Roof rack test: quantifying the aero penalty
 

Originally written: 03-25-2007, 08:44 PM

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Waaaay back in the summer of '06 I did a few runs to look at the effect roof racks have on fuel economy.

Originally, this came up in a thread about a cyclist using his Yaris to carry bikes to races he was attending.

(Note: zooms don't work here - go to MetroMPG.com if you want to see the bigger versions.)


http://metrompg.com/posts/photos/blackfly-rack-mtb.jpg

I finally got around to posting details of the "test" (not exactly rigorously scientific) and some pics on MetroMPG.com. Here's the summary:

From: Mini-experiment: the wrath of roof racks (MetroMPG.com)

The comparison was between:

• A - no roof racks
• B - empty roof racks
• C - a mountain bike on the roof racks

Results:

• Average of all 'A' runs: 55.24 mpg (US)
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• Just adding (empty) roof racks reduced fuel economy to 48.2 mpg (US), or -7.0 mpg / -12.7%.
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• Putting the mountain bike on the rack absolutely destroyed the mileage! Down to 40.1 mpg (US), or -15.1 mpg / -27.3%
  
  

Also posted some corroboration about the severity of the effects from other people:

Such as the Prius owner who saw his mileage drop from 47 to 36 mpg (US) with a kayak up top.


And Peakster's mini-experiment posted here at GS about the effect of driving with your hatchback wide open: hatch-propped open vs. hatch-closed

For all the gory details: http://metrompg.com/posts/roof-racks.htm

Could I actually fit the bike inside? Yes.

There's room for a couple of bikes inside with the back seat folded (front wheels & seats off the bikes). I just did that for the test.

I only carry stuff on the roof in town.

I once drove to the east coast with a Laser sailboat on top of my Accord though. http://www.gassavers.org/images/smilies/redface.gif

Since doing this test, I've actually carried long loads *under* the car on highway trips. I can do this because the car has ridiculously high ground clearance, and the loads were easy to strap "up" clear of suspension & exhaust.

Both times it was a windsurfer mast (a couple feet longer than the car).

Just have to be careful for steep driveway angles with the longer loads!

This thread has generated another one...

How to reduce drag of surfboard roof racks?

I can also corroborate this drastic hit in economy -- I used to have an '86 Mazda RX-7. On the highway, it would normally get around 26 mpg at 70mph but the addition of a roof rack + mountain bike dragged the gas mileage down to 19mpg on the same route.

Hi drcoopster - thanks for the additional info.

Here's my pence....

A friend of mine was talking about getting a truck - to carry stuff.... Really though, he rarely carries much - occasionally a kayak... So far, using a thrift store blanket (recycling?) and loaning him some of my tie down straps, the kayak has been happy on the roof of his Accent.

Sure, a truck would be "easier" - but I convinced him that adding roof racks will be more utility than he needs... And even if he leaves the racks on all the time (I'm not saying he's lazy... but....), the fuel hit won't be nearly as much as the fuel hit of going from an Accent to any Truck.....

The wrath of roof rack and roofboxes has been independently tested by Edmonds and Consumer Reports. The impact was 17-25% depending on car, speed type of carrier. I have been developing an idea based on shortened boat tail plates tested on semi trailers, integrated in a cargo carrier for suvs, wagons, hatch backs, and sedans.

Essentially a back mounted aerodynamic cargo carrier. It eliminates the ~20% mpg penalty of roof top carriers and has improved mpg 5-10%, in preliminary tests with the boat tail design. We use it for camping, family road trips, ski equipment etc. AeroPACKusa is the product. I would appreciate your comments. Thanks!

Comment: Try advertising your product in the success stories forum... or somewhere more suitable, instead of brining up threads that have long since been forgotten. It would get more attention in a more suitable forum.

BTW... I don't remember exactly, but I don't think you're supposed to directly advertise based on the terms and conditions of the site. You might want to read over that again. Maybe I'm wrong, but either way, it will be more noticeable in a forum that was made for it.

http://farm4.static.flickr.com/3552/...45a97f0b_b.jpg

Ya think these guys noticed any change in MPG and wind noise ?

I like it. If you ever make anything like that for a surfboard let me know.

Also just brainstorming if you created a quickly removable, easily secured tie-down system and sold it fairly cheap you could make a killing here in Southern California targeting the surfing industry. The current selection of 'inexpensive' racks is not that great at all.

lucidexp: you might be on to something. I remember another EM member way back when who signed up specifically to noodle about a better way to deal with his surfboard roof rack.

Also: I missed it when Mikel09 originally posted, but I think he might have contacted me first to ask if he could mention his product on the forum. (And I said, "sure.")

Being a fellow surfer and having my truck die on me I have had to resort to having a car, but dont want to give up surfing because of it.

I have looked around and tried a few but several current version of removable racks are really, really bad. I have seen some flatable roof racks but they sell for around $100 and for that you are better off getting a 'real' rack.

I have the design tools and a degree in architecture, willing to give the design away since I just want a solution.

Guys this sounds interesting. Are you thinking about a roof mount or back mount surfboard carrier? What are your thoughts on the concept?

New thread! (Please, if you're going to do it.)

i know this is an old thread but im wondering would it make any difference if i make a roof rack and mounted it on an aero cap?? would that be worse that just keeping my truck cap and putting the bike inside?

My brother has an F250 with an aero topper. It's described here: http://ecomodder.com/forum/showthrea...250-16798.html

Since then, he added four eyebolts to the topper for tying down a 13 foot canoe. The canoe lays upside down on the topper with one end over the cab and the other end sticking out back a little bit. He has not noticed a mileage decrease with that setup, so any mileage hit is less than 1 MPG.

I think his success is due to the canoe being down tight against the topper.

CR adds some data: Honda Accord, Toyota Camry
 

Updating this 11 year-old thread...


Consumer Reports asked:
What's the Impact of Carrying Stuff on the Roof?

They tested with a  2013 Honda  Accord at 65 mph / 105 km/h.

• 42 mpg US - baseline with nothing on the roof
• 37 mpg - empty bike rack
• 35 mpg - empty bike rack with wind deflector (made it quieter, but increased drag)
• 27 mpg - two bikes on the rack (didn't specify deflector or not)

They also tested a 2008 Camry with a large car-top carrier...

• fuel economy dropped by 5 mpg

Source: https://www.consumerreports.org/fuel...l-economy-now/


Moral of the story: carry stuff inside the car or on the back whenever possible!


http://ecomodder.com/forum/attachmen...1&d=1528809339

I put a pair of Kayaks on my Civic hybrid to drive to the lake near my dad's house. I made ~48mpg getting to his house (11.5 hour drive). Going from his house to the lake and back (1.5 hours) with the Kayaks on, the drag was so great I had to use 4th gear to maintain 60mph on the backroads without flattening my hybrid battery... I made about 35 mpg.

Huge difference. Note: I had an aero-clean car (no rack) for the drive to his place, the Kayaks were put on one of those gutter strap-down roof racks he had at his place.

THAT's amazing.

Car and Driver - 2022 Kia Carnival test
 

Not often you get to update a 15 year-old thread with additional relevant info ...


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https://ecomodder.com/forum/attachme...1&d=1675977837

Car and Driver magazine tested a 2022 Kia Carnival minivan with & without aero crossbars at 75 mph / 121 kmh.



https://www.caranddriver.com/news/a4...t-gas-mileage/

minus-3mpg
 

Wow! I would not have thought it was that significant.
And to think that thousands go by Denton every day at that speed.:(

I wonder why the mpg hit is so high. It’s not like it adds that much front area. Didn’t expect turbulence to tank things that hard

Nothing has changed since 2007? Maybe the only thing that hasn't changed. :)

Yeah roof racks are horrible. Anything on the roof is bad.
Even putting the light bar on my leaf reduces highway range by up to 20% it's almost as bad as pulling my 4x8 flat bed trailer empty.

The numbers check out. My parents would lose 20%-25% fuel economy running a not-very-aerodynamic cargo carrier atop the minivan growing up. 25-26 mpg would become 20 mpg.

I wonder how many kids who drive around with empty "fashion statement roof baskets" on their cars have any idea how much they're costing them.


Not just fuel use, but top speed and high speed acceleration, too!


https://ecomodder.com/forum/attachme...1&d=1676076231

I need something like that with magnetic feet for the few times a year one would really come in handy.

Needs a Coca-Cola cooler, skate boards and old suitcases for maximum effect.

I'd like to have a Jim Dandy rack for my Superbeetle. There are reproductions.

https://www.meubagageiro.com.br/admi...6ufh-jpg_t.jpg
http://www.meubagageiro.com.br/admin...6ufh-jpg_t.jpg

Like HHOTDI found in 2013, I like to think the enpty rack would ameloirate the Beetle's too-fast taper.