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Mini-experiment: the wrath of roof racks
Posted Saturday, March 10/07 in Mods & Tests
I went snowboarding a few weekends back (with 5 other people via diesel van), and couldn't help thinking about aerodynamics & efficiency.
Partly that's because I'm a lousy snowboarder. To keep from coming to a dead stop on the flat sections of the (easy) runs I tend to ride, I sometimes have to crouch way down low (minimizing my frontal area) to maintain to enough momentum to "coast" to where the run drops off again.
But mostly it was because of what I saw in the ski hill parking lot: a sea of roof racks and roof-top carriers. I wondered how many people were aware of the magnitude of the fuel consumption penalty they cause. I wasn't entirely sure myself, so I did a quick comparison and saw some dramatic results first-hand.
Roof rack test overview ...
The 9th commandment ...
Despite seeing this point made regularly, they never provide enough information on just how big a deal it is. The stated MPG penalty isn't consistent (if it's stated), and when it is stated, it varies wildly (eg. 5% to 20% mentioned in 2 of the above links).
So I decided to do a quick experiment to see what I could learn.
The (not very rigorous) test: "A-B-C" ...
I did these roof rack runs as part of the warm-up for last year's side mirrors experiment.
The comparison was between:
It should be said this wasn't a rigorous test. I only did one bi-directional run for each of the B and C conditions. So, while the results were dramatic, you'll have to take them with a grain of salt large enough to require roof-top transport.
Air Temperature (ATMP).. 67.6 F / 19.8 C; Aug 23/06; 7:30 - 8:30 PM
@ 88 km/h (54.7 mph)
A = no roof racks
A/B/C ... West ... East ... Average MPG (US) / L/100 km / MPG (imperial)
A ..... 55.0 ... 55.3 ... 55.2 / 4.26 / 66.29
Observations... move that commandment!
Maybe the roof rack warning should be higher up those fuel economy tips lists.
Admittedly, every car/rack combo will be different. But still, wow! I was stunned. I would never have predicted the fuel economy hit would be that big.
But even without the benefit of the ScanGauge telling me what was going on, the extra drag of the mountain bike was immediately obvious to me, by listening to the engine. I run my ignition timing somewhat advanced, which makes the motor more prone to pinging under higher loads. At 55 mph, when the cruise control periodically tugged on the accelerator to maintain the set speed for the test run, I could hear some pinging. Cringe.
But when I think about it, the roof top penalty shouldn't be that surprising. After all, racks both increase frontal area and Cd. The effect is bound to be significant.
That said, not all racks are made equal, and on top of that (pardon the pun) different loads will have different Cd & A characteristics. As you can see in the photos, my racks are definitely "old school", with square tube construction, and slab-faced pillars. Also, I tied the bike down with nylon straps that extended out to the sides and hummed and buzzed in the wind. A dedicated bike carrier (no straps) designed with aerodynamics in mind would undoubtedly perform better.
Still, I'm not the only one who has seen a significant MPG hit from roof-top carrying:
Anectodal corroboration ...
I spoke to a guy who's an avid cyclist who regularly travels to out of town races. He used to carry his bike on top of his Yaris until I showed him the results of my roof rack test.
His next trip was made with the bike on a rear carrier (where the bike is mostly in the car's wake). His results: the best ever fuel economy for a trip, 10% above his lifetime average:
"Best (ever) milage WITH bike on back :)
From a Prius forum:
"I am not sure about roof racks, but I took my kayak on the roof for a 700 mile trip and my milage dropped from 47 mpg to 36 mpg."
... and in the same thread ...
"Normally I get around 51 without the racks and 48 with the racks (and nothing else)."
Even carrying a large load inside a car may prove disastrous (for fuel economy) if it forces you to leave the rear hatch open, as Ecomodder.com member Peakster illustrated. He ran an A-B comparison of hatch-propped open vs. hatch-closed at three different speeds. Here's what he found:
Again, it's not that surprising, when you consider the open hatch both increased frontal area and Cd. (Although it was worse than I predicted.)
One more observation: it seems to me the majority of "styled" roof top boxes are aerodynamically backwards. The blunt end should face forward, with the taper at the rear.
Carrying options ...
What do you do, if you have large stuff to transport?
darin AT metrompg D-O-T com, or here