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Trailer Side Skirt

The potential of trailer side skirts was confirmed by numerous track tests, which showed fuel savings up to 7.5%.

Important obstacles stand against the adoption of innovative technologies, of which one of the most important is the lack of practical information coming from independent sources. The objective of these fact sheets is to inform those in the trucking industry about new and emerging advanced technologies that have been tested through the Energotest program (run by FPInnovations), which can help increase fuel efficiency and, at the same time, reduce the environmental impact of freight transportation in Canada.

Description

The objective of trailer skirts is to reduce aerodynamic drag (resistance created by the air flow) under the vehicle and consequently, overall fuel consumption. Trailer side skirts are built from one or several panels installed on trailer sides, effectively extending them closer to the ground.

Trailer side skirts can offer significant fuel savings, and they are recognized as a fuel-saving technology under the United States Environmental Protection Agency’s (EPA) SmartWay^SM program, which classifies trailer side skirts as being either a “trailer side skirt”, with savings of at least 4%, or an “advanced trailer skirt”, with savings of at least 5%.

FPInnovations track-tested 17 devices in previous years in the framework of Energotest^TM campaigns, and the potential of this approach was confirmed: trailer skirts showed fuel savings up to 7.5%.

In addition to the fuel savings, trailer side skirts improve vehicle appearance, reduce sensitivity to side winds, improve stability, and reduce tire spray, which can cause visibility issues for other drivers. The benefits of trailer side skirts are most apparent in the presence of cross-winds.

Scope of Application

Trailer side skirts are used on tandem or tridem van semi-trailers in Class 8 tractor-trailer combinations.

They are best suited for line and regional hauling, which are characterized by long trip lengths and higher average speeds. Because they make the truck more aerodynamically efficient, maximum benefit is only realized at higher cruising speeds.

Return on Investment

The economic impact of the various fuel-saving measures is evaluated based on the payback period, which is calculated by dividing the total additional cost of a modification by the annual net savings it provides. The aftermarket purchase costs of trailer side skirts vary from $1 400 to $2 500 with installation costs of $300-$500 (installation time can vary from 3 to 6 man-hours). The purchase cost, when including it as an option in the purchase of a new trailer, can be lower. The estimated payback period range is 10 to 18 months. The following is an example of a typical payback calculation example:

• The total purchase and installation cost of a pair of advanced side skirts is $2 000 per semi-trailer.
• The certified track-test fuel savings is 6 % obtained at constant high speed.
• For an annual mileage of 200 000 km for the tractor, and considering that one tractor uses two semi-trailers, this represents 100 000 km per semi-trailer per year.
• We assume that the track-test results would apply only under similar conditions, such as trips done on roads with a posted maximum speed of 80 km/h or more: 80% of the annual mileage of the semi-trailer, which gives 80,000 km.
• We assume an average mixed fuel consumption of 34 L/100 km.
• The annual fuel savings would be 1 632 L: 
   
• For the diesel fuel price of $1.10/L, the annual savings would be: 1 632 L x $1.10/L = $1 795.
• The payback period would be: $2 000 / $1 795 = 1.11 years or 13 months.

Specification Considerations

During Energotest^TM campaigns, trailer skirts show fuel savings from 0 to 7.5%. Therefore, not all skirts are equal and their design and installation can have a major impact on their performance. Installing trailer skirts in the optimal position provides the best fuel savings.

In tests, trailer skirts with curved surfaces showed 3.5% lower fuel savings than similar size skirts with angled straight sections. A longer skirt is not always better since geometry plays a critical role. Straight trailer skirts running parallel to the trailer's long axis, at a small angle to this axis, or a combination of the two designs, will offer the best performance. Skirts that are closer to the ground and longer offer better results, but may have clearance issues.

To increase the lifetime of the device, inquire about protective coatings, material composition, and means of attachment: metallic devices may rust and non-metallic devices may degrade through exposure to the environment.

Side skirts can be bent if drivers go over high curbs or snow banks. This can result in dents, which can interfere with the streamlining of the trailer and therefore reduce their effectiveness. To reduce damage from severe road conditions, choose trailer skirts that are made from stiff yet flexible materials.

Regulatory Issues

There are no specific regulatory requirements, but the device’s weight increases tare and marginally reduces payload allowance.

California requires that trucks operating in the state be equipped with aerodynamic devices, such as trailer skirts. These requirements vary depending on the age of the tractor and trailer. More information can be found at http://www.arb.ca.gov/cc/hdghg/hdghg.htm.

To qualify as a US EPA Certified SmartWay^SM trailer, the trailer should be equipped with side skirts, among others.

Maintenance Considerations

Install according to the manufacturer’s specifications: correct installation is critical for optimum efficiency. Some manufacturers and dealers provide installation, or training on the installation.

Regularly check that the trailer skirts have not been damaged and are secure.

During the winter, check that snow or ice has not accumulated on the device, as its aerodynamic efficiency will be impaired and the weight of the ice itself could damage it.

Some trailer skirts are built from multiple panels that are bolted together so that they can be adapted to a variety of trailers; therefore, individual panels can be replaced if damaged. When installing the component panels, the bolts must be loose enough to allow the skirts to expand and contract.  If the mounting bolts are too tight, then the trailer skirts can buckle and become wavy with changing temperatures.

References

Council of Energy Ministers. 2009. On the road to a fuel-efficient truck: A guide for purchasing aerodynamics for heavy-duty tractors and trailers. Natural Resources Canada, Ottawa, ON.
http://fleetsmart.nrcan.gc.ca/index.cfm?fuseaction=rfet.index (accessed November 2010).

Department for Transport. 2007. Freight best practice: Aerodynamics for efficient road freight operations. Queens Printer and Controller of HMSO 2007.

Surcel, M.-D. 2010. Energotest 2010: Fuel consumption track tests of fuel-saving technologies. FPInnovations, Pointe-Claire, QC. Internal Report 2010-10-28. 93 p.

Surcel, M.-D.; Michaelsen, J.; Provencher, Y.  2008. Track-test evaluation of aerodynamic drag reducing measures for Class 8 tractor-trailers. SAE 2008-01-2600. SAE 2008 Commercial Vehicle Engineering Congress and Exhibition, October 7–9, 2008, Rosemont – Chicago, IL.

Surcel, M.-D.; Provencher, Y.; Michaelsen, J. 2009. Fuel consumption track tests for tractor-trailer fuel saving technologies. Paper no. 2009-01-2891. SAE 2009 Commercial Vehicle Engineering Congress & Exhibition, October 6–8, 2009. Rosemont – Chicago, IL.

Web site of SmartWay Transport . Verified aerodynamic technologies.
http://www.epa.gov/smartway/transport/what-smartway/verified-technologies.htm#advanced (accessed November 2010).

Date modified:

2012-03-09

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