The question of how to best manipulate the airflow around a truck and trailer combination has divided the transport equipment industry for almost a decade now and led to the formation of a whole new market segment that is solely dedicated to challenging the common conception of trailer design.
With Stemco’s acquisition of US company ATDynamics – which has long pioneered the trailer tail movement in the US – and even more conservative European companies like Jost and Wabco jumping on the bandwagon at the last IAA Show in Germany, the debate is picking up pace once more at the moment.
But consumers in both regions are still skeptical as to whether the use of drag reduction devices will actually generate a measurable Return Of Investment – especially since most of the research that is publically available has been financed by those who benefit from selling them.
That’s why a San Francisco-based team managed by Lawrence Livermore National Laboratory (LLNL) Senior Scientist, Kambiz Salari, stepped to the plate to find out just how efficient the different technologies for sale really are. Salari’s team conducted a survey of 256 US fleets and found that five per cent of them are currently using a gap reduction or boat tail device, while four per cent have tried trailer skirts to reduce fuel consumption.
In light of the Stemco deal, which was only made public in February, his findings are now more topical than ever as they challenge some of the long-standing urban myths around aerodynamic improvement: According to Salari, the majority of companies that are currently using drag reduction devices do experience an increase in vehicle fuel economy, but it still doesn’t incentivise them enough to further invest in the idea. Salari found that only two to seven per cent of all companies plan to invest in aerodynamic fairings in the near future, with just 27-30 per cent even willing to consider the move.
As a result, Stemco still has some work to do if the ATDynamics product is to take off at a large scale, even though the foundation has undeniably been laid. To build on it, Salari says there are several obstacles left to overcome – including price, weight, maintenance and access. And then there is a fifth issue, which has been identified by a research team based in Melbourne, Australia – credibility.
David Burton, who manages the wind tunnel at Melbourne’s Monash University, believes that the promotion of unrealistic fuel efficiency figures has held back the young aerodynamics market. “I think this is hurting the cause. It’s important for purchasers to look for independent test data,” he says.
To provide such data, Salari and Burton have conducted a range of independent wind tunnel tests to evaluate just how well drag reduction devices actually perform. Both say that wind tunnel testing is the closest to reality when testing the effectiveness of aerodynamic attachments, as they allow for a controlled variation of wind speeds and vehicle yaw angles, which is crucial to achieving accurate data.
While wind tunnel testing is relatively common at a scale of 1:8 or even 1:10, working with small-scale models can negatively affect the outcome due to the so-called Reynolds number effect, which says that a change in model size can change the test result.
As a result, both teams chose a different approach to find out how well aerodynamic equipment really works in the field. To mitigate the scaling issue, Burton, for example, used a 1:3 model of a cab-over truck pulling a dry van that also addressed a second issue many studies have ignored in the past. “We believe that a useful test of truck-trailer aerodynamics needs to consider the entire vehicle,” he explains. “A poorly matched truck roof fairing has the potential to increase fuel consumption. That’s why we think the truck must be the starting point to ensure a smooth interaction with the trailer. Measuring aerodynamic devices on their own won’t give you a good result.”
Salari’s team also acknowledged the interaction between the powered vehicle and the trailing equipment – but it didn’t have to scale down at all. The LLNL test was carried out at the National Full-Scale Aerodynamics Complex (NFAC) at the NASA Ames Research Centre in California, which provides a test area of 24m x 36m that can easily accommodate a full size truck and trailer combination.
Salari tested three truck and trailer combinations at a speed of 93 km/h: a 2008 Navistar ProStar sleeper cab with a Wabash 16.2 m straight-frame box trailer, a 2008 Navistar ProStar day-cab with the same Wabash trailer, and a day cab truck pulling a 16.2m drop-deck dry freight van built by Kentucky Trailer. Salari also had access to a large-scale turntable that allowed him to yaw the entire vehicle and simulate crosswinds at a sweep range from -9° to +9°. 
With independent research from Australia and US, it is now safe to say that trailer skirts are still the most effective single device for reducing drag. Even special models that were designed with access in mind and therefore compromise on drag manipulation still improve airflow noticeably.
Salari’s team found an interesting detail, though: They tested two skirts that were very similar in their basic construction, but yielded vastly different results – revealing that even small changes to a device’s height and attachment angle can almost double the estimated fuel saving. “This demonstrates that even slight variations in design can yield substantial changes to performance,” Salari states.
Varying the angle of the plates that form an ATDynamics-esque boat tail device also creates different results, Salari says. “While the correctly installed boat tail devices resulted in a reduction in drag, there was less change in airflow found as the angle of the plates increased. Plate lengths and how the tail is attached to the trailer will make an impact on its aerodynamic performance,” he concludes.
Salari’s team also tried to find out how the gap between truck and trailer affects the airflow around the combination, with an astounding result: According to Salari, reducing the gap to about 0.61m can be more effective than adding specially designed fairings or extenders – potentially making a case for adjustable fifth wheel couplings, which have recently become en vogue.
Australian scientist David Burton explains, “The most important thing to get right is the transition between the cab and trailer. Once this is done, side skirts and boat tails provide good drag reductions.” He argues that both studies were able to confirm that considering the whole combination as one unit will yield the best results in terms of airflow optimisation and that a combination of aerodynamic devices actually leads to savings larger than the combined total.
Salari speculates that this could be due to the fact that tackling the issue from multiple angles will decrease the overall amount of flow separation, resulting in a thinner boundary layer on the surface of the vehicle. “When this thinner boundary layer approaches the boat tail, the flow carries additional momentum near the surface of the trailer and is therefore better able to remain attached to the boat tail plates, thereby producing more positive trailer base pressures,” he says. “However, if you get the flow at the front wrong, you can also reduce the benefit of the components at the rear.”
US scientist Salari also believes that in order for aerodynamic equipment to become more accepted by operators, a second generation of devices must be designed to meet tightening industry and Government standards while still providing a decrease in fuel consumption. “This can only be accomplished through a coordinated effort of the trucking industry, aerodynamic researchers and Government regulators,” he says.
With a new wave of aerodynamic devices about to hit the market and the segment receiving more attention than ever before, now could be just the right time to facilitate that process, he says. But unfortunately there are still regional differences to overcome. While the US law is more liberal regarding the addition of boat tails, for example, EU regulations limit the overall length of a combination and would force those willing to apply such a device to sacrifice payload.
Given that additional obstacles like price, weight, maintenance and access remain, many fleets are unwilling to take the leap of faith and trial aerodynamic trailing equipment just yet. To convert them, manufacturers are now experimenting with new, lightweight materials like fiberglass, with Melbourne scientist David Burton saying they could be just on the right path.
According to Burton, the difference in manufacturing materials, from a finish perspective, is unlikely to be an important parameter to influencing the effectiveness of the devices as long as they are rigid enough to hold shape at highway speeds. “In some cases regulations force us to think of more creative drag reduction methods, such as inflatable boat-tails,” he says – officially giving the starting signal for a new innovation race that could go beyond the mere development of trailer add-ons.
Burton says if there is one thing both studies have found it is only an integrated design approach will yield true benefits for the future of aerodynamic vehicle design. Agrees Salari, “We need to be more aggressive in reshaping the trailer. Companies have done an excellent job in improving aerodynamics by concentrating more on trucks, [but we now need to look at] modifying both tractor and trailer shapes.”
Much of that process is still left to the designers’ imagination, but as Salari and Burton’s teams continue to publish fresh independent data, next generation truck and trailer design could take a very different shape than expected.
