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As the table and illustrations show, numerous articulated combination sets
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were trialled, with results ranging from a 4.7% loss in efficiency to a 5,5% gain.
The trials started with the set-up of a control model a standard 4.5m-high Lawrence David trailer equipped with standard under-run bars and tractor unit with standard cab deflector. The co-efficient of drag (Cd) value was 0.565 in direct frontal conditions and 0.889 with the 100 yaw angle.
To demonstrate the importance of the cab deflector, the same set-up as the control was re-tested, but without the cab deflector. This resulted in a 4.7% drop in fuel efficiency. Based on a vehicle operating 60,000 miles a year and bulk fuel at £1 a litre, that's an extra cost of £1,430 a year.
The best improvement was achieved with a combination of optimised air deflector, wide-radius, front air deflector, deep side skirts between axles and at the rear, and a rear roof slope. This offered fuel savings of 5.5% (and potentially £1,680 a year) per vehicle.
Next best was the '
Brian Timpson, technical so-called teardrop' director at Lawrence David design, demonstrating potential savings of 5.1% and £1,550 per year. Interestingly, this was virtually identical to the savings possible by using a standard 4.1m-high trailer, indicating that if you don't need (or can't use) the roof space in a teardrop trailer, then the benefits may be easier to gain elsewhere.
"There's been a lot of hype about these teardropdesign trailers and their potential benefits," says Lawrence David technical director Brian Timpson, "but our experience with our original fuel-saving curve trailer and the research results indicate that while there are savings to be had, there are simpler ways to get the same benefit."
Other notable findings from the research point to marginal improvements gained by buckleless curtains a £210 annual fuel saving, from a £2,000 outlay, would not appear to be a decent investment and no measurable benefit from the addition of trailer under-skinning. The methodology for the research was fairly straightforward, explains Timpson. "We wanted to research the aerodynamic efficiency of a number of trailer types and look at the impact that other devices might have on fuel efficiency.
"We used the CFD modelling to look at head-on wind and a 10-degree yaw angle, simulating a crosswind to build the drag co-efficient figure for each type.
"With the help of MIRA, using its fuel savings estimator, we've put together a financial model to see how these different systems will impact on overall operating costs. The results make for some very interesting reading." •