1929-01-01

Effect of Six-Wheel Vehicles on Highway Design 290076

TWO distinct phases of the subject are the physical and the economic, both of which are included in the conclusions stated in the paper, based on investigations made by the Bureau of Public Roads.
It is as pertinent to inquire what effect the highways have on the motor-vehicle as to inquire what effect the motor-vehicle has on the highways. Mutual adjustment must be made if real economy is to result.
Two general conclusions that may be drawn from the observations presented are that the six-wheel vehicle offers a desirable and effective answer to (a) the problem of the load in excess of the normal desirable limit of weight for the four-wheel truck, and (b) the problem of the load equal to the heavier four-wheel truck in areas where road conditions do not permit the maximum wheel-load concentration.
Deductions regarding deflection, deformation and tension in concrete pavement by single and multiple wheel-loads are listed, and references are given to reports on truck-wheel impacts conducted by the Bureau with the cooperation of the Society. Data from the impact tests indicate that, for two trucks carrying the same load and identical except for the rear-end construction, the unsprung component of the impact reaction of the six-wheel vehicle is about one-half that of the four-wheel vehicle.
Necessity for the construction of the greatest possible mileage of paved highways in this Country has resulted in American engineers building roads that are carrying the greatest tonnage on the lightest-built roads that is being attempted anywhere in the world; and the motor-vehicle industry would not have it otherwise. Proof exists that economical transportation is secured, except in limited areas, by a net-load limit of 5 tons; and the wheel-load concentration called for by a vehicle having this capacity is within the safe limit for rural pavements of modern standard types. A large percentage of the mileage is not safe for greater loads, and lower-class roads, constituting 90 per cent of the total mileage in the Country, are incapable of carrying trucks with 8000-lb. wheel-load concentration at certain seasons.
Solution of the problem is not load limitation below the economic requirements of transportation, but limitation of wheel-load concentration by permitting the use of six-wheel trucks fitted with pneumatic tires.

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