Destructive Factors.
—From the above it appears that there is a general opinion that there should be some common measure for the destructive effect of vehicles upon road surfaces. As yet no unanimity of opinion has crystallized. While density of traffic influences the surface wear of the road crust—considerably in the case of earth and gravel, less for macadam and asphalt, and still less for brick and concrete—the actual weight of the wheel load seems to have a much greater destructive effect. The impact due to speed and irregularities of the road surface, the resiliency of the tires, the proportion of sprung to unsprung weight, and the shoving forces of the wheels all have their effects which are usually in some way connected with either the weight or the speed, or both, of the vehicle. The many experiments now being carried on by the United States Bureau of Public Roads, and the several states may furnish data from which a practical measure will some day be devised. Mr. Older, Chief Highway Engineer of the State of Illinois, under whose direction the comprehensive investigational and endurance tests under way in that state are being carried on, recently stated to a party of visitors, of which the author was one, that in his opinion weight, including impact, is the prime factor in the destruction of a pavement. Wear is of very minor importance, temperature and weather is of considerable importance.
Road surfaces must be considered as bodies acted upon by forces. Some day the stresses produced by these forces will have been analyzed, then will it be possible to standardize the importance of the several vehicle loads. At present it is known that the weight of the load and the weight of the pavement itself are under some circumstances sufficient to produce cracks in the pavement and disruption of the road crust. Bearing tests and bending tests are being devised to measure the effects of such loads. Road crusts, earth, gravel, macadam, asphalt, brick, concrete, are to varying degrees elastic bodies and when loaded they give, as an elastic band stretches, a spring shortens, or a bow bends, until the internal stresses reach a limiting point where the crust is broken or permanently distorted. It is well known that the effect on an elastic body of a suddenly applied load is twice as destructive as the same load gradually applied. And when the action is an impact the destructive effect may be very great indeed, depending on the physical properties of the impinging bodies. But however the load is applied, whenever the internal stresses reach the limiting strength of the material of which the road crust is composed it will go to pieces. The sudden application of the load by fast driving is a sort of impact. The stresses produced by this impact are now being studied. Much good is expected to come toward the solution of the problem of destructive vehicle influence from these researches.
Another effect of speed is noted on the more or less viscous materials of which road surfaces are composed. The pushing of the wheels against the surface causes wrinkles which continue to grow until the wrinkles become waves entirely across the pavement. Such waves may also be produced by expansion and contraction due to changes in temperature, but are probably always accentuated by wheel pressure. Side thrust of wheels often produces longitudinal waves in viscous road crusts.
In the classifications given no one seems to have considered the proportion of sprung and unsprung weight in the motor car. There can be no doubt but that the resiliency of the springs relieves the pavement of very much of the shock of impact. This is illustrated by an attempt to drive a nail into a springy board. It can hardly be done because the springiness of the board uses up, absorbs, the work of impact. A mechanical statement is, the work of impact equals the change in kinetic energy, or algebraically stated
Fs = Wv22g
when the entire energy has been absorbed. Here F is the acting force and s the distance through which it acts, Fs, is the work done by the force F. W is the weight of the ram or moving body (vehicle, wheel load), v the velocity of impact and g the acceleration of gravity, a factor that enters the equation in the expressing of mass in terms of weight. Solving this equation for F there results,
F = Wv2 2gs,
which shows that the smaller s is the greater the force of impact F. When s is made long by means of a spring the force F becomes smaller. This is illustrated by the old method of catching a baseball without gloves—the hands were allowed to go backward so that the work of stopping the ball was spread over a greater distance, the impact force thus becoming so small it did not sting the hands.
The effect upon the road, and also the vehicle, is like that of the hammer which hits a nail on the anvil. The nail is flattened, pounded to pieces very soon. But if the nail were not placed upon the solid anvil but upon a slab of springy steel, it might be pounded all day without doing it much harm, the spring at all times absorbing the shock. So with the weight of the vehicle largely sprung the damage to the roadway is comparatively small. Therefore, it would seem, as though a fair classification would take into account the springs of the vehicle.
The pneumatic tire, and the cushion tire and wheel, each act as springs and shock absorbers in varying degrees. In some of the censuses, pneumatic or solid tires were noted, and very many of the earlier noted whether rubber or steel tires were used.
Just how far all these things should be taken into account is questionable. Whether or not just as good results would not come for even a simpler classification is not yet determined. It might be that only the heavy loads and their frequency is all that need be considered if the destructive effect of traffic alone is aimed at.
The great amount of pleasure riding and the tremendous desire for such riding should be considered in laying out a system of roads and in the selection of a type of road, therefore all passenger cars and motor cycles should be counted and given an influence number.