Aerodynamics is an easy concept to understand. Here is a simple test to show you how aerodynamics affects your truck going down the road. While driving on a road with little traffic at road speed, roll down your window and with your hand flat, stick your arm out the window putting the palm of your hand forward. Your hand and arm will be blown backwards. Now, make a fist and put your arm out of the window. There is no push against your hand leaving it and your arm stable due to your curved fist providing less resistance and allowing the wind to veer around it. Multiply the force you felt against your flattened hand to equal the force against the front of your truck and trailer. This is the concept behind aerodynamic designs found in nose cones, curved hoods, sleepers and other air deflection devices.
A class 8 truck uses roughly 50-70% of its fuel to fight wind velocity in trucks without aerodynamic design. That is a lot of fuel to pay for achieving little. Realizing the fuel costs, the first thing to change by truck designers was building higher sleepers and adding a slope to them. This took the place initially of a nose cone on the trailer and allowed the wind to flow over the top of the cab and trailer. Side gap fairings on the truck came next to stop the drag caused by wind collapsing behind the cab and hitting the nose of the trailer. Kenworth came out with the T-600 slope nosed truck and a few years later, other truck manufacturers followed suit starting to round out the hoods on some models.
While trailer manufacturers came in behind truck manufacturers in designing aerodynamic trailers, the nose cone was soon seen on dry vans. Reefer manufacturers started to round out nose-mounted reefer units using the unit itself to deflect the wind. This was the status quo for many years.
With fuel going to $5.00 a gallon in some places, EPA scientists, studying how to effectively save fuel and reduce greenhouse gases, found that the drag from wind going under the trailer and wind collapsing behind the trailer was immense. Trailer skirts and tail wings were invented, as were other devices to mount under the trailer to help the wind move along easier. One driver interviewed about the side skirting on his trailer said his fuel economy increased 8/10 of a mile per gallon after installing them.
California leads the way in requiring fuel saving devices on trucks and trailers. Starting out requiring these devices on trailers for California-based trucks at first, California will soon require them on any truck running the state’s highways.
Many trailers are seen with the trailer skirting, but not so many with the tail wings. Due to the size required to reduce collapsible drag and the logistics to the driver of having to navigate into small dock spaces, many companies have bypassed these fuel savers. However, work is being done to design newer models.
Other things seen in truck manufacturing to reduce drag are the lower, more rounded bumpers, mirrors, fairings over fuel tanks and wheel covers. Fuel economists and EPA scientists would like to close the gap between the tractor and trailer even more, perhaps all of the way, but this is not feasible as it would affect the turning radius of the unit.
One thing that a driver who is buying a truck can do without extra cost is to spec his truck for the type of trailer they will be pulling. While a high stand-up sleeper is desirable by most and works well pulling a dry van or reefer, it might not be ideal for tanker or flatbed trailer use. For those types of trailers, a mid roof or even a flat top would lessen collapsible wind drag between the tractor and trailer.
For more information on the EPA’s Smartway system of reducing fuel consumption and greenhouse gases, go to their website http://www.epa.gov/smartway/technology/index.htm .