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2002 Pair: 3.0 VDC Wag & 2.5 Limited Sedan
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parked my 2002 roof rack cross bars long long ago.

used them one time since. (I store them with the bolts taped in them with painters tape in a garage corner).

gained about 1-2mpg without them, ...and lost a WHOLE LOT of wind noise.
 

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Hmmmm...if I can gain 2MPG by removing my roof rails I might just do that. They don't say if they have the crossbars though as mine are not deployed!. Any thoughts????
 

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2008 OB Limited 2.5i, Portland OR USA
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It would be interesting to see other cars tested as well with this same equipment/cargo setup.

My theory: Cars with a relatively aerodynamic profile and low CD to start out will probably suffer the most percentage drop in MPG, compared with cars that don't have as good a starting point for CD. And it might not be just the sum of the two resistances (car plus roof rack/carrier/cargo); there might be some weird compounding effect going on.
 

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Hmmmm...if I can gain 2MPG by removing my roof rails I might just do that. They don't say if they have the crossbars though as mine are not deployed!. Any thoughts????
From the article: "The control test—the TourX without roof rails installed—revealed that adding roof rails alone decreased the car’s fuel economy by about seven percent, or a negative -2 mpg change."

If I had to bet, I'm guessing they got their terms mixed up. Buick's website says they come standard with roof rails, and I doubt they took the factory ones off when they did the test, especially since it doesn't give separate results for Nothing on the roof, roof rails installed, and cross bars installed.

My guess when they say adding roof rails decreased MPGs by seven percent, they're actually talking about cross bars.
 

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It would be interesting to see other cars tested as well with this same equipment/cargo setup.

My theory: Cars with a relatively aerodynamic profile and low CD to start out will probably suffer the most percentage drop in MPG, compared with cars that don't have as good a starting point for CD. And it might not be just the sum of the two resistances (car plus roof rack/carrier/cargo); there might be some weird compounding effect going on.
Agreed. There are more factors than people realize. I remember reading about a college competition to get the highest MPGs out of a car and they used masking tape over every panel gap and seam to help their mileage. I'm all for better MPGs, but don't think I want to take it that far. :grin2:
 

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I'd be interested to see how they did the kayak mounting and what type of kayak. I have never seen anything close to a 28% drop. I typically still average over 30 mpg (self calculated) with mine on top, and I've used three separate vehicles to transport it.
 

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Two things:
1) Their results were based off of simulations resulting from wind-tunnel data, rather than real-world experience. I'm not debating that adding stuff on top of the car affects fuel economy, I most certainly agree it does; just that real-world data may not match with their claims. It could be better, it could be worse. The inflatable pool float should collapse when met with highway speed airflows, but I have to wonder if they measured the increased resistance at a low speed and extrapolated the effects to higher speeds. That would be an invalid assumption.

2) The solid items with the large impacts leads me to think about the forces created that are transmitted through the item (such as the MTBs) to the mounting hardware, to the roof racks and finally to the vehicle roof and structure. Based on their test data, these are non-trivial forces. I think about how most bikes are held in with usually just the fork mounts and a band going over the rear rim. That doesn't seem like much to hold against so much wind force. But based on the millions of miles traveled by bikes atop cars, we know it works!
 

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'03 outback limited, '01 Outback Limited, '01 Legacy L wagon, '96 Legacy Brighton wagon
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Agreed. There are more factors than people realize. I remember reading about a college competition to get the highest MPGs out of a car and they used masking tape over every panel gap and seam to help their mileage. I'm all for better MPGs, but don't think I want to take it that far. :grin2:
You'd be surprised at how much drag even bugs and decals will add.

Standard practice on Indy cars nowadays is to not use decals - all the logos you see on the cars are actually paint - because the edges of the stickers create vortices, increasing drag. Makes a couple MPH difference at 200+ MPH.

4 decades ago, Mercedes set a bunch of closed-course records with their C-111-3 ( with a Cd of .183!) that when the runs were done, they wiped the accumulation of bugs off of it and gained back another 4mph!
 

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Their results were based off of simulations resulting from wind-tunnel data, rather than real-world experience. I'm not debating that adding stuff on top of the car affects fuel economy, I most certainly agree it does; just that real-world data may not match with their claims. It could be better, it could be worse.
This is entirely true, and I likewise am a bit skeptical about the part of the article that translated these results into real world MPG numbers.

But on the other hand, it is an excellent way to compare setup "A" to setup "B" and infer which one will do better on a given vehicle. A cargo basket with open exposed cargo, versus an aerodynamic cartopper, is the classic comparison (although that one is a no-brainer). A better one might be factory low mount crossbars versus aftermarket higher bars on the same cartopper.
 

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Back when I had a Saab 9-3 (2.0l turbo), I measured MPG several times on the same ~300 mile highway trip with and without a roof rack and two road bikes mounted at the forks with the front wheels off. (This gives less wind resistance than full upright mounting.)

Without the rack, the car averaged 31 MPG on this route. The roof rack alone (a Thule with the big plastic aero deflector on the front) caused a loss of about 1-1/2 MPG. The rack with the road bikes caused loss of about 4 MPG. Mountain bikes are less aerodynamic than road bikes, so a pair of those probably would have lost another 1 or 2 MPG, but I never measured that.
 

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It would be interesting to see other cars tested as well with this same equipment/cargo setup.

My theory: Cars with a relatively aerodynamic profile and low CD to start out will probably suffer the most percentage drop in MPG, compared with cars that don't have as good a starting point for CD. And it might not be just the sum of the two resistances (car plus roof rack/carrier/cargo); there might be some weird compounding effect going on.
What?


I'm an Aeronautical engineer and there is no better, or exact, data than wind tunnel data because the environment is completely controlled. And can be duplicated. Acquiring measurable aerodynamics outside your home is not as exact because of the constant changing uncontrolled conditions like wind, moisture and atmospheric air pressure.


Gas mileage does give us a crude idea of different drags, but there is always some doubt.


So lets move on. Of course roof racks add more drag. Anything that causes a disruption of air blow causes drag. And typically the bigger the rack, the more the drag. My roof rails actually look pretty aerodynamic without the crossbars, but the rails on the OB Tour look a lot more aerodynamic.


While I agree bugs cause drag (we ironically call it parasitic drag), the speeds of the typical car won't cause much measurable bug drag. Because drag increase at the square of speed (dynamic pressure), cars with a consistent speed over 120 mph can see some measurable bug drag. Strangely, depending how the car body is shaped, sometimes bug drag can actually smooth out the overall flow and improve performance. Next time you look at the wing of an airplane, you might see little bitty deflectors or wings mounted in a roll. Called vortex generators, they actually help the aero dynamics/


As for what get the best possible mileage from packing the roof; the rounder it is, the better the airflow. A ball is more aerodynamic than a box the same size. Some of the roof racks I've seen on Outbacks aren't very round.


Beary
 

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...there is no better, or exact, data than wind tunnel data because the environment is completely controlled. And can be duplicated. Acquiring measurable aerodynamics outside your home is not as exact because of the constant changing uncontrolled conditions like wind, moisture and atmospheric air pressure.
Absolutely. However, the way aerodynamic data translates to MPG isn't straightforward, given factors like rolling resistance/tire pressure, vehicle and cargo weight, hill profile of terrain, driving style, weather, etc. The original article seems questionable in how it made the translation from aerodynamics to fuel efficiency.

That's why I gave a real-world example of MPG impact for my Saab, averaging the MPG for the same car, same driver, same route driven multiple times in each configuration with and without a roof rack and bikes, over the course of multiple years.
 

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Absolutely. However, the way aerodynamic data translates to MPG isn't straightforward, given factors like rolling resistance/tire pressure, vehicle and cargo weight, hill profile of terrain, driving style, weather, etc. The original article seems questionable in how it made the translation from aerodynamics to fuel efficiency.

That's why I gave a real-world example of MPG impact for my Saab, averaging the MPG for the same car, same driver, same route driven multiple times in each configuration with and without a roof rack and bikes, over the course of multiple years.
Ah! Gotcha. You may be over thinking the article. The subject is only comparing the gas mileage efficiency (or rather inefficiency) of the outdoor equipment. The car is only used as a baseline, or starting place, to give the equipment comparisons a perspective of measurement.

They didn’t calculate the car gas efficiency from the wind tunnel drag, they used the EPA published efficiency. They subtracted the car baseline wind tunnel drag coefficient from the total wind tunnel drag of the car and outdoor equipment tested together. Then they divided the equipment drag by the total drag to calculate a percentage for the equipment drag. They multiply that equipment percentage times the EPA mileage to find mpg loss caused by the equipment drag.

I hope that makes sense. It seemed simple in my head.

Beary
 

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Ah! Gotcha. You may be over thinking the article. The subject is only comparing the gas mileage efficiency (or rather inefficiency) of the outdoor equipment. The car is only used as a baseline, or starting place, to give the equipment comparisons a perspective of measurement.

They didn’t calculate the car gas efficiency from the wind tunnel drag, they used the EPA published efficiency. They subtracted the car baseline wind tunnel drag coefficient from the total wind tunnel drag of the car and outdoor equipment tested together. Then they divided the equipment drag by the total drag to calculate a percentage for the equipment drag. They multiply that equipment percentage times the EPA mileage to find mpg loss caused by the equipment drag.
And that proves my point. It's as though they were calculating the impact on fuel efficiency assuming that aerodynamic drag is the only contributing factor, which it obviously isn't. Unless you know how much the car's aerodynamic drag contributes to it's overall inefficiency, how could you possibly calculate how much impact changing it has on fuel efficiency.
 

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What? [regarding "compounding effect"]
All I meant by this was that the measured characteristics of the car alone or the roof apparatus alone might not be a precise indicator of the two characteristics together. Nor, by inference, can they precisely tell the MPG hit, which you've probably well addressed. But what it can do is establish whether setup "A" is better than setup "B", when all of the components are present.

I'm aware there are nonlinear effects here, and also that turbulent flow can really drastically alter the mathematics behind all of this. But it's been 40 years since I've been around any fluid dynamics, and that was only for liquids in a laminar flow, not gaseous, and not turbulent. So I'll defer to your expertise here.
 

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And that proves my point. It's as though they were calculating the impact on fuel efficiency assuming that aerodynamic drag is the only contributing factor, which it obviously isn't. Unless you know how much the car's aerodynamic drag contributes to it's overall inefficiency, how could you possibly calculate how much impact changing it has on fuel efficiency.
Again, the article is not about the car, it's about the equipment.


The writer's are very simply giving comparison numbers to show buyers of similar equipment an idea of how some equipment is more efficient than the other.


Aero engineers do the same calculations everyday to learn how much different pieces of equipment added or taken off the fuselage change the total efficiency.


I have no idea of other conditions that you speak of could have any effect on their results. Air is air. Weight, tire resistance or the full moon would have no effect on the drag of a ball sitting on the roof. Rain or snow would have some difference of drag on the ball or box, but it would be constant on all the equipment, so it wouldn't effect the delta or difference of drag between the ball or box. The ball is always more efficient. If gas mileage is your concern, then the ball is always the better choice to stick on the roof of your car. Any car. Any condition.


Beary
 

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Again, the article is not about the car, it's about the equipment.

The writer's are very simply giving comparison numbers to show buyers of similar equipment an idea of how some equipment is more efficient than the other.

Aero engineers do the same calculations everyday to learn how much different pieces of equipment added or taken off the fuselage change the total efficiency.

I have no idea of other conditions that you speak of could have any effect on their results. Air is air. Weight, tire resistance or the full moon would have no effect on the drag of a ball sitting on the roof. Rain or snow would have some difference of drag on the ball or box, but it would be constant on all the equipment, so it wouldn't effect the delta or difference of drag between the ball or box. The ball is always more efficient. If gas mileage is your concern, then the ball is always the better choice to stick on the roof of your car. Any car. Any condition.
I'm not disagreeing with anything you've said. (As my former boss used to put it, "We're in violent agreement.")

The title of the original article was "Here's How Much It Hurts Your Mileage When You Strap Canoes or Bikes to Your Roof", but the testers didn't measure any mileage, just aerodynamic drag, and then the authors apparently made the faulty logical leap that changes in drag would yield the same % changes in fuel economy.

Anyway, here's a related question for you: Is there any merit to using vortex generators, like I've seen on some performance cars, at automotive speeds?
 

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Back when the vehicle was newer and I paid closer attention to such things, it felt like I could notice the difference with the cross beams attached vs. put away. Not much of a difference, but a difference nonetheless.


For sure, I measured a long trip once of 1,200 miles each way. One leg with nothing on the roof, the other with large suitcases tied down on the roof. Similar weather conditions, mostly flatish interstates. A huge difference in MPG. 27 without and 18 with the suitcases. Like I said, huge.
 
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