle in the
centre, then a little open space around it, and then the clutch frame
_bb_, which is connected loosely, not rigidly, to the drum. By this
arrangement the pressure is distributed evenly upon the three rollers
_d_, _d_, _d_, outwardly at three points against the casing, and in no
event is the work done by a single roller. This device worked as well
as any of this class I have tried; but the patterns are for sale at a
very reasonable price. The main trouble I found in this contrivance and
all other roller clutches was, that the great pressure disintegrated
the oil, making a paste that would cause the rollers to slip in spite
of everything.
If it were not that another American, a cycle-maker, has apparently
made a success of a roller-clutch, I should be tempted to warn all
experimenters against it as a thing that “stingeth like a serpent and
biteth like an adder.”
[Illustration: Loose centre roller-clutch.]
Under a bench in a shop not far from the geographical centre of
England may still be found about a bushel of friction-clutches of
various and ingenious forms, which future historians in the art will
find very interesting. Should any one wish to enter the arena as a
searcher for the true friction-clutch, let him first examine these
specimens, and he will start several years ahead. The nearest approach
to a success which the writer has fallen upon is illustrated below for
the purpose of helping those who may wish to carry on the search, or
experiment in clutch-cycles,—if any should think it worth while in view
of the alleged success of the American above referred to. The clutch
illustrated below was contrived by a fellow-laborer in the field. The
drawing represents the device in a crude form; some improvements having
been necessary to complete it.
_B_ is a cog-wheel within another, _A_, the latter fast to the
wheel-hub, and the former to the clutch-drum. A wedge, _E_, follows
between the wheels, whence it will be seen that they can revolve, in
relation to each other, in one direction only.
[Illustration: Scott wedge-clutch.]
For those who wish to study this question more minutely, Kempe, on
link motion, will be found a valuable work in connection with the
construction of levers in any art, when it is desired to obtain a
motion in a straight line from an oscillating or circular.
In the way of conclusion, reverting to the possibilities of direct
application of these remarks to the actual purchase and use of cycles,
I wish to say, in regard to the mechanical difficulties in this
matter of lever and clutch machines, that so long as the use of oil
is necessary, I have very grave doubts if a thoroughly satisfactory,
noiseless friction-clutch for use on cycles will ever be invented.
TANGENT VS. DIRECT SPOKES.
[Illustration: Tangent spokes.]
The subject of Tangent _vs._ Direct Spokes, or Direct vs. Partial
Tangent, is one on which so much has been written and said within
the last few years that it is probably well understood in the main
by all enthusiastic wheelmen, but a few points may not come amiss to
the beginner. In the first place, there is no such thing as partial
tangency. A tangent spoke is tangent, and that is all there is about
it. A tangent is a definite thing, and means a line normal to a radius
at the circumference; at least, we can accept this definition as well
enough suited to the cycle art. And, in speaking of tangency, we ought
rather to say tangent hub than tangent wheel, since the spokes are not
tangent to the rim of the wheel, but to the hub. All cyclists know very
well, nevertheless, what is meant by partial tangency in the cycle art,
and I will therefore use the term. If a long spoke went straight from
one point in the rim to another nearly opposite, and just touched the
outside circumference of the hub in one place, it would make two purely
tangent spokes. (See cut.) As, for instance, _a b_ and _cd_ make all
together four spokes, _af_, _bf_, _de_, and _ce_. If a spoke runs from
any point, _a_, _c_, _b_, or _d_, to any point on the circumference
of the hub between _f_ and _e_, it will not be a full tangent spoke.
The distinctive characteristic of a full tangent spoke is that, when
the force tending to revolve the wheel is applied, it pulls from the
point on the hub which would recede most rapidly from that point in the
rim to which the other end of the spoke is affixed. Hence, the common
expression that “a tangent hub gives a direct end-pull on the spokes;”
but so does any other hub, if the spoke is swivelled into it. With a
direct spoke screwed into the hub, the weight of the man is sustained
by a direct end-pull, and a slight power is transmitted to the rim by
the resistance to flexure or bending in the spoke tending to revolve
the wheel, and it will be found in practice that any hub with a direct
spoke will turn independently of the rim far enough to increase the
distance slightly between the ends of the spokes so as to really make
an end-pull as in the tangent spoke, but evidently the hub must revolve
a great way in order to increase the length a very little. Here comes
in the advantage of the tangent spoke, for, in order to turn the
hub within the rim, the spoke has to stretch an amount equal to the
distance a point on the circumference of the hub moves. To represent
this in popular terms, if the hub turns one-eighth of an inch, the
spoke has to stretch that amount if tangent, whereas the necessary
increase in length of the direct spoke is almost imperceptible.
One point must not be forgotten in this matter, which redounds to
the credit of the absolute direct spoke. It is that the driving
strain passes through every spoke from the hub to the rim, whereas,
in a tangent or partial tangent spoke, the strain is resisted by only
one-half of the entire number. This defect is partially remedied by the
late plan of soldering the spokes together at the points of crossing,
this binding together being what really makes the tangent spokes so
strong in resisting buckling, to which they were very liable before
the soldering process was used. I am inclined to think that the midway
or partial tangent hubs are the best, as they seem to combine all of
the possible advantages, but the plan of crossing the spokes just once
is, in the light of my experience, very bad, as it seems to combine
the faults of both with the advantages of neither; they should be more
nearly full tangent than direct if varied from the midway position at
all. The small eighteen- or even thirty-inch wheel is good enough, if
well made, with either direct or tangent hubs, especially in the one
not used as a driver.
[Illustration: Old bone-shaker wheel.]
The soldering of the spokes together, and other difficulties in the
way of screwing them into tangent hubs, has led makers to adopt the
plan of screwing them into the rim; this seems unavoidable, but is not
very desirable, if for no other reason than that the wheel getting wet,
the screw threads are apt to rust off and strip. With brass, aluminum,
or bronze nipples, however, this difficulty can be to a great extent
overcome.
Tangent wheels are as old as the industry of cycling. Starley, of
Coventry, is said to have experimented and shown, many years ago, that
a tangent wheel with silk spokes would resist the revolving strain on
the hub equal to a direct wire spoke, and the _Scientific American_
gave an illustration of a tangent hub in their issue of September 1,
1877.
The cross bar in the old bone-shaker made practically two tangent
spokes, and pulled from the rim, so to speak, as will be noticed in our
essay on hobbies.