On Aerodynamics

The Effect of Aerodynamics on Tractor Trailers

Understanding Tractor Trailer Aerodynamics

The fuel efficiency of semi-trucks is influenced tremendously by aerodynamics. Most of the power generated by a class 8 tractor (50% to 70%) is utilized to overcome drag forces while operating at highway speeds. Aerodynamic fairings have therefore become standard equipment on modern tractors. Aerodynamics for trailers on the other hand have been largely ignored until recently and represent the greatest area for potential improvements left on the tractor trailer combination.

The drag caused by a trailer's box shape is a severe detriment to the vehicle's overall wind resistance. NASA studies had proven back in the 1980's that streamlining the front, rear and undercarriage of a rectangular vehicle can reduce fuel wasting drag by over 30%. The key to achieving these savings has been recent work to identify the most important and practical areas to realize these gains.

Side Fairings

Freight Wing Side Fairings Undergoing Testing

The area underneath a trailer represents the greatest opportunity for significant and practical semi trailer aerodynamics improvements. This statement in not only based on Freight Wing’s pioneering work in this area, but is supported by 18 different skirting products that have recently been able to meet the 4% minimum SAE J1321 type II testing requirement of the EPA SmartWay’s verified aerodynamic technologies list. Freight Wing welcomes this new competition as it validates what we have been doing for years in promoting this important fuel saving strategy.

Trailer skirting technology is very effective in redirecting airflow and crosswinds around the drag inducing rear wheel, axle components and cross members to provide improved laminar air flow around the trailer. The new state of the art wedged skirting geometry developed by Freight Wing and featured on the Aeroflex fairing has introduced the industry to levels of performance that can not be ignored. With up to 7% fuel savings available from this new system, the competitive edge gained in long haul operations with the Aeroflex is truly remarkable.

Semi Trailer Side Fairings

Gap Fairings

Freight Wing Semi Trailer Gap Fairings

The gap between tractors and trailers is another high drag region that is easily streamlined. Ideally, this space would be sealed completely. Tractor manufacturers have made great progresses towards this ideal through aerodynamic roofs and side extensions, but additional improvements are impractical as the remaining gap is crucial to truck operation as it enables a truck to turn.

Because a sizable 24” to 48” tractor trailer gap still exists even in the most efficient set up, air still enters the gap to hit the trailer's front face directly, especially in cross winds. The Freight Wing Gap Fairing solves this problem by protecting the top and side edges of the trailer’s face with curved airfoils that leave plenty of room for turning. This is a proven area for practical improvements as gap fairings are inexpensive, lightweight and pose no operational problems.

Rear Fairings

Although streamlining the rear of the box trailer has obvious potential for improvement, realizing these gains are extremely difficult due to the operational constrains of rear door configurations and the large structure required to achieve even a modest drag reduction. Unfortunately, a practical rear fairing system has remained a tough design challenge that many have tried, but none have been able to consistently prove in legitimate SAE J1321 testing situations or in field operations. Freight Wing does offer a Rear Fairing which can provide fleets an inexpensive option to experiment with this emerging technology.