How to build a better trailer

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Don-Bur’s revolutionary Teardrop semi-trailer has produced remarkable results for Marks & Spencer. But, asks Malory Davies, does it herald a new era in trailer design?

It’s not often that you hear the claim that a trailer can carry more goods and reduce fuel consumption at the same time – but that is exactly the claim Marks & Spencer is making for the new “Teardrop” trailers that it is now bringing into service.

Marks & Spencer is so pleased with the new design that Simon Ratcliffe, general merchandise logistics director, says: “The Teardrop trailers are another step in our efforts to take our UK and Irish operations carbon neutral under Plan A, M&S’ five-year eco-plan. Don-Bur’s revolutionary design has helped us not only reduce fuel usage and carbon emissions, but also allows us to carry more stock per trailer, cutting the number of journeys we need to make.”

The problem for most people is that it is counter-intuitive. It seems obvious that if the trailer is bigger then it is going to take more fuel to push it along the road.

The vast majority of trailers on the road today are brick-shaped and have all the aerodynamic qualities of a brick. It is not uncommon to see air deflectors on cab roofs to guide the air flows over the top of the trailer. And in the last couple of years there has been a move to make the trailer roof slope down towards the cab.

This seems almost too simplistic but tests repeatedly show that it does cut fuel consumption. The problem of course is that it also reduces the carrying capacity of the trailer.

Don-Bur has been supplying sloping front trailers under the Eco-Stream name for some time, points out Richard Owens. However, he says, Don-Bur’s research has shown that if you can fill the space that is lost then it will probably be more cost-effective to choose the conventional brick shape. For many applications, that makes the sloping front trailer a non-starter.

Nevertheless, the success of that design started Don-Bur on further research into the aerodynamics of trailers. Owens says it came to the conclusion that the three worst areas of turbulence are the tractor and the tractor-trailer gap, under the chassis and the area behind the trailer.

Turbulence and drag is caused when airflow is forced away from the surface by rapid directional change in bodywork such as corners or protrusions, forcing straight (or laminar) airflow away from the surface. Dependent on the speed of the vehicle, it may take several moments before the air can reach equilibrium again.

This research led to the work on the teardrop shape. Owens points out that the critical step forwards was to consider the entire tractor-trailer combination as one so that the impact that the whole shape has on aerodynamics, from the moment the air reaches the front bumper, to a point several metres behind the trailer, has been taken into consideration.

Don-Bur first unveiled its thinking at a seminar for customers and as a result of that Marks & Spencer started to take an interest. The challenge was to take a standard single deck trailer and give it the aerodynamic teardrop shape.

The result is a 13.6m long box van trailer bodywork that follows the streamlining of a teardrop, and internal volume increases from 78 to 86 cubic metres – an additional 10 per cent.

The roof has a specially designed full-length curve, retaining a four metre height at the front to match the DAF CF tractor unit but rising to 4.5m and gradually dipping to 3.7m at the back.

Don-Bur has achieved the smooth curve of the roof by using large radius cant rails that have been specially rolled. The shape reduces the area of turbulence at the rear of the trailer

In addition, the top of the front bulkhead rakes forwards to reduce turbulence between the tractor and trailer while remaining within the swing clearance. Full-wrap GRP skirts have been specified which effectively manage the air under the chassis. The substantial reduction in drag more than compensates for increase in size.

M&S also specified Technolite aluminium panelling, resulting in an unladen trailer weight of just 6,860 kilos – a saving of 640 kilos on its existing trailers. Technolite is made up of an aluminium structure foil honeycomb core, faced with aluminium sheeting and has been used in the aircraft industry for many years. Don Bur says this can be cost-effectively recycled, does not deteriorate (even when damaged), is easy to repair and gives a smooth engineered finish.

The Teardrop trailers are designed to be used on trunking operations for general merchandise, transporting stock between M&S suppliers and distribution centres.

As a result, testing of the prototype fell to DHL Exel Supply Chain, which manages the M&S contract for Marks & Spencer. It devised two tests. In test one, two trailers (Teardrop vs. comparison trailer) were run at the same time; one behind the other at a distance so that one would not be affected by the slipstream of the other. Both trailers carried the same payload weight. To average any driver or tractor variances, the drivers were swapped in equal timescales, as were the tractor units. The comparison trailer was an existing M&S tandem axle 13.6m box van with similar specifications, without the Teardrop shape or Technolite panelling.

The vehicles started in London running 700 km empty and 700 km fully loaded (14t payload) predominantly at night. DAF CF 75.360 tractor units were used – both Euro 4 and 5.

The second test was based on standard network operations starting at the Leicester sorting depot and covering a total of 4,600 km. A mix of DAF CF 75.360 Euro 3,4 and 5 tractors was used.

DHL reported an overall fuel saving of 10.14 per cent in addition to an increase in load of 16 per cent – equivalent to 10,560 extra pairs of knickers.

Marks & Spencer calculated that these savings equate to a reduction in carbon dioxide emissions of 20 per cent, when compared to a standard cab-trailer fleet operation.

The retailer has put its money where its mouth is and ordered 140 of these trailers – and it reckons that in total the fleet will reduce its carbon footprint by 840 tonnes every year.

The teardrop design is generating a lot of interest which is not really surprising given the potential for cost savings. However, Owens points out that the system does have its limitations. Getting the air to flow smoothly over the cab and trailer is essential which makes the cab height an important element. The trailer is taller than normal which might be a problem if the route includes low bridges. Reefers present a particular problem because the fridge unit interferes with air flow between the cab and the trailer.

And drawbars with swap bodies have so far eluded the ingenuity of the designers. The demountable bodies would be different shapes which would severely limit the flexibility of the system.

The additional capacity of the trailer is in the teardrop roof shape which limits its availability. The cost effectiveness of the trailer would depend on how much of that space could be used.

But even with all those caveats, the savings identified by Marks & Spencer suggest that there could be substantial potential for savings from the teardrop shape. Are we looking at the shape of trailers to come?


Take one drop of water…
Here’s an experiment you can do in the office. Pour a drop of water onto a desk. Position your mouth level with your desk and blow gently over the droplet. You should see the droplet from a teardrop shape lhighightning the fact that it is an aerodynamically efficient shape.

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