WHEEL COVERS (TRAILERS)
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.
As trailer aerodynamic devices add weight the critical trade-off between aerodynamic performance improvement and increasing tare weight will depend on the weight-sensitivity of the fleet.
Difficulty getting accurate comparisons between devices
The different methods used to test and analyze aerodynamic devices can make it difficult to do accurate comparisons.
Variances among aerodynamic manufacturer information
Information published by the device manufacturers varies greatly in content, format, and detail.
The need to optimize tractor/trailer ratios
The tractor-trailer ratio will indicate the trailers actual annual mileage and the return on investment from the adoption of trailer aerodynamics.
Questions about durability and reliability
There are no standards for structural integrity of aerodynamic devices. To survive normal daily operations and should be subject to regular driver inspection and other audits.
This list represents some manufacturers of wheel covers but it has not been vetted to ensure product availability.
- Aerotech Caps
- Flow Below
- Real Wheel Covers
- Solus Solutions And Technologies
COMMON FLEET STRATEGIES
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
WHAT FLEETS ARE SAYING
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
NACFE has developed several tools to help fleets make their decision about trailer aerodynamic devices.
- The Confidence Matrix, informs fleets of NACFE’s confidence in the technology being studied vs. the payback the fleet should expect to receive from the technology
- A simple payback calculator has been developed for aerodynamic devices
- An appendix of SmartWay verified trailer aerodynamic devices can be found at the end of the full Confidence Report
- Trailer aerodynamic devices save fuel. There is significant data showing fuel savings for the various technologies. The priority for device adoption by fleets is side, underbody and gap and then other devices. The EPA SmartWay program has made noteworthy progress since its inception in 2004, providing the industry with a structure for cataloging and ranking trailer aerodynamic devices. It should be considered a foundation for further improvement in performance evaluation.
- Devices have matured and will continue to improve. Skirts have become lighter, less expensive and more robust improving their payback. Other devices are maturing but need continued development to improve their total cost of ownership. There is a widespread recognition of the further improvements and efficiency gains that stand to be achieved in trailer aerodynamics.
- Unique challenges exist. These include trailer-to-tractor ratio which limit the miles per trailer, some cases of the trailer aerodynamics purchaser not buying the fuel and lastly, devices should be driver passive: no driver interaction is required to deploy or stow. There are solutions to these challenges.
- Performance for each fleet is difficult to determine. Performance of any device is subject to many variables and each operator will likely have their own experiences. But the standardized test methods are directionally useful in evaluating different devices and combinations of devices. A range of products are now readily available that offer proven savings. As these products have matured, so has the industry’s understanding of the need for improvements in the way fuel efficiency performance is measured and allocated. Advances in test and analysis continue to be made, but the tools available today tend to report performance judged under controlled, focused, operating conditions rather than representing the full range of operations possible in industry. Although most fleets can measure tractor efficiency very closely, they do not have the tools to monitor the trailer efficiency at all.
- Regulations will drive greater adoption. GHG Phase II and CARB rules will drive much greater adoption of trailer aero devices in the near future, taking them from being add-on options to being standard equipment. The Greenhouse Gas Phase II emissions rules are likely to significantly influence trailer aerodynamic technology adoption. The rules have been released but await litigation proceedings before they can be implemented at this time. California’s existing CARB rules, which are linked to EPA SmartWay designated technologies, are already influencing some investment decisions. However, the primary motivation for aerodynamic technology investment remains a business one, with fleets demanding a two year or less payback for technologies.
- Aerodynamic devices must work without driver intervention. History has shown that devices that need driver intervention — such as first generation trailer tails — are not effective solutions as drivers do not deploy them 100% of the time. Second generation rear devices are addressing some of the challenges of the earlier versions of those devices. Future aerodynamic devices must work without needing driver involvement in their operation.