Roof rack test: quantifying the aero penalty

[MetroMPG]

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.)

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

[MetroMPG]

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.

[MetroMPG]

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).

[MetroMPG]

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

[MetroMPG]

This thread has generated another one...

How to reduce drag of surfboard roof racks?

[drcoopster]

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.

[MetroMPG]

Hi drcoopster - thanks for the additional info.

[trebuchet03]

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.....

[mikel09]

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!

[Christ]

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.