For tipper trailers and other trailers where significant twisting loads are present, this must be taken into account when upgrading the trailer chassis. The torsional stiffness of a chassis is determined by the design and position of the cross-members and the presence of cross-ties. Reducing the web thickness of the chassis main beams will have a very limited effect, while reducing the cross-member thickness will affect the chassis torsional stiffness significantly. To avoid stability issues, design changes can be introduced to achieve matching or even improved torsional stiffness compared to the original design.

By introducing profiles with closed cross-sections for the cross-members, the stiffness in torsion is significantly improved, as in the graph and illustrations below. 

Comparison of the torsional resistance of an original design trailer chassis with open section cross-members (1), to the same chassis with reduced web thickness (2), a design with closed cross-member profiles (3) and a design with double cross-ties (4).

However, for optimum material utilization, the position of the cross-members is equally important. The redistribution or introduction of one or two additional cross-members influences the overall torsional stiffness. In general, the cross-members should be focused towards the rear part of the chassis. However, this shift has been exaggerated in practice many times. By moving cross-members forward or introducing an additional cross-member in a strategic region of the front part, the overall behavior can be significantly improved. Since a small rotation in the front part results in big displacements of the rear, increasing the torsional stiffness of the front could improve the overall performance.

Introducing cross-ties is another effective measure. To ensure optimum material utilization, it is important to design the cross-tie to only carry tensile loading in one bar and allow the other bar to buckle. Therefore, the bars should be slender and not welded to each other at the center. To demonstrate the effect of these measures, a comparison was made of the twisting angle resulting from a torsional moment applied at the rear of a common tipper chassis. The results from the calculations represent a unique case, but clearly illustrate the impact of these measures on the chassis stiffness in torsion. In all calculated cases, the total mass of the cross-members remained constant. That is, for the case where closed cross-sections were used, the thickness of the profiles was reduced. The results show that a reduction in web thickness results in a minor decrease in torsional stiffness compared to the original design, while introducing closed cross-members or a double cross-tie significantly improves the stiffness.