I have seen the 3% to 7% figures widely cited by skirt manufacturers, regulators with a self-justification agenda and fleets that have a desire to look good in the eyes of the EPA and CARB. But what about real-world sources? Who is out there actually driving skirted trailers that rave about the same 3% to 7%?
Until I see a convincing number of real-world reports from the people who actually drive skirted trailers, I will remain skeptical of the 3% to 7% figure.
Well, just a basic knowledge of aerodynamics should tell you that there will be an improvement. A Gap Fairing alone or Air Tabs on the side of the tractor can add 2% to your fuel economy. The same knowledge should tell you that ridiculous claims of 10-20% would be unlikely. The most notable exemption of tractor-trailers traveling less than 50,000 miles a year should tell you that significant time spent at lower speeds and shorter distances doesn't get you much benefit, and that longer distance and higher speeds is where you will see the most benefits, which is perfectly in line with the aerodynamics. The question is, how much?
Testing by several third parties, including NASA in their wind tunnels, and testing at the Lawrence Livermore Laboratory show a consistent increase in aerodynamics of skirted trailers across the board of five percent. That five percent increase in aerodynamics translates to between 3-7% increase in fuel economy depending on speeds and distances traveled. So this is an aerodynamic benefit, like Air Tabs, which will yield very small results, if any, at low wind resistance speeds and distances. It's much like the benefits of nitrogen or low rolling resistance tires, where the benefits are minimal over time at short distances, but are greatly increased with distance.
Clearly, the trailers driving closer to 50,000 miles a year are going to be closer to the 3% figure, while those traveling 125,000 or more will see a higher percentage, and team trailers will see an even greater number. Schneider reports 3%, but they report an overall change in the fleet, not in the individual trailers with skirts, so that's likely to be on the low end. Walmart reports 6%. Con-Way reports 5% on their trailers with skirts, so does C.R. England, US Xpress, and Fremont Contract Carriers. But even using the most conservative figure of 3%, that can mean tens of thousands of dollars over the live of a trailer, in many cases literally paying for the trailer itself.
Con-Way, Pepsico and a few others are also also experimenting with a base system that includes side skirting and something called an "under-tray" that is a fairing under the trailer. The under-tray system minimizes drag caused by the low-pressure wake behind the trailer and its suspension system, primarily by injecting high-energy airflow into the trailer wake, not unlike the "vortex" caused by Air tabs. The under-tray system was developed by a company called SmartTruck, in cooperation with NASA, the EPA's Oak Ridge National Laboratory facility (mainly through use of their Cray supercomputers). In the past 2 years, Con-way Truckload has installed under-tray equipment on more than 2000 trailers. They're happy so far. Con-Way Truckload has a fleetwide average MPG of 6.5, and the trailers with the combination of side and under-tray has yielded an average of 7% in fuel economy. A combination of the under-tray and side fairings, on the back edge and roof of the trailer (Air Tabs or similar), delivers a 10% improvement in MPG for Con-Way and Pepsico. Using super-singles adds another .75-1.0% on top of that.
The aerodynamics are real, and so are the benefits. But there are the downsides, like difficulty of maintenance, and whether or not a driver will actually crawl under the skirting to do a pre-trip inspection.
But again, these are tractors with 53' trailers, who travel long distances at highway speeds, and the achieved benefits simply are not going to translate in the same way to shorter trailers, those who don't drive as far, and to straight trucks. Knowing basic (and some advanced) aerodynamics tells me that an expedite straight truck that travels long distances will see an improvement in fuel economy with improved aerodynamics, but also knowing the aerodynamics, I would be shocked if it is even as high as 3%, which is the low end of a trailer. The math suggests that it's much closer to one percent, or slightly lower.
Then there's the cost-benefit analysis which doesn't bode well for skirted straight trucks, not only for the small increase in economy versus the repair and maintenance costs, plus down time, but the increased wear and tear from heat since side skirts and other aerodynamic technologies also block the proper flow of cool air over temperature-sensitive tires and brake systems. Side skirts are quite likely THE point of diminishing returns on a straight truck where the benefits of improved aerodynamics will cost you more in repair and maintenance of the skirt, tires and brakes, than you'll ever save in fuel.
I'm not skeptical at all about the 3-7% benefit with a trailer, but like Phil, I'd want to see some at least real-world anecdotal evidence on a straight trucks, and I'd want to see it over time to see just how well the rear tires and brakes handle such a dramatic decrease in airflow, considering they are designed specifically for a certain, proper airflow. There's no question that reducing wind resistance on any vehicle will increase fuel economy, but the cost-benefit must be strongly considered. In that context Air Tabs are certainly worth it, as is slowing down and other driver considerations, along with tires with low rolling resistance, and as much as Phil resists the notion, nitrogen. But skirts on a straight truck? I dunno about that. One new set of brakes or tires just six months earlier cancels the benefits of skirts. So does missing a good load while maintaining or repairing the skirts. Much like taking a brand new medication just approved by the FDA, this isn't one that I'd be wanting to jump into and be a part of the large scale clinical trials. I'd want to wait and see.
