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Polite Reminder: Roof Racks Will Kill Your Electric-Car Range

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2012 Nissan Leaf with roof rack fitted

2012 Nissan Leaf with roof rack fitted

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We’ve passed spring break, the weather is getting warmer and you’ve turned your weekend attentions to camping trips, DIY projects and outdoor sports.

For many Americans, that means digging roof racks out of their winter storage and securing them on top of the family car for the season. 

If that car is electric however, you’ll want to think twice: It could severely reduce your car's range.

That’s because fitting roof racks to almost any car increases both the frontal area of the car and its drag coefficient. 

In short,  a roof rack changes the shape of the car it is fitted to, making it less aerodynamic and increasing the amount of energy needed to push it along. 

Even with an empty roof rack fitted, gas car fuel efficiency can be reduced as much as 12 percent. Carrying something like a bicycle could reduce gas-mileage by 30 percent. 

In a gasoline car capable of 300-500 miles per tank, a reduction in fuel economy of between 10 and 30 percent will require you visit the gas station more often, hurting your wallet more than anything else. Unless you're going into the wilderness, you'll still reach your destination.

In an electric car with a range of only 80 miles or so per charge however, a 30 percent reduction in efficiency means finding somewhere to recharge every 50-60 miles. That could seriously affect your trip planning. 

We experienced the dramatic effect roof bars have on electric cars first-hand recently, thanks to a weekend trip to buy a new shelving unit. 

Driving to the store in a 2011 Nissan Leaf with Thule roof rack fitted, we noticed the roof rack’s dramatic effect on the car’s range after a few minutes on the freeway. 

Instead of predicting a range of between 50 and 60 miles for a three-quarter full charge, our car predicted nearer to 35. 

We arrived at the store after 10 miles, with the Leaf using around a quarter of its charge to drive that short distance. 

2012 Nissan Leaf (RHD)

2012 Nissan Leaf (RHD)

Enlarge Photo

After fitting the 95 pound shelving unit to the Leaf, the 10-mile trip home burned through almost all of the remaining charge. 

Interestingly, neither handling nor performance seemed that badly affected by our load, with our test car happy to match other cars on the freeway in terms of acceleration and speed.

However, range did suffer, arriving home with enough charge for an estimated 5 more miles of travel. 

That day, since unplugging our Leaf with a 100 percent charge that morning, we’d covered just 43 miles. As a reminder, 23 of those miles had been covered without any roof rack fitted, using up just a quarter of the available charge.

Admittedly, our experience combined poor weather, freeway driving and a car laden with two adults, two children and a 95-pound load -- but it illustrates how dramatically load can affect range. 

Our advice? 

If you’re planning on fitting a roof rack to your electric car, remember that weight, and size of your load will affect how far you can travel on a single charge. 

If you’re going longer distances, remember to plan extra recharging stops to account for the loss in range from your heavy load.

Oh, and don't leave roof racks on your car when you don't need them.

+++++++++++

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Comments (7)
  1. That is a shockingly larger reduction in range. I wonder how accurate the range indicator really is.
     
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  2. Indeed. Of course, the thing we forgot to mention was that the box (seen above) is much less aerodynamic than the roof boxes you can get at most outdoor stores like REI. With less frontal area and a more aerodynamic shape, they should help range.
     
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  3. Still, the percentage of frontal area hasn't increased that much with the package in the photo. It really surprises me that the impact is so large.
     
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  4. The one known and predictable downside I'll have with the Renault Fluence ZE vs my current 5 door (newer shape) Honda Civic is picking up visitors at the airport which I do 3 or 4 times a year. My astonishing Honda can hold two full suitcases side by side, the Fluence will take only one.

    I'll probably pick up a set of bars from the UK (they cost double here) for those trips. I don't think range will be an issue at all.

    It would also be interesting to test the old A/C vs all the windows down myth and see if modern A/C is less of a draw than the extra drag from open windows.
     
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  5. I have a rear-mounted hitch rack on my EV, and did a couple of measurements. A small cargo box mounted close to the rear of the car seemed to reduce range about 3% (although it's hard to measure accurately at that level). A bike added 15%; an amount that surprised me.
     
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  6. I too use a hitch mounted rack for cargo, like heavy coolers, suitcases, etc., on my Leaf. Works great with negligible aero impact.
     
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  7. The range finder in the Leaf is designed to freak you out of using too much energy. I have seen the range meter say 9 when I was 21 miles away from home, and when I made it home, it said 8. This was mountain driving in bad weather, so I gained a lot of energy going downhill, while going as fast as I considered safe. I have also seen the car tell me I had just 19 miles of remaining range when I had half of a battery remaing, and 35 miles to go. No problem making it that time either. Driving style rather than the roof rack likely gave you such a reading on the range meter. If you didn't trigger a low battery warning, so you still had 20% charge. Drive gentler, and you will see a range increase, I have seen 120 with an 80% charge.
     
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