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The provinces of central and eastern Canada have tolerated the use of liftable axles on heavy trucks for many years, and about one quarter to one third of all heavy trucks, a majority of the truck population that hauls the heaviest loads, are so equipped. These trucks have been configured for the most part simply by means of a bridge formula, and their performance as vehicles has been compromised to generate the highest possible gross weight. This paper presents some theoretical highway impacts of typical heavy trucks that make use of liftable axles and operate in central and eastern Canada. It then outlines some of the practical impacts on pavements observed from weigh-in-motion data. It appears that where liftable axles are widely used, driver behaviour plays a significant role in consumption of the pavement resource, in addition to freight volume.

Canada's Memorandum of Understanding on Vehicle Weights and Dimensions (M.o.U.) now defines the configuration of eight vehicles, based on regulatory principles derived from assessment of vehicle dynamic performance against objective measures that correlate with accident involvement. This process screened out a wide range of trucks equipped with liftable axles that operate in central and eastern Canada at considerably higher gross weights than most of the trucks meeting the strict conditions of the M.o.U. The paper uses computer simulation to assess the stability and control characteristics of some straight trucks and tractor-semitrailer combinations with liftable axles against the same performance standards used as the basis for the M.o.U. It examines vehicle performance with liftable axles properly loaded, with them raised, and with alternative self-steering systems for the wheels of the liftable axle.

The double drawbar dolly eliminates articulation of the conventional single drawbar converter dolly to improve the stability of multiple trailer truck combinations. It is usually provided with a self-steering axle to improve maneuverability. This paper describes double drawbar dolly types, factors in their design, and findings from tests of a specific dolly and computer simulation results pertaining to a range of dolly concepts. Performance topics investigated include low and high speed offtracking, stability characteristics and loads at the dolly hitch. The results showed vehicle performance is highly dependent upon friction and stiffness in the dolly self-steering system. Both test and simulation showed that compared with the single drawbar dolly, a double drawbar dolly having properties equivalent to that tested would significantly improve vehicle stability.