Roof racks a drag on fuel economy

April 26, 2016, Lawrence Berkeley National Laboratory
Both loaded and unloaded roof racks create drag, reducing your vehicle’s efficiency. Credit: istock.com/Sisoje [L], flickr.com/Chris Young 43

As you get ready to hit the road this summer, with the kids loaded inside and the bikes strapped to the roof of your car, you may want to stop and consider that the roof rack on your car may be costing you as much as 25 percent more in gas.

In the first study of its kind, Lawrence Berkeley National Laboratory (Berkeley Lab) researcher Alan Meier, working with Yuche Chen of the National Renewable Energy Laboratory, have estimated the fuel consumption penalty of this popular and fast-growing vehicle add-on. They found that in 2015, roof racks nationwide were responsible for 0.8 percent of light-duty , or 100 million gallons of gasoline.

Their study was published recently in the journal Energy Policy in a paper titled "Fuel consumption impacts of auto roof racks." In addition to projecting the fuel consumption penalty into 2040, at which time usage of roof racks is estimated to increase by about 200 percent in the United States, they also calculated how effective various policy and technology measures would be at mitigating the penalty.

"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. "In this case the fuel consumption of vehicles with after-market accessories isn't captured in the test procedure."

Use of roof racks requires vehicles to expend more energy due to aerodynamic drag. While there have been studies of their impact on individual vehicles—depending on the configuration, the fuel consumption penalty can be 0 to 25 percent on passenger cars—this is the first study to estimate impacts at the national level. Moreover, use of roof racks is projected to increase given national travel trends.

"A national perspective is still needed to justify policy actions," the authors write. "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."

Meier and Chen used a bottom-up approach to collect data, including using online forums and crowdsourcing as data collection sources. For example, roof rack usage rate estimates were based on nationwide highway video surveys conducted by the authors and workers recruited through Amazon Mechanical Turk.

"It was a way of doing a national survey on the cheap," Meier said. "We considered using some other sources, like toll booths, but we found the quality wasn't good enough to figure out what was on the roofs."

They also created an energy inventory model that included rack usage rates, vehicle stock, vehicle miles traveled through 2040, and vehicle-level roof rack fuel consumption penalties. They considered roof racks both loaded and unloaded. (See photos.)

The researchers then conducted a sensitivity analysis to understand how the national penalty would change with varying inputs, for example, whether vehicles are driven on the highway or not and whether racks are loaded or not. They found that unloaded cross roof racks (with crosspieces, as pictured) driven on the highway was the scenario that made the biggest difference; this is because the total miles traveled with unloaded racks is four to eight times higher than that for loaded racks.

"These results suggest that some fuel-saving policies should focus on reducing the number of vehicles driving with empty racks," the researchers write.

So next they analyzed the impact of possible policy, technology, and behavior changes. For example, manufacturers have found that it is possible to make roof racks with greatly improved aerodynamics. A policy to require energy labeling of roof racks could spur greater changes, the researchers note.

Even greater energy savings would come from removing roof racks when not in use. Meier notes that they could be designed so as to be easier to remove. The researchers estimated that a government policy to minimize unloaded roof racks (admittedly extreme) in combination with more energy-efficient designs would result in cumulative savings of the equivalent of 1.2 billion gallons of gasoline over the next 26 years.

Explore further: Federal policy reverses benefits of alternative fuel vehicles

More information: Yuche Chen et al. Fuel consumption impacts of auto roof racks, Energy Policy (2016). DOI: 10.1016/j.enpol.2016.02.031

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12 comments

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Nik_2213
not rated yet Apr 26, 2016
Snag, though, is the push to streamline cars has diminished both their cargo capacity and tail-gate dimensions. Where do you put a mountain-bike or bikes ? Some vehicles will take a tail-rack, the rest must make do with a roof rack. Short of a bulky, aerodynamic clam-shell, like an over-sized surf-board enclosure, what can you do ?
kochevnik
not rated yet Apr 26, 2016
It is a silly concern. So much energy falls on the planet lost as heat. The real concern is engines emitting carcinogens into the air and burning building materials, instead of using captured solar fusion power

In San Francisco new buildings must now include solar power generation
phprof
1 / 5 (3) Apr 27, 2016
I see more garbage back door science here. This article reflects the basic failure to use direct scientific observation, experimentation, and analysis. I would be ashamed to publish these results.

With the science critic out of the way, now for philosophy. This is a sign of the totalitarian nothing is ever good enough mentality. First it's smaller more fuel efficient cars. Then it is more biking but the bike trails are too far from work and home so one uses a rack but then that isn't good enough so they complain about that. Then it's a requirement to have untenable, unreliable, and environmentally unsafe "green" energy sources on your home and/or business. It all leads to one end. A few people deciding all for the rest and a police state to enforce it.
antigoracle
not rated yet Apr 27, 2016
Roof rack tax. /sarc
antialias_physorg
5 / 5 (3) Apr 27, 2016
Where do you put a mountain-bike or bikes ?

And here I was thinking riding a bike was the point...not lugging it around with your car. Silly me.

The point they are making here is that you should take the RACK off when you're not using it (unless you need to drive around with a bicycle strapped to it 24/7 - but I can't really see a scenario that would warrant this)
kochevnik
not rated yet Apr 27, 2016
And here I was thinking riding a bike was the point...not lugging it around with your car. Silly me.
Fast way to die on many old, congested roads
antigoracle
not rated yet Apr 27, 2016
The point they are making here is that you should take the RACK off when you're not using it

Come on, that's the sort of exercise that builds the wrong muscles.
PPihkala
not rated yet Apr 30, 2016
I also think that when roof racks are easier to mount and unmount, there will be less miles traveled with empty ones. I myself try to use them only loaded, because I know that they increase the fuel consumption. So off they come as soon as the load is off them.
rrrander
not rated yet May 01, 2016
Greenies want everyone stuffed on public transit, like animals. Except them of course.
antialias_physorg
not rated yet May 01, 2016
Fast way to die on many old, congested roads

Then don't ride on roads. Many countries have enough bycicle ways to get around safely. If yours doesn't then lobby for them.

In any case: In countries where riding bicycles is common drivers are aware of them. It's a lot safer than you think.
ogg_ogg
not rated yet May 01, 2016
25% ? No one should wonder why science and scientists aren't taken seriously. This crap is exactly why anti-scientists have a point.
Da Schneib
not rated yet May 01, 2016
The point they are making here is that you should take the RACK off when you're not using it (unless you need to drive around with a bicycle strapped to it 24/7 - but I can't really see a scenario that would warrant this)
I never drive with my integral roof racks installed unless I'm carrying something on them. Seemed relatively obvious to me. I think lots of people are lazy and don't think aerodynamics applies to them. :p

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