Fuel is the second largest expense for trucking companies next to driver pay, so obviously, saving fuel is keenly important for every fleet operator! Trailer aerodynamic devices can help truckers do just that. Sometimes.
Where trailers lose efficiency
Three areas in a semi-trailer’s vehicle configuration account for the vast majority of aerodynamic inefficiency…the gap between the truck and trailer, the trailer underbody, and the rear of the trailer. There are many products available that address each of these areas, but the single most tested, scientifically verifiable, and popular industry solution is the installation of trailer side skirts (also called fairings) to reduce underbody drag.
According to various industry research sources, trailer side skirts can save a trucker as much as 5% in the cost of fuel in normal OTR highway operation. In conjunction with other fuel saving improvements like low rolling resistance tires and tire pressure management systems, fleets may see an up to 15% improvement in the cost of fuel. But, that's not always the case...
Side Skirts today 
The good news is that trailer skirts have evolved over the last 20 years to be lighter in weight, lower in cost, and more damage resistant, all resulting in skirts becoming quite popular. More than 50% of all new 53’ vans are now estimated to be built with side skirts installed. In addition to the ROI business case based on fuel savings, the growth in the use of side skirts is driven by state and federal regulations focused on reducing carbon emissions from the trucking industry. This includes the California Air Resources Board (CARB) and the US Environmental Protection Agency (EPA) Greenhouse Gas rules. California currently requires newer, longer 53’ dry van and refrigerated trailers sold in or entering the state to have side skirts or other approved underbody alternatives. Other states have recently adopted or are considering measures similar to California’s.
Not all trailers are included in the current state and federal regulations. Trailers shorter than 35’, flatbeds, container chassis, trailers with rear or side lift gates, trailers with side-mounted pull-out ramps or steps, and drop frame/belly box vans are some of the configurations exempted as “non-aero” trailers. Continued government regulation promoting trailer aerodynamic devices in addition to skirts is expected to increase in the years ahead.
When they make sense - and when they don’t
Despite the compelling argument for trailer side skirts or other aerodynamic devices in most operations, they may not be a solid win for every fleet unless you operate in a jurisdiction that requires them. While skirts have been proven to be effective at any speed, they are clearly more effective at higher, sustained highway speeds. Low average speeds or excessive trailer pooling days extend the payback period making it harder for skirts to provide a desirable ROI. Maintenance costs due to skirt damage can increase in metropolitan areas with tight operating environments. Retrofitting older trailers may not be practical given the remaining useful life limitations on the ROI payback period.
The maximum benefit of side skirts is achieved with trucks traveling straight into the wind. However, crosswinds (e.g. yaw) can impede the aerodynamic benefit and even create more resistance and drag on the trailer. In addition, fleets with more deadhead (e.g., empty) miles use inherently less fuel than those with full loads thereby adding more “miles to payback” for those running the higher empty miles.
Other than fuel savings, anecdotal performance benefits of trailer side skirts reported by users include increased road stability, splash and spray reduction, improved driver handling, and lower in-cab road noise.
Summing it up
So, if you run your long trailers at highway speeds most of the time, adding skirts will likely reduce fuel costs and other possible benefits. However, if you run at slower speeds more often, or if your trailers sit idle in the yard or shipper pools for long periods, you may want to do some financial analysis and/or run a field test before committing to a broader fleet conversion.
An excellent outline for calculating ROI on trailer aerodynamic devices can be found in the “Confidence Report on Trailer Aerodynamics” at the North American Council for Freight Efficiency (NAFCE) website.
If you need help determining whether side skits make sense for your unique situation, reach out to us! Our team is here to help you develop a plan that's right for you.
Useful Resources –
- U.S. Environmental Protection Agency, Smartway
- SmartWay Verified List of Aerodynamic Devices
- North American Council for Freight Efficiency
- California Air Resources Board, Truck and Bus Regulations
