This paper investigates options for lightweighting a 13.5 m long flat-bed road trailer chassis through the application of composites and structural optimisation. Trailer load cases are first understood through finite element modelling in Abaqus. A parametric model of a typical 13.5 m trailer built from steel I-beam was developed, using Python scripting and the finite element package Abaqus. The model has been used to show that the conventional steel I-beams could be reduced in weight by 28% (140 kg). The model has been expanded to analyse composite trailer structures. It showed that approximately 1, 300 kg of weight could be saved by shape and material optimisation in a composite trailer, while mechanical performance is maintained to an acceptable level and increases in raw material costs are minimised.