The aerodynamics associated with rotating tires and wheels are complicated by many factors. Wheel covers, which are inserted into the wheel of all tires, streamline the airflow around tires. They provide a net benefit to the fuel economy of the vehicle, but the improvement is small.
The cost, weight, and installation time of wheel covers is relatively small compared to other products. One set of four covers for a trailer complete with mounting bracketry may add 20 to 50 pounds to the trailer.
Multiple wheel cover designs are available. The primary design challenge has been making it possible to remove the wheel covers easily so that the wheels are accessible for inspection, maintenance, and repair
1% to 10% depending on the devices chosen
This list represents some manufacturers of wheel covers but it has not been vetted to ensure product availability.
Each area of the trailer represents an opportunity to reduce aerodynamic drag.
Both industry and government aerodynamicists have shown that the maximum aerodynamic improvement comes from a combination of sealing the tractor/trailer gap, sealing the trailer underbody, and adding a boat tail.
As each device is added, the performance of other devices will be impacted. The airflow over each device changes the operating conditions for the other devices. The performance of a combination of devices will not simply be the additive total of each device operating alone. However the greatest aerodynamic drag reduction comes from using devices in three main areas: gap, underbody, and rear. Addressing the aerodynamics of all three points of drag should give the greatest fuel savings for the vast majority of fleets
The overall perception of the savings offered by trailer aerodynamics is positive. “They are really effective devices now,” one fleet owner said.
Fleets stated that aerodynamic device construction, design, and materials have all vastly improved in the past five to seven years. They have become lighter and more robust.
Some fleets feel that drivers have become more accustomed to having aerodynamic devices on trailers and when combined with fuel economy incentive programs, actually appreciate having them.
Fleets were uniform in stating that the devices should “require no driver intervention.” One fleet owner said, “Any statement that starts with ‘All the driver has to do is…’ should be questioned.”
Fleets have been investing in trailer skirts as their first choice for aerodynamic improvements. However, now having done that they are looking at the next steps and are debating the merits of tails versus other options.
The study team developed several tools to help fleets make their decision about trailer aerodynamic devices.
• There is significant data showing fuel savings for the various trailer aerodynamic devices. The priority for device adoption by fleets is skirts, tails, front, and then other devices.