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In 2020 there were roughly 40 million trucks registered and used for business according to the American Trucking Association’s Economics and Industry Statistics, which does not even include the trucks used for government or agricultural purposes. The operation of these 40 million trucks supports the country’s economy however, it is also a major source of fuel consumption. By virtue of this, even small changes that improve the fuel economy of these trucks can save the industry Billions annually and improve the productivity of the country at large. But do small changes like this exist? Retired president of Lockheed Martin Energy Systems, Gordon Fee, explained that the relatively simple invention of trailer skirts can do just that; save the logistics and transport industry Billions of dollars in fuel expenses. The computer modeling system that produced this solution is of great significance to not only transporters, but also every dependent industry such as import-export operations, grocery, pharmaceuticals, chemical production, and car manufacturing to name a few. Addressing the aerodynamic drag on trailers using tractor-trailer skirts and tails can improve the fuel economy of commercial vehicles.
Aerodynamics
Truck aerodynamics in a basic sense refers to the force on a truck that resists its motion as it increases speed. Aerodynamic drag, according to STEMCO, accounts for roughly 65% of a tractor-trailer’s fuel consumption. High-tech modeling of trailer movement confirms that the resistance due to the shape of the trailer creates excess drag. This is the reason why airplanes and rockets are not shaped like cubes because even though the rectangular floor is efficient for maximizing hauling storage in a trailer, the edges are not fuel-efficient for moving at high speeds due to the subsequent drag. Physicists and truckers alike can, and should, apply this basic principle of physics to the benefit of national efficiency.
Purpose of Skirting a Truck Trailer
The panels under the trailer are called trailer side skirts, aerodynamic panel skirts, air fairings, or side panels. Regardless of the name, the panels effectively guide air around the sides of the trailer rather than through the undercarriage to reduce drag. The size, material, durability, and weight of trailer gap devices vary greatly depending on the manufacturer and the type of trailer it is designed for. There are concerns regarding the possible downsides to trailer skirts due to added weight, but the North American Council for Freight Efficiency assures that “Skirts have become lighter, less expensive, and more robust improving their payback.” The addition of trailer skirts is a net positive investment, and the payback period is getting shorter over time.
Another reliable source supporting the use of trailer skirts is Susan King with the American Trucking Association, who explained that side panels “reduce drag on the undercarriage and wheels of the rig, thereby improving fuel efficiency.” The association estimates a 5 to 15 percent fuel-saving window, with 10 percent as the average saving rate. Skirts for trailers are made with a variety of materials, some are manufactured using cardboard or aluminum and some use rubber or glass-reinforced thermoplastic composites. The difference in materials accounts for the longevity of the product, its flexibility, and its impact resistance. The price of trailer skirts varies from roughly $2,000 to $6,000, but a reassuring estimate from the EPA and NHTSA is that “… an operator of a semi-truck can pay for the technology upgrades in under a year and have net savings up to $73,000 over the truck’s useful life.” As is goes for trailer skirts, the evidence indicates that they are successful in improving the fuel economy of commercial vehicles.
Purpose of Trailer Tails
The panels that hang off the rear of the trailer are often called rear tail fairings or trailer tails. Due to the pockets of turbulence that develop behind and under trailers, the amount of energy needed to pull them increases. Trailer tails serve to reduce the drag that festers at the tail-end of the truck, thus decreasing the amount of energy needed to haul the trailer. Tail fairings are collapsible and are often very easy to close and open. Most designs do not present a burdensome task for the driver when they go to load or unload their freight, although some do. Trailer tails also seem to stabilize the trailer in windy conditions, improving safety for the driver and other vehicles on the road. The below visual representation of drag at the rear of the truck with and without tails proves that tails have an effect on the aerodynamics of the trailer’s movement.
Other Notable Devices
- Trailer Gap Devices: Air gets caught in the gap between the tractor and the trailer, and alongside the air in the undercarriage and at the trailer tail, air in this gap is a culprit of aerodynamic resistance. Reducing this space to less than 18 inches will limit drag and improve fuel economy. Some gap devices cover the gap in its entirety, while others simply reduce the space in the gap by protruding – nose cones are a prime example of this as they are a rounded mound on the trailer that sticks out towards the truck, but does not completely close the gap.
- Mudflaps: While it is not feasible to determine the exact percentage of fuel savings, mud flaps do reduce aerodynamic drag by reducing space for airflow under the trailer. Some mudflaps are vented, meaning they have patterns of holes that allow the passage of air but still deflect water and debris. There is a variety of mudflap designs and it’s important to pick the appropriate width because an improperly sized mudflap is actually seen to increase aerodynamic drag.
- Trailer Underbody Devices: Another way to address drag in the undercarriage is to attach aerodynamic devices to the underbody of the trailer. These products provide easier access to the undercarriage than full skirts while at the same time not sacrificing effectiveness. These have a drawback however because they are more vulnerable to weather conditions. Snow and ice can build up on the underbody device and increase the weight of your load, it also becomes less effective at improving truck aerodynamics when hindered by the weather.
- Wheel covers: The purpose of wheel covers is to ease airflow around the tires, which does work in principle and is a net benefit however, the improvement is narrow and difficult to measure. Wheel covers have the perk of being relatively lightweight and cheap compared to other aerodynamic technologies, but they do present an additional step when attempting to inspect or repair trailer tires.
Are Trailer Skirts Required?
Trailer skirts are not legally required for every kind of trailer. However, California is the only state that currently has laws on the books mandating the use of trailer skirts. California’s compliance laws concern only 53-foot box-type trailers (dry or refrigerated). The California Air Resources Board requires that “…any long haul box trailer operating in the state must have both aerodynamic devices and tires that can help reduce rolling resistance…” as advised by the EPA’s SmartWay Program. Although aerodynamic technologies are not legally required nationally, they should be strongly encouraged, if not to save billions of dollars annually, but because these measures improve safety on the road. Visibility for the trucker, and other drivers, is hindered by the uptick of dirt, dust, and snow at the rear of the trailer – this is partially a consequence of aerodynamic drag.
Important Things to Remember
It seems to be the case that the benefit of adding trailer skirts and tails is maximized at high speeds. It also means that to seize the fuel economy improvement, the trailer needs to be moving at least 60 miles per hour according to ATDynamics. The reason for this is that lower speeds produce less drag and subsequently result in less fuel saving as a result of the skirts. This is not to say that the only way to realize the benefit of trailer skirts is to always travel at that speed, but rather that the return on the investment is best seen when the vehicle is moving at high speeds more often. Furthermore, trailer skirts do add weight to the truck which is an additional reason why local transporters are much less likely to find a use for them. For some fleets, a notable obstacle to the adoption of trailer skirts is that those who use multiple trailers for each tractor limit the number of miles driven per year for each set of skirts and tails. This means that the payback period is increased for each tractor-trailer skirt in a large fleet, although the safety benefits are still present which is a respectable accomplishment in its own right.
Summary
The objective of aerodynamic devices is to limit air turbulence on the tractor-trailer. Trailers are shaped the way they are to serve their purpose of hauling freight; however, this does not provide the best aerodynamic conditions for efficient transport. For example, bullets have a rounded or pointed head to better enable them to pierce through the air – militaries are not out here rocket launching Rubik’s-cube-shaped grenades. Changing the shape of the trailers would be unfeasible and more expensive than simply adding the shaping equipment to compensate for the unwanted drag. Trailers with aerodynamic technology, especially skirted trailers with boat tails, have improved stability and fuel efficiency by mitigating aerodynamic drag in transit. An important takeaway for individual truckers and transport companies alike is that the transport industry is the backbone of the U.S. economy and improving efficiency in this sector by even one percent annually is a worthy cause. Fortunately, this can be done with the relatively simple addition of trailer skirts, tails, gap devices, and more.
