Aerodynamics Part III

By Allen Penticoff

Penticoff1This is the last in this three-part series on aerodynamics. Previously, we had the basics (with a bit of flawed math – to be explained later) and a look at the aerodynamics of camping trailers. This part will address the changes in aerodynamics of the over-the-road freight industry – better known as tractor-trailers or semi-tractor trailers. Since the technically correct term is “tractor-trailer” that is what I shall use here.

Outside of aviation, no industry has probably benefited more from aerodynamic improvements than the freight/trucking industry. Up until recently, tractor-trailer rigs were utterly devoid of aerodynamic and environmentally friendly efficiencies. The tractor bodies may be shaped like a dice cube and the trailer behind it was shaped like a Velveeta box (only us ancients know what this is about, so let’s call it a long box). Enter high fuel costs and concerns about global warming. The governments of the world are concerned about global warming and who puts what into the atmosphere. Freight hauling enterprises are concerned with the cost of fuel going sky-high. Enter the engineers. Long had the engineers known that tractor-trailer rigs could use some cleaning up – now was their chance to do something about it. Shoving a box down the highway at 70 mph requires a lot of energy and thus fuel. That’s dollars to the trucking companies and pollutants to the government. In town at 20 mph, 28 percent of a tractor-trailer’s power goes to overcoming aerodynamic drag, while 72 percent goes to the rolling resistance of tires and other equipment. Bump that speed up to 70 mph and the aerodynamic drag and rolling resistance swap factors; 70 percent of power goes to overcoming drag and 30 percent to other factors. Clearly, since the bulk of tractor-trailer time is spent on the highway at high speeds, some significant savings in fuel consumed can be made by making their passage through the air more efficient.

Penticoff2First off, 25 percent of the total aerodynamic drag of the whole rig is the tractor itself. From the cube-shaped tractor we’ve now evolved tractors with pointy-ish front ends, smooth fenders, integrated fuel tanks, faired headlights, sleeper cabs with high tops that deflect air over the trailer and other improvements in getting the impact air around the tractor and for it to blend with the trailer. In fact, 25 percent of the drag is created by the turbulent interface of the tractor to the trailer. Anything that can deflect the airstream up and over or around the front of the trailer improves its efficiency and the faster they travel the more important this becomes.

The other 50 percent of aerodynamic drag comes from the trailer. Try to visualize the air flowing over and around the tractor… it’s eight rear tires present themselves to the air and create turbulence (suction) behind them. Undercarriages and the trailer’s own eight tires present more impact resistance to the air and more turbulence behind them. Enter the trailer “skirt.” With this simple skirt of thin metal draped under the trailer between the tractor’s tires and the trailers tires, air is prevented from circulating behind the tractor’s tires, reducing turbulence – the undercarriage is not exposed to direct air flow, and the impact of air against the trailer’s tires is very limited. All that is left is the turbulence behind the trailer – which in itself is a whopping 25 percent of total drag. That’s 25 percent for the interface, 25 percent for under the trailer and 25 percent for behind the trailer.

While I daydreamed about inflatable pointy tails for trailers, working engineers figured out a way for an origami of simple flat panels to greatly reduce the drag/turbulence/suction of the trailers. One of the more popular designs is called “Trailer Tails” sold by Stemco. These panels fold out from the back of a trailer to create an easier path for the air flow to exit off the trailer. Canted in slightly, the air follows this path so that the immediate vortex coming off a 90-degree angled edge is reduced dramatically. The reduction in drag is significant. A Trailer Tail in itself will reduce a modern tractor-trailer rig’s diesel fuel consumption by 14.4 percent at 70 mph – that’s a lot of money and a lot of air pollution.

There are other benefits besides increased efficiency. The reduced turbulence makes the trailer more stable. The spray is reduced during rainy weather, improving visibility for the driver and those who follow them. The increased stability means the driver is fighting the trailer less – decreasing driver fatigue. This all sounds win-win to me. In fact, having recently had some long interstate highway road trips – dealing with following or passing modern, updated and “trailer-tailed” trucks is much less the problem it was in the recent past with the old “box” designs. They are hardly worth trying to draft for increased fuel efficiency (and a terrible thing to do anyway – we’ll leave that to NASCAR drivers). Stemco’s website has some good video showing the differences in old versus new trucking aerodynamics.

In 2004, a government (EPA) and the trucking industry organization formed SmartWay® Transportation Partnership, creating a voluntary industry/government partnership to promote reduced fuel consumption and green house gas emissions. SmartWay develops test protocols and verifies performance of vehicles, technology and equipment that can reduce fuel consumption and global warming emissions. Trucking fleets, are seeing the advantages of being SmartWay certified as shippers seek to reduce their own climate impact. For an aerodynamic devise to get SmartWay certification, it must reduce fuel consumption by 5 percent. Generally, trailer skirts and tails reduce fuel consumption by 5 percent or more each.

While this has not been an exhaustive look at trucking aerodynamic efficiency, because some wind tunnel analysis and other products have shown promise – including all electric tractor-trailers, we have come a long way and can be thankful for that. I hope to see more efficiencies along these lines as the aerodynamic improvements are a good cost-benefit to the truckers.

I’ll conclude this series with a response to letter writer, Ed Schott (TRRT 10/26-11/1). Mr. Schott’s message was technically accurate. I delved deep into books and online aerodynamics resources. None had simple solutions. But I believe Mr. Schott’s message was fundamentally correct but over-explained and confusing. The essence is found in a book by Larry Reithmaier – Mach 1 And Beyond, page 31 – “…parasite [and form] drag is directly proportional to the square of the speed.” Unless, dear reader, you are VERY good at complicated math, your take away is, and you can quote me, “The faster you go, aerodynamic drag increases dramatically.” The curve looks like the national debt – ever arching upward the further you move along the chart. That’s why driving slower saves you fuel cost and the atmosphere pollution.

Please visit for the online version of Mr. Green Car with photos and a table of changes in speed/drag/resistance for a tractor-trailer provided by Stemco.

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