However, it is clearly still more beneficial to reduce pressure drag, regardless of vehicle configuration.
The authors of one study concluded that some vehicle combinations can show an increase of 40% in friction drag with only a corresponding increase of 8% in pressure drag.
Or put in other terms, the drag coefficient on an LCV is only marginally more than half of the sum of the drag on the two vehicles it replaces when wind angles are at zero degrees.
As vehicle length increases, the percentage contribution to overall drag from friction drag rises slightly since there is so much more planar surface aligned with the wind, yet the blunt front face of the vehicle remains unchanged.
asked the National Research Council (NRC) to undertake a literature review to assess various aerodynamic drag reduction technologies for heavy-duty vehicles.
The literature review evaluates the fuel consumption and GHG reduction potential of aerodynamic enhancement devices for tractor-trailer combinations and intercity busses.
However, this is still 40% of a very small number, and 8% of a very large number but the fact remains that increasing vehicle length increases the relevance of frictional drag reduction strategies and has much less effect on pressure drag.
The size of the gap between the lead and trailing trailer plays a significant role in the amount of drag experienced by the combination vehicle, particularly at higher yaw angles.
Although friction drag occurs along the external surfaces of heavy vehicles, particularly along the sides and top of buses and trailers, its contribution to overall drag is small (10% or less) and is not a strong candidate for drag-reduction technologies.The report serves to aid in identifying potential knowledge gaps about the performance of these devices in Canada.The recommendations included in the report by the NRC are for deliberative purposes only, and do not necessarily reflect Transport Canada’s position or views in the subject.Ideally, a study could be conducted whereby a variety of gap fillers, side skirts and boat tails are sequentially added to the LCV in order to determine if the effects of these devices on LCVs is similar to their effect on conventional vehicles.Canadian Motor Vehicule Safety Standards (CMVSS) compliant mirrors are responsible for approximately 2% of the overall drag on a conventional tractor and trailer.
A study concluded that the percentage contribution of pressure drag on the baseline vehicle was 93.3% whereas the contribution of pressure drag on the LCV was 91.7%.