When it was first discovered that a smooth-faced driving wheel,
running on a smooth-faced rail, would "bite," the era of iron railways
and locomotive engines may be said to have fairly commenced. The
correction of a single radical error was, in this case, the dawn of a
new system of travel, so extensive in its growth and marvelous in its
results, that even the wildest dreamer could not, at that time, have
imagined the consequences of so simple a discovery.
A popular and somewhat similar error regarding the bite of wheels on
rough and uneven surfaces, has also prevailed. We say popular error,
because engineers have not shared it, and it has obtained, to any
notable extent, only among those unfamiliar with mechanical science.
The error in question is, that hard-surfaced wheels will not bite on
a moderately rough surface, sufficiently to give an efficient tractile
power. It seems strange that this error should have diffused itself
very extensively, when it is remembered that a certain degree of
roughness is essential to frictional resistance. The smoothness of the
ordinary railway track is roughness compared to that of an oiled or
unctuous metallic surface; and it has been amply demonstrated that
the resistance of friction, of two bearing surfaces depends, not
upon their extent, but upon the pressure with which they are forced
together. A traction wheel, of given weight, resting upon two square
inches of hard earth or rock, would develop the same tractile power
as though it had a bearing surface of two square feet of similar
material.
On very rough and stony ways, however, another element practically of
no importance on moderately rough ways, like a macadam surface or a
concrete road, where the prominences are nearly of uniform hight, and
so near together as to admit between their summits only very small
arcs of the circumference of the wheel; comes into action. This
element is the constantly recurring lifting of the superincumbent
weight of the machine. Even this would not result in loss of power,
could the power developed in falling be wholly applied to useful work
in the direction of the advance of the engine. The fact is, however,
that it is not so applied, and in any method of propulsion at present
known to engineering science, cannot be so applied. Above a certain
point where friction enough is developed to prevent slip, the more
uneven the road surface is, the greater the power demanded for the
propulsion of the locomotive. And this will hold good for both hard
and soft-tired wheels.
What then is the advantage, if any, of rubber-tired wheels? The
advantages claimed may be enumerated as follows: increased tractile
power, with a given weight, secured without damage to roadways; ease
of carriage to the supported machinery, whereby it--the machinery--is
saved from stress and wear; and economy of the power, expended in
moving the extra weight required by rigid-tired wheels, to secure the
required frictional resistance. The last-mentioned claim depends upon
the first, and must stand or fall with it. The saving of roadway,
ease of carriage, and its favorable result upon the machinery, are
generally conceded.
A denial of the first claim has been made, by those interested in the
manufacture of rigid-tired traction engines and others, in so far
as the rubber tires are employed on comparatively smooth surfaces;
although the increased tractile power on quite _rough_ pavements and
roads is acknowledged.
This denial is based upon results of experiments performed on the
streets of Rochester, England, between the 9th October and the 2nd
November, 1870, by a committee of the Royal Engineers (British Army),
with a view to determine accurately the point in question.
Care was taken to make the circumstances, under which the trials
took place, exactly alike for both the rubber and the iron tires. The
experiments were performed with an Aveling and Porter six-horse power
road engine, built in the Royal Engineers' establishment. The weight
of the engine, without rubber tires, was 11,225 pounds; with rubber
tires, it weighed 12,025 pounds. Without rubber tires it drew 2.813
times its own weight up a gradient of 1 in 11; with rubber
tires, it drew up the same incline 2.763 times the weight of engine,
with the weight of rubber tires added; showing that, although it drew
a little over 2,200 pounds more than it could do without the rubber
tires, the increase of traction was only that which might be expected
from the additional weight.
It is claimed, moreover, that the additional traction power and
superior ease of carriage on rough roads, secured with rubber tires,
is dearly bought at the very great increase in cost, of an engine
fitted with them, over one not so fitted.
This is a point we regard as not fully settled, though it will not
long remain in doubt. There are enough of both types of wheels now in
use to soon answer practically any question there may be of durability
(upon which the point of economy hinges), so far as the interest on
the increased cost due to rubber tires, is offset against the greater
wear and tear of iron rimmed wheels. It is stated, on good authority
that a rubber tired engine, started at work in Aberdeen, Scotland,
wore out its tires between April and September, inclusive, and when
it is taken into consideration, that the cost of these tires is
about half that of other engines, made with solid iron rimmed driving
wheels, it will be seen that, unless very much greater durability than
this can be shown for the rubber, the advantages of such tires are
very nearly, if not more than, balanced by their disadvantages.
The fact that one set of tires wore out so soon does not prove a rule.
There may have been causes at work which do not affect such tires
generally, and it would be, we think, quite premature to form
favorable or unfavorable judgment, of relative economy from such data
as have been yet furnished.
The difference in the current expenses of running the two most
prominent types of engines, with hard and soft tires, now in use, does
not affect the question of rubber tires, unless it can be shown that
these tires necessitate, _per se_, such a form of engine as requires
a greater consumption of fuel, and greater cost of attendance, to
perform a given amount of work.
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