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The paper aims to resolve the practical problems of asynchronous brake lock-up which acts on the wheels of statically indeterminate tractors with an air suspension. First, two kinds of deflection model are calculated, both of which are under the combined effect of the brake force and the vertical force. One model considers the leaf spring suspension that is mounted on the front axle, and the other considers the air suspensions that are mounted on both the midlift axle and the drive axle. Second, a 12-degree-of-freedom braking force distribution model of the statically indeterminate multi-axle tractor–semitrailer is constructed on the basis of the foregoing preparation. Third, the ideal braking forces of each axle in the model are simultaneously calculated and the actual braking forces of each axle are measured by road tests. The curves of the ideal and the actual braking force distribution ratios at different brake rates are described. The contradistinctive outcome indicates that the excessive braking force of the midlift axle results in a premature brake lock-up, which is the main reason why wheels lock asynchronously. Finally, because of the analysis of the distribution ratios of the ideal and the actual braking forces, the paper outlines how to redesign the braking force distribution of the tractor–semitrailer according to ECE R13, SAE J992b, and JASO C514. The paper also proposes a definition of and calculational method for the fitting degree (FD), and the FD of the midlift axle is improved from 61.2 per cent to 91.8 per cent. Compared with the original tractor–semitrailer, not only is the actual braking force distribution more reasonable but also the brake stability is improved.

The paper makes a study on the relation between power consumption decline of air compressor and oil saving of heavy-duty vehicle. First, on the basis of theoretical mathematical equations and actual bench test, the paper clearly analyzes the effective and ineffective shaft powers of air compressor. Second, road test is carried out so as to describe the relation between the load rate of air compressor and the average speed of vehicle. Finally, based on the research outcomes, the paper puts forward an effective approach of reducing power consumption of air compressor which synchronously improves the oil saving of heavy-duty vehicle.