Bottom Line
Against the wind

By Paul Abelson
senior technical editor

 

When Bob Seger sang he was “still runnin’ against the wind,” he had no idea he would be wailing the anthem of tens of thousands of big truck owners still pushing a big hood down the road.

One of the greatest fuel wasters in over-the-road trucking is aerodynamic drag. At speeds below 50 mph, it is not a significant factor, but as speeds increase, so do the drag’s effects.

Over the past 30 years, we’ve seen truck builders develop very sophisticated aerodynamic tractors, with large radius rounded corners and fairings to direct air over and around trailers.

Until the start of the 21st century, the overwhelming majority of research has gone into tractor aerodynamics, leaving the trailer largely ignored. Now, it is the next target for improvement.

The idea of aerodynamic improvement is to leave the air with as simple a path around the vehicle as possible. The fewer the changes in direction, the less force – or fuel for that matter – is required. Initial efforts were concentrated on the tractor because it is the first part of the truck to encounter air resistance, and the payoff was significant and immediate.

Today, the tractor accounts for only about 25 percent of the total aerodynamic resistance. The remaining 75 percent is in the trailer. Of course, these are approximate figures for a dry van or refrigerated trailer. Platform and tank trailers have other considerations.

Of the 75 percent, about 30 percent occurs at the nose, in the gap between tractor and trailer. Another 35 percent is at the rear of the trailer, and the remaining 35 percent occurs below the trailer, at the suspension, axles, landing gear and cross members. Products have been developed to address each of these areas.

Before describing some of these, let me advise you to be frugal in your purchases. Frugal does not mean cheap. You need to understand your needs and choose the right product to fit them.

If a product will return your investment in a year, and then keep paying you back after you’ve paid for it in full, it is a penny-wise purchase. But if a lower price attracts you away from better quality and the product wears out or breaks after a few months, your choice was not a prudent one.

THE GAP
The quickest, easiest and least expensive way to reduce drag at the nose is to shorten the gap between tractor and trailer. Back when most tractors had spring suspensions, stretching out your ride “West Coast style” made a great deal of sense. It improved the ride, and with diesel at 25 cents a gallon, no one paid that much attention to mpg. But for some of those rigs, the net effect was to almost double the frontal area, a key contributor to drag. Today’s air suspensions ride well, so the need to stretch it out isn’t there anymore.

Use a full air deflector. If you don’t already have cab side extenders, get some at your dealer. With rubber rear inserts, there shouldn’t be any problem with trailer interference. Still, some air is bound to get into this turbulent gap. To control it, Silver Eagle offers vortex traps from Solus Engineering.

Nose Cone makes a series of aerodynamic trailer noses for the front of trailers. They were originally developed for rigs without tractor aerodynamics, but testing later proved that, because of cross winds, there were measurable mpg improvements when used with aerodynamic tractors. Many tractor air shields also have adjustable trim tabs top center. These should be angled to deflect air so it meets the trailer roof just behind the leading edge. Rounded top lips are available to smooth the transition of air from tractor to trailer. 

DOWN BELOW
To keep air turbulence from under the trailer, Laydon Composites and Freight Wing have trailer side skirts that can be retrofitted to existing trailers.

First-generation skirts were rigid. They were often damaged by curbs and backing into pits. They collected snow and ice, adding weight. The latest versions are pliable, resist impact and abrasion damage, and do not hold snow. Approved by the EPA SmartWay program, and proven in

SAE/TMC tests, they effectively increase fuel mileage.

Nose Cone offers a Belly Cone that shrouds the rear tandem and suspension. Also worth considering, Eco-Flaps provide air an easy path behind wheels. They, too, had SAE/TMC tests. Field results showed 2 to 6 percent improvement for very little outlay.

TAIL END
At the 2008 Mid-America Trucking Show, ATDynamics introduced the TrailerTail, a device that folds flat and stores against the doors, and extends four feet beyond the trailer when deployed.

By simulating a long, tapered streamlined tail, the  TrailerTail improves fuel economy 5 to 6 percent by reducing suction drag. It is legal, according to the FHWA, citing 23CFR658.16, “Exclusion from Length and Weight Determinations.”

Other aerodynamic aids for trailers include vortex generators from Solus and Airtab. They direct air flow to emulate a streamlined tail. Some of the largest fleets in the country have improved fuel economy more than 15 percent by using combinations of trailer add-ons.

Besides saving fuel, there are other benefits from improved trailer aerodynamics. Stability in cross winds is improved while splash and spray are reduced, increasing visibility around the truck. LL

 

Paul Abelson can be reached at truckwriter@anet.com.