TRACTIVE POWER OF HORSES.
The loads that a horse can pull upon various road surfaces will not necessarily be proportional to the resistance offered by the surface to traction, as the tractive force that the horse can exert depends upon the foothold afforded by the surface. The ability of a horse to exert a tractive force depends upon the strength of the animal, upon his training. for the par ticular work, and whether he be accustomed to the surface upon which he is travelling. , The work of dif ferent animals is therefore subject to considerable varia tions, and only very rough approximations are possible in giving average values of the work a horse may do under differing conditions.
The tractive force that may be exerted by a horse, at moderate speeds, varies approximately inversely as the rate of speed; or, in other words, the power that a horse can exert through any considerable time is nearly constant for varying velocities. Thus it may be assumed, as an average value, that a horse working regularly ten hours per day can put forth a tractive effort of 8o pounds at a speed of 250 feet per minute on an ordinary level road surface.
For the power of the horse we then have Power = force X velocity = 8o X 250 = 20000 lbs. per minute.
For any other rate of speed, as 200 feet per minute, we would have 20000 ± 200 = pounds as the tractive force exerted by the horse.
If the period of daily work be lessened, the power that may be developed will, increased, either by increasing the load or the velcity.
The tractive force that a horse is able to exert decreases very rapidly as the rate of inclination increases. This is due both to the expenditure of power by the horse in lifting his own weight up the grade, and to the less firm footing on the inclination. The effect of differences in the foothold afforded by various pavements is very marked in the loss of tractive power upon grades.
In the table below are given the loads that an average horse may be expected to continuously haul up different inclinations, on various road surfaces, at slow speed. These figures, while of little value as an
absolute measure of what may be done in any par ticular case, are of use as a rough comparison of the relative tractive properties of different surfaces and grades. The effect of grades upon tractive effort will also depend upon the condition in which the surface is maintained, and upon the weather. Snow and ice in winter, or the damp and muddy condition of some pavements in wet weather, have a very considerable effect to diminish tractive power.
In general, the tractive effort that a horse may exert is approximately proportional to the weight of the horse and the averages above given correspond to light animals; many horses are capable of exerting double the pull mentioned.
A horse may frequently exert for a short time a tractive force about double that which he can exert continuously; hence, when short grades occur steeper than the general grades of the road, loads may often be taken over them much heavier than could be carried if the steeper grade prevailed upon the road.
On ordinary country roads in dry weather the amount of load that can be hauled is usually deter mined rather by the grades than by the nature of the surface. Unless the gradients are very light the amount of load that can be carried on a broken-stone surface does not differ greatly from what may be taken on a dry and hard earth road. In improving a road by substituting a hard surface for a surface of earth the gradients and location should therefore always be carefully studied, with a view to deriving the full practical benefit from the hard surface in the light traction that it may require with easy ruling gradients.