Box trailer design reduces drag while retaining practicality

Interpreting data gathered using the latest computational fluid dynamics simulation techniques, aerodynamics experts at MIRA applied their expertise to improving the poor aerodynamic performance of a standard 4.5m box trailer, seeking to preserve its load capacity but reduce drag to the levels of a smaller 4m equivalent.

Dr Anthony Baxendale, senior manager for advanced engineering at MIRA, said: ’From the outset we wanted to design a solution that recognised the operational considerations of business but still deliver tangible performance gains in financial and environmental terms.’

To do so, the engineering team used CAD geometry and ’morphing’ techniques to modify the trailer shape and maximise aerodynamic efficiency without compromising overall load space. They then added underbody panels to the cab and trailer to further minimise drag — the result of which was a reduction of up to 30 per cent while avoiding the complex surfacing and space constraints of ’tear drop’ designs.

MIRA’s aerodynamicists calculate that the resulting fuel savings and reduction in CO₂ emissions (of between 15 and 20 per cent) mean the cost of the adaptations to the trailer could be recouped by fleet operators within 12 months. The enhancements can also be applied to curtain-sided trailers — making it highly commercially viable and highlighting why MIRA’s design was recognised with an award from the Low Carbon Vehicle Partnership in November.

MIRA’s efforts to retain practicality when redesigning the 4.5m trailer saw the rear-door aperture reduced only marginally — to the height of a standard 4m box trailer — for ease of loading. Underbody panels enable easy access for routine maintenance on brakes and axles, and apertures created in the side skirts allow operators to check the wheel nuts.

The engineers also optimised the crosswind behaviour of the trailer — with the final design achieving a reduction in both lift and sideforce — in a move that improves stability. The wake sizes were also significantly reduced, which reduce the amount of spray generated by the design on wet roads.