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Figures of merit describing the performance qualities of multiple-trailer vehicle combinations (for example, rearward amplification) are usually determined from either full-scale vehicle testing or computer simulation analysis. Either method is expensive and time consuming, and restricted in practice to organizations with specialized equipment and engineering skills. One goal of a recent study, conducted by the University of Michigan Transportation Research Institute and sponsored by the Federal Highway Administration, was to use basic vehicle properties to develop simple formulations for estimating the performance qualities of multiple-trailer vehicle combinations. Several hundred computer simulation runs were made using UMTRI's Yaw/Roll program. Five common double-trailer vehicle configurations (defined by trailer lengths and axle configurations) were studied. Each of the five vehicles was subject to fifteen parameter variations.

Tilt-table testing is one means of quantifying the static roll stability of highway vehicles. By this technique, a test vehicle is subjected to a physical situation analogous to that experienced in a steady state turn. Although the analogy is not perfect, the simplicity and fidelity of the method make it an attractive means for estimating static rollover threshold. The NHTSA has suggested the tilt-table method as one means of regulating the roll stability properties of light trucks and utility vehicles. One consideration in evaluating the suitability of any test method for regulatory use is repeatability, both within and among testing facilities. As a first step toward evaluating the repeatability of the tilt-table method, an experimental study examining the sensitivity of tilt-table test results to variables associated with methodology and facility was conducted by UMTRI for the Motor Vehicle Manufacturers Association. This paper reports some of the findings of that study.

The handling-performance capability of most large commercial vehicles operating on US highways is generally established by the limits of roll stability. Especially for heavy trucks, suspension properties play an important role in establishing the basic roll stability of the vehicle. For all highway vehicles, the limit of static roll stability is established first by the ratio of half-track width to center-of-gravity height, and then by the compliant responses of the vehicle, which lead to outward motion of the center of gravity in a turn. Three suspension properties, roll stiffness, roll-center height, and lateral stiffness, influence this motion significantly. This paper discusses the basic mechanisms of static roll stability and highlights the role of suspension properties in establishing the roll-stability limit. Facilities and procedures for measuring key suspension properties are described, and data from the measurement of ninty-four heavy-vehicle suspensions are presented.

A method for calculating the probability of wheel lock occurring during braking with passenger cars is presented. The method combines (1) the probability distribution of vehicle deceleration during braking, (2) the probability distribution of tire-road friction exhibited by the general road system, and (3) the braking efficiency level of the vehicle, to predict the probability of the occurrence of wheel lockup during a given braking event Results are presented employing the constituent probability data available in the open literature. These results predict that lockup events are very rare for typical drivers operating vehicles of 80% braking efficiency or better. As braking efficiency falls, the frequency of occurrence of lockup rises rapidly. At braking efficiency levels as low as 50%, typical drivers would experience lockup about once a month or once every 35 miles of travel on wet roads.

Many commercial vehicle manufacturers are now using computer simulation as an aid in assuring compliance with FMVSS 121. One particularly important aspect of such a simulation is the analysis of the interaxle load transfer during braking due to tandem axle dynamics. In this paper, two analyses of the four spring tandem suspension with short load leveler are presented. One, a simplified model, serves to illustrate the mechanism of interaxle load transfer and to identify the design parameters which affect this phenomenon. Improved accuracy is obtained by the second model through the inclusion of frictional forces that develop at the points where the leaf springs contact the frame. Comparison of effectiveness test data indicates good agreement between empirical and simulated results when this advanced suspension model is used in conjunction with existing commercial vehicle simulations.