Highway Legislation and the Six-Wheel Truck 280068

ASSERTING as a premise that highway legislation should be purely a matter of economics, the author draws a comparison between the costs of building a cheap road and hauling with 2½-ton trucks and building a heavier road and hauling with 5-ton trucks. He shows by this illustration that the latter proceeding is the more economical.
Most States permit gross weights of vehicle and load that make it possible to haul pay-loads of about 5 tons. If 5-ton trucks show a saving in transportation costs over 2½-ton trucks, still larger capacity four-wheel trucks might show a corresponding saving over 5-ton trucks, from which it might be argued that all roads should be built sufficiently strong to carry the heavier vehicles and loads without damage. But it is pointed out that there are a great many secondary roads on which traffic is light, and that it is uneconomic to build roads and roadbeds stronger than is warranted by economic needs.
Laws must therefore be enacted on a scientific basis and, if experience proves that new transportation units make possible the hauling of heavier loads and result in economic gain with less road destruction, the legislators should enact highway laws favoring such constructions.
Ten economic advantages listed for the six-wheel truck include economy in transportation, reduced road-impact forces, less traffic congestion and increased safety. Each of the advantages is explained and some are illustrated. Contentions of the author regarding reduced impacts and distribution of road-surface pressures by 3-ft. separation of rear axles are supported by citations of reports by the Bureau of Public Roads. Recommendations of the Supervisors Association of California that the gross-weight limitation of four-wheel motor-vehicles be maintained at 22,000 lb. and that for vehicles having six or more wheels be fixed at 34,000 lb. are quoted. Reference is made also to legislation in 11 States of the Union and in several foreign countries favoring six-wheel trucks.
The author feels that legislators would benefit their States by enacting laws that will permit such trucks to carry greater loads than four-wheel trucks, and which, instead of specifying gross weight, would fix the maximum weight per axle or per wheel, specify that the two rear axles shall not be more than 4 ft. apart, and that a “balanced suspension” shall be used to obtain the benefits of six-wheel construction.
Old and more or less obsolete bridges that are unsafe for the greater gross loads should be rebuilt, as the expense is justified by the greater transportation economies that will result.
Speed laws, it is asserted, should be modified also in conformity with scientific facts.
Discussion of the paper deals with both the construction and performance of the six-wheel vehicle and with effects of four and of six-wheel trucks upon the highways. The favorable attitude of the Governor and the highway engineers of Illinois toward bills in the House and Senate of the Legislature to permit greater gross weight on six than on four wheels, as a result of an investigation made by engineers of the State Highway Department, is shown by quotations from letters. Questions are raised and answered regarding the claims for reduced unsprung weight and superior traction of the six-wheel vehicle and regarding effective lubrication of the worm drive in the dual rear axle.
Greater loading and unloading time necessitated by a 10-ton vehicle in city and suburban work, as compared with a tractor and semi-trailer unit, is cited as an economic factor.
Statements that any pavement will last indefinitely are deprecated, but experience is said to have shown that loads of 10 and 20 tons can be carried satisfactorily on 8-in. concrete foundations under both flexible and rigid pavement. Reports of the Highway Bridge Committee of the American Road Builders Association and of the American Society of Civil Engineers hold that bridges on primary highways should be designed to carry trucks having gross loads of 20 tons.


Subscribers can view annotate, and download all of SAE's content. Learn More »


Members save up to 40% off list price.
Login to see discount.
Special Offer: With TechSelect, you decide what SAE Technical Papers you need, when you need them, and how much you want to pay.