PLATFORM TRAILERS may appear low to the ground, but their very nature makes them somewhat more vulnerable to rolling over during sharp turns, according to analysis by the engineering company of Ruhl and Associates.
In an effort to calculate the rollover threshold of platform trailers, the company has developed a mathematical model that management believes is of value to platform trailer designers, dock loading managers who specify load placement, and traffic safety managers who train drivers.
"The fact that flatbed trailers are compliant in roll should not surprise us," said Dr Roland L Ruhl, president of Ruhl and Associates and an adjunct professor in the department of general engineering at the University of Illinois at Urbana-Champaign. "The lack of box cross section when compared to a van trailer, bulk commodity trailer, or a tank leaves them with a significantly lower cross sectional moment of inertia. This results in lower torsional stiffness."
Roland A Ruhl drove home the point in a presentation to the Society of Automotive Engineers Truck and Bus Meeting in Cleveland. Speaking on the subject of platform trailer rollover, he showed a photograph of a flatbed trailer coupled to a tractor. The rear of the trailer was on its side, yet the tractor was still upright. Between the kingpin and the rear of the trailer, the platform had twisted almost 90 degrees .
For platforms, Ruhl said, trailer torsional stiffness can be more important to rollover resistance than is the stiffness of either the suspension or tires-variables that are key determinants of rollover stability of other types of trailers.
Changes in size and weight limitations have had differing effects on platform trailer rollover resistance, Ruhl observed. Increasing the width limit from 96" to 102" has helped increase roll stability, but the move to longer trailers-all other things being equal-has lowered it. Ruhl said that when a platform trailer is lengthened, trailer design may need to be modified to make sure that the torsional stiffness of the trailer is sufficient.
"The task of a flatbed trailer designer is not an easy one," Ruhl said. "Economics in the trucking industry increasingly demand lower trailer weight to allow greater payloads. The use of aluminum-particularly in the frame rails-confronts the designer with a material selection of significantly lower modulus of elasticity and yield point. And the defacto geometic standards in the trucking industry greatly restrict the designer's options. Platform heights, fifthwheel heights, axle width, tire and wheel size, all are standardized."
A Roll Model
Ruhl and Associates has developed a model for evaluating roll stability that considers the following variables:
Tractor suspension and frame compliance
Moment release across the fifthwheel due to an articulation angle between tractor and trailer.
Discrete loads at two locations on the tractor and five locations on the trailer.
Trailer torsional stiffness specified for longitudinal zones.
Trailer suspension. The trailer can be configured either with a single or tightly-spaced tandem-axle location or with two single axles spaced 120" apart.
The model is based on Microsoft Excel, with some custom programming done in Microsoft Visual Basic. The model is also useful for dock management and traffic safety managers who need to know the effect of a particular loading pattern on rollover stability. For additional information, contact Ruhl and Associates, 1906 Fox Drive, Champaign IL 61820.