Benefits gained by distributing truck weights and loads among six wheels rather than four, include less liability to cause road destruction, greater carrying capacity and more economical operation. The author classifies the causes of road destruction under headings of excessive loads on tires, impacts between road and tires, traction effects of wheels, and braking effects and says that the remedy is to reduce load or to correct improper weight-distribution. Impacts probably contribute most destructive effects. He describes in detail what happens when a truck traveling at a given speed strikes a road obstruction and how impact forces are exerted and cites tests by the Bureau of Public Roads to show that when a truck equipped with solid rubber tires and traveling at 16 m.p.h. hits a road obstruction 1 in. high, the impact on the road surface is seven times the load on the tire; that is, for an 8500-lb. load on each rear wheel, the intensity of blow imparted to the road surface is nearly 60,000 lb. Average impact is about four times the static load; with pneumatic tires it is about 25 per cent more than static load.
Impact intensity depends primarily upon mass and upon acceleration during upward and downward flight of the wheel and the axle; upward, when the wheel hits an obstruction and flies up; downward, when the wheel comes down after reaching maximum upward travel, acquires velocity under the force of gravity and spring pressure and hits the road surface another blow before coming to rest. Regarding unsprung weight, the author states that the greater the weight of the wheels, axles and tires and the stiffer the springs, the greater the impact. Less unsprung weight and more flexible springs reduce impact. The truth of this is illustrated and explained.
Engine propelled tractive effort causes driving wheels to tend to slip. The greater the speed and the smaller the ratio of load on driving wheels, the greater the tendency for wheels to spin and grind away road surface; this tendency is lessened when a truck has four driving wheels or if the load on the driving wheels is relatively large.
The first advantage gained by distributing a given load over a greater number of wheels is the reduction of load on each wheel. With the type of six-wheel construction under detailed consideration, the two rear wheels on each side are tied together by a wheel-connector that is swiveled at center where it is attached to the spring above. When one wheel is raised, the tendency is to raise the chassis only one-half the distance that the chassis rises in ordinary four-wheel construction. Hence, when passing over an obstruction that raises one wheel to a certain height, since the center of the connector tying the two axles together is raised only one-half this height, the wheels can go over larger obstructions than is possible with four-wheel construction without raising the chassis to more than one-half the distance, provided both wheels on the same side are not raised at the same time. The springs between the axles and the chassis are flexed only one-half the usual amount, even though the wheel should rise the same distance as for a four-wheel truck; therefore, six-wheel construction lessens impacts between the wheels and the road considerably.
Comparing percentages of pay load to dead weight of truck, Mr. Favary quotes for the six-wheel truck a total load of 34,000 lb., a pay load of 20,000 lb., a dead weight of 14,000 lb. and a pay-load to dead-weight ratio of 142 per cent; for a modern 5-ton four-wheel truck with a dump body and a hoist, a dead weight of 11,000 lb. and a pay load of 10,000 lb., or a similar ratio of only 91 per cent.
Supposing a demand for the transportation of 1000 tons of merchandise per day over a stated route, the author estimates the needed number of six-wheel trucks weighing 7 tons each and having a pay load of 10 tons each to be 100, or a total dead weight on the road of 700 tons; and the number of four-wheel trucks weighing 5½ tons each and having a pay load of 5 tons each to be about 200, or a total dead weight on the road of 1100 tons. In such case, an extra 400 tons per day must pass over the road with four-wheel units and, even if but 180 of these were needed to equal the performance of the six-wheel units described, it would call for 80 more trucks and 80 more drivers.
Impact tests by the Bureau of Public Roads show the impact forces of a six-wheel truck loaded with 6 tons to be lower than those arising from an ordinary 2-ton truck, both being equipped with pneumatic tires. Also, the ordinary 3 to 5-ton solid-tire Army truck exerts a maximum subsoil pressure of 6½ lb. per sq. in.; the 5 to 7½-ton six-wheel truck exerts a subsoil pressure of but 2 lb. per sq. in., each truck with a 10,000-lb. load.
After making an illustrated mathematical analysis of impact forces due to four-wheel 5-ton and six-wheel 10-ton trucks, Mr. Favary discusses the steering qualities of six-wheel trucks; some steered by two front wheels, others by four wheels. When the distance between the two rear axles is more than a given amount, steering through four wheels is necessary to avoid a sliding action between the tires and the road surface when turning. When the two rear axles are as close together as 40 in., no sliding action takes place, as is proved by tests; consequently, in the six-wheel truck no steering linkages are required for the middle wheels, driving as well as braking is through the four rear wheels and steering by front wheels only.
In conclusion, Mr. Favary compares the capabilities of four-wheel and six-wheel trucks. Among the advantages of the six-wheel truck, he mentions that the reduced load on each wheel results in lessened static and subsoil pressure, reduced impact forces, improved traction, reduced tendency toward wheel spinning and skidding, increased economy in freight transportation, and a smaller number of trucks on the road to carry a given tonnage, and states further that fewer trucks represent an important factor in decreasing road destruction, increase the traffic capacity of existing roads and minimize traffic congestion.