other styles. The design of many of these non-dry vans is far less amenable to incorporation of all or even some of the aerodynamic devices found helpful on the dry van. Polk has provided a dataset for all trailers by body style for the state of Florida (Table 5-7). The results are summarized here as a surrogate for the national averages.

Aerodynamics of Van Refrigerated

The trailer-face-mounted refrigeration unit will dictate different gap treatment solutions than those used with the dry van. The refrigeration unit requires an airflow to provide engine combustion air, engine cooling air, and refrigeration condenser cooling air. On the other hand, the refrigeration unit itself may provide an aerodynamically significant reduction in drag, by virtue of providing some vortex control and associated pressure increase on the tractor rear face. Of course, the trailer skirt and trailer base areas are similar to those on a dry van, so similar aerodynamic features can be used. Freight Wing, Inc. has fitted skirts to a refrigerated van (see Figure 5-18), but test results for this trailer type are not yet available.

Aerodynamics of Flatbed Trailers

This trailer body style is among the most problematic of the semitrailer family because the space above the deck carries an endless assortment of products and implements, many with grossly unsymmetrical geometries. The trailer skirt area is judged to be as amenable to aerodynamic treatment as the dry van. One trailer aerodynamics supplier, Freight Wing, has fitted a flatbed with a skirt but has not yet tested the configuration. Note that the trailer model in Figure 5-19 also has a spread axle, which somewhat complicates skirt addition and may limit its potential effectiveness.

TABLE 5-7 Florida Trailer Population by Body Style

Body Style

Population (%)

Auto transporter




Container chassis


















Van refrigerated






SOURCE: Personal communication between L. Hart and C. Salter, committee member, June 2, 2009.

Aerodynamics of Container Chassis

This body style appears substantially similar to a van trailer. A removable container box is attached to a skeleton chassis consisting of a frame, a king pin, and an axle bogie. The difference from an aerodynamic point of view is that this style is equipped with square corners and many external ribs. These ribs provide the requisite strength to the container box to deal with the handling forces when loaded containers are lifted on and off the chassis frame but also can add to the trailer Cd (see Figure 5-20). The trailer skirt area is judged to be amenable to aerodynamic treatments, similar to those of the dry van. One trailer aerodynamics supplier, Freight Wing, has fitted a container chassis with a skirt but has not yet tested the configuration (see Figure 5-21). Unfortunately, typical empty-chassis handling and stowage practices will put trailer skirts at high risk of damage.

Aerodynamics of Tank Trailer

Since tanker trailers are normally operated at the maximum legal weight limit, the design is constrained to achieve minimum tare weight within the structural demands of the unit. While the tank itself is typically cylindrical, often with a hemispherical or somewhat rounded front face, the functional needs often result in an external skeleton of pipes, tubes, and so forth, to facilitate product loading/discharge and personnel protection when accessing certain operational devices. Further, the rear shape of the leading tractor poorly matches the trailer face in aerodynamic terms. Standard high-roof sleepers or day cab air deflectors are too high for tanker-trailer applications. Trailer skirts can be fitted as the dry van. One trailer aerodynamics supplier, Freight Wing, has fitted a tank trailer with a skirt, but has not yet tested the configuration (see Figure 5-22). Because any weight added for aerodynamics will reduce the load that a tanker can carry, these features are not likely to be financially attractive to any operator unless regulatory allowances are made.

Aerodynamics of Auto Transporter Trucks

The population of this body style is relatively small. This design style also has a complex nonaerodynamic array of structural tubes deploying the moving floors. Interestingly, one transport company, Precision Motor Transport Group (PMTG), has created a variety of trailer configurations incorporating curtain sides, rounded noses, and boat tails. This carrier specializes in transport of upscale sedans, and the auto capacity is inferior to the more standard designs. PMGT’s trailer system can hold six to eight sedans, compared to nine to eleven for a typical transporter trailer. The aerodynamic performance must be substantially superior to the traditional design, but no data are available at this time.

PMGT’s solutions raise the question of what could be done to improve other current body styles that have poor

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