(The abstract linked in the story is here:
Fuel consumption impacts of auto roof racks)
Roof racks create drag, cost gas -- study
Ariel Wittenberg, E&E reporter
GreenWire, April 26, 2016
Bad news for anyone planning on strapping their bikes or kayaks to their car for a day trip this summer -- roof racks significantly limit fuel economy.
A new study from the Lawrence Berkeley National Laboratory, published in the journal Energy Policy, estimates that roof racks can cost drivers as much as 25 percent more in gas.
Berkeley researcher Alan Meier, working with Yuche Chen of the National Renewable Energy Laboratory, estimated the fuel consumption of the vehicle add-on.
Nationwide, they found that roof racks were responsible for 0.8 percent of light-duty vehicle fuel consumption -- 100 million gallons of gasoline -- in 2015.
"I've always been intrigued by energy consumption that was somehow overlooked or ignored because, for example, it wasn't in the test procedure," Meier said.
Placing a roof rack on a car creates an aerodynamic drag, which requires vehicles to expend more energy. Depending on the rack's configuration, it could create a fuel consumption penalty of zero to 25 percent on a passenger car.
"For comparison, the additional fuel consumption caused by roof racks is about six times larger than anticipated fuel savings from fuel cell vehicles and 40 percent of anticipated fuel savings from battery electric vehicles in 2040," the researchers wrote.
An abstract for the study explains that the researchers used a "bottom-up" approach to estimate national fuel consumption of roof racks. The duo were able to assess the fuel consumption impacts for several different vehicles using a computer model that incorporates real-world data and vehicle stock information. The researchers also created an energy inventory model to include rack usage rates, vehicle stock, vehicle-level roof rack fuel consumption penalties and vehicle miles traveled through 2040. The study considers the drag of roof racks both when they are loaded and when they are unloaded.
The pair wrote in their report that policymakers should consider crafting regulations to reduce the number of drivers traveling with empty roof racks on their cars. They also say a policy requiring energy labeling of roof racks could spur such a change.
Meier noted that making roof racks that are easier to remove could also prompt drivers to take them off their cars when they are not in use.
The study estimates that a government policy to reduce the use of unloaded roof racks, in combination with more energy-efficient rack designs, could save 1.2 billion gallons of gasoline over the next 26 years.