Aerodynamics
Truckers readily pay for tractor fairings, but few will even consider the fuel-saving gains possible with aero improvers for trailers.
Tom Berg
Equipment Editor
Aerodynamic packages for tractors? Sure. Why not? Even if a set costs thousands of dollars, tests and years of operating experience show that a fairing on the roof, skirts covering tanks and boxes, and cab or sleeper side extenders all cut air drag and save fuel money.
Accordingly, manufacturers design the best fairings possible because they know they'll save operating expense - and they know customers will buy them.
That's not true of trailers, which usually face the wind in bare-bones attire. Trailer builders say fleet customers are just too price conscious to consider anything that could save them more fuel money.
The latest tractor example is the new Volvo VT880, the builder's entry into aggressive, owner-operator-pleasing style (see story on page 102 in this issue). It's got a bold nose and big engines, but it purposely sidesteps "heritage" style typical of "long-and-tall" tractors in favor of aerodynamics.
Instead of bolting the chrome exhaust stacks to the cab sides just behind the doors, the stacks are mounted behind the sleeper - still visible in all their glory, but shielded by the cab, at least when the air flow is directly from the front. And the air intakes are flush against the hood's sides (albeit with chrome-plated "toaster" grilles and amber LEDs) instead of being part of can-type external air cleaners. Not every buyer will like this approach, but those who use it will save some fuel money, Volvo designers insist. Some other "large car" tractors can be spec'd this way, and a few individualists do so.
Even the industry's dominant "large cars" with few aero touches - Peterbilt's 379, Kenworth's W900 and Freightliner's Classic - are usually sold with large roof fairings to customers who pull vans or reefers. Trailers seldom get the equivalent, except for reefer units whose cabinets are tapered to reduce wind drag.
Why Not Trailers?
In the early 1980s, when interest in fuel economy was high, Peterbilt Motors built an experimental rig with a very short tractor-trailer gap, and with side skirts and a "truncated" boat tail applied to the van trailer. Careful on-highway testing proved the aerodynamic trailer added 1.5 miles per gallon to the rig's fuel economy. How much would that save at today's fuel prices of $2 and more per gallon? Do the math based on your typical trailer's annual mileage.
Engineers acknowledged that the deep side skirts could easily be battered on driveway aprons and high-crowned railroad crossings, and that not many operators would accept the special boat tail, even though it allowed fairly normal forklift loading of cargo. Besides, Peterbilt didn't make trailers (and still doesn't), so they dropped the project.
Today, the gap between tractor and trailer is still too long on many rigs, when shortening it could reduce turbulence and save fuel money. And trailer aero improvers are available but still largely ignored.
Nose Cones, Side Burns, Tail Cones and Air Tabs are add-on devices that we've written about in the recent past (see July '04 HDT). A Nose Cone applied to the front of a van (whether trailer or truck body) smooths its passage in frontal and cross winds, and reduces turbulence between a tractor and trailer, the company claims. Savings of a half a mile per gallon are typical and drivers notice improved handling, users have told us.
Nose Cone Manufacturing Co. also offers Side Burns, which are rounded panels fitted to the forward edges of a trailer's vertical front corners. These cut drag even more and are said to improve fuel economy by an additional 5%. Another product is the Tail Cone, installed on the rear of car-hauling trailers (but not practical on most vans that load and unload at docks).
Air Tabs, small wishbone-shaped "vortex generators" affixed near the trailing edges of the sides and roof of any box-type body, detach air from those surfaces and reduce the vacuum usually produced at the rear doors. Truckers who use the inexpensive, easily installed products testify that they measurably cut fuel costs and keep the rear doors fairly free of dirt and snow - indications that the drag-producing vacuum is indeed reduced.
Yet trailer builders we've talked with say very few customers inquire about such products. And if they're told about the aero improvers, customers refuse to spend any money on them.
Some cite the fact that their trailers spend a lot of time sitting at terminals or in customers' yards; others say they're afraid the devices will get banged up and cost money to fix, and still others simply aren't interested.
Basic Details
Even if the trailer sits much of its life, buying a basically smooth vehicle will save fuel money every time it moves. (And if it doesn't move much, how can it make any money?) And some aerodynamics are possible on non-van trailers. For example:
• Smooth tops - Dump truckers complain about high fuel prices just like everybody, but most of them don't actually do much about fuel economy. Few may realize that using tarpaulins to tightly cover the load not only keeps the contents from blowing into motorists' faces but also improves aerodynamics. That's even more true of open empty trailers (and presumably truck bodies).
Pulltarps Manufacturing says its distributor in the United Kingdom commissioned wind tunnel studies by MIRA Ltd., an automotive testing facility, whose findings might surprise many truckers. With a tarp extended over a load, fuel use was 3% less than without a tarp. When the tarp was stretched over the empty body, fuel savings were 11%. It follows that if a dump rig is loaded for half its running miles, the fuel savings would average 7%.
Pulltarps meanwhile says its products use a rounded and beveled leading edge that eases air over the tarp and guides the air down the trailer's length. And it recently added a spoiler atop the rearmost bow that stretches the fabric tightly to the rear, further smoothing the air flow.
• Smooth sides - Dump truckers might also save some money by running vehicles like East Manufacturing's Genesis dump trailers, which use clean sidewalls of polished aluminum instead of outside posts. Without the drag of external posts, Genesis trailers can yield fuel savings of 5% to 10%, equivalent to an extra 0.25 to 0.5 mile per gallon, East claims. Depending on speeds, hauling distance, fuel prices and other factors, the per-vehicle savings could be as high as $3,000 a year.
• Smooth undersides - There's not much that can be done to improve the aerodynamics of flatbed trailers, but East is trying. Its underbelly crossmembers are rounded on the bottom to reduce the drag of air as it flows past the beams. And the flooring panels are enclosed. In both cases, airflow still has places to hang up, and engineers have not tested the designs for actual fuel savings.
Flatbedders might not think about it, but tarping their loads can improve aerodynamics because the tarp takes the sharp edges off corners of squarish cargo. Many flatbedders and reefer operators run wide-spread tandems to more easily distribute load weight; some install a bright-metal toolbox to fill the gap between the axles, and that box has the added benefit of smoothing air flow past the wheels and tires.
As the saying goes, every little bit helps.
