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Rack on top of vehicle and MPG

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Rack on top of vehicle and MPG

Old 01-06-15, 05:32 PM
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cajunpedaler
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Rack on top of vehicle and MPG

I drive a Honda Odyssey. I have roof racks with a nice Yakima rack for 2 bikes. On one of our long road trips, we hauled the bikes up top (had bikes inside too) and it seemed our gas MPG's suffered. Has anyone else ever notice a decrease in MPG hauling bikes on top?
Thanks.
Perry
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Old 01-06-15, 05:45 PM
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I'd be surprised if it didn't. You're adding a lot of drag with bikes on top. Even the empty rack will affect MPG slightly.
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Old 01-06-15, 06:09 PM
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Science of Cycling: Aerodynamics & Wind Resistance | Exploratorium

Engine efficiency - Wikipedia, the free encyclopedia

The first site computes 3.2 kW for a 170 pound cyclist going 65 miles per hour on level ground with no headwind. If I didn't have a really bad cold that turns my brain into tofu, I'd work out how 3.2 kW translates into fuel consumption. You also need to know the heat of combustion of gasoline.

If nobody else has figured this out by the time I get better, I'll come back and finish it.
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Old 01-06-15, 06:23 PM
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Originally Posted by cajunpedaler View Post
I drive a Honda Odyssey. I have roof racks with a nice Yakima rack for 2 bikes. On one of our long road trips, we hauled the bikes up top (had bikes inside too) and it seemed our gas MPG's suffered. Has anyone else ever notice a decrease in MPG hauling bikes on top?
Thanks.
Perry
You're not alone. The bikes are sailing through a 60mph wind. Anyone who's ever seen a sailboat tug at an anchor with only bare poles, knows how much force can be involved.

Bike on a roof will greatly increase the aerodynamic drag of any vehicle, with the resulting fuel use penalty.
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Old 01-06-15, 06:33 PM
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A Thule rack with four trays drops the mileage on my 2004 Silverado 1-2MPG. All four bikes on it drops it another couple MPG.
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Old 01-06-15, 06:36 PM
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Originally Posted by Gresp15C View Post
The first site computes 3.2 kW for a 170 pound cyclist going 65 miles per hour on level ground with no headwind. If I didn't have a really bad cold that turns my brain into tofu, I'd work out how 3.2 kW translates into fuel consumption. You also need to know the heat of combustion of gasoline.
This is an erroneous figure since the aerodynamic drag for a bike and rider is largely due to the rider, not the bare bike. That said, a roof rack alone does decrease gas mileage and a rack with a bike or two has a significantly higher effect. Quantifying this will be an empirical experiment as it will depend on the rack, the bikes and the car they are on.

The heat of combustion of gasoline is about 20,000 BTU/pound or about 123,000 BTU/gallon. However, an auto engine has a thermal efficiency in the 35% range so factor that into your calculation when you do find the correct drag/wattage number.
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Old 01-06-15, 07:52 PM
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A roof rack carrying nothing will decrease gas mileage over a smooth roof. Take a look on EBay or Craigslist and pick up a fairing for the front bar. It will reduce the wind noise and help the gas mileage.
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Old 01-06-15, 08:48 PM
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Get a fairing. I skimped when first setting up my rack. It turns out that an empty rack with no fairing has more drag than with a fairing and 3 bikes.
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Old 01-06-15, 09:04 PM
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I will only use racks that come off easily and take them off when I am not using them. I did cheat a little and pay a full time penalty of both air resistance and noise by installing the tracks for them. I replaced my Toyota All-Trac with its fairly roomy back with a Prius C. Since I own a house and this is my only car, I need it to carry bikes, sheets of plywood, etc. And gutters have gone the way of the dodos. Good side of all this is that Yakima racks go on and off with those tracks really easily.

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Old 01-06-15, 09:08 PM
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With Yakima Landing Pads on the roof (attached to the factory threaded holes in the roof) and matching towers for the crossbars, a roof rack can be attached or removed almost instantly. Quick release mechanism in the towers locks securely to the Landing Pads. Driving around with no roof rack is more efficient than with a roof rack, with or without a fairing.

Put one or more bikes on the roof rack and mileage will suffer. That said, the overall mileage I get with a Mazda5 is considerably better than what I got with a Mazda MPV, even though the 5 sometimes carries a bike on the roof, while the MPV sometimes carried a bike inside the van.
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Old 01-06-15, 09:36 PM
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Originally Posted by HillRider View Post
This is an erroneous figure since the aerodynamic drag for a bike and rider is largely due to the rider, not the bare bike. That said, a roof rack alone does decrease gas mileage and a rack with a bike or two has a significantly higher effect. Quantifying this will be an empirical experiment as it will depend on the rack, the bikes and the car they are on.

The heat of combustion of gasoline is about 20,000 BTU/pound or about 123,000 BTU/gallon. However, an auto engine has a thermal efficiency in the 35% range so factor that into your calculation when you do find the correct drag/wattage number.
d'oh!

I'm laughing out loud as to how I could have overlooked that! Chalk it up to being sick.
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Old 01-06-15, 10:14 PM
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Gas here is now under $2 - don't worry about it
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Old 01-06-15, 10:34 PM
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The smaller/weaker the engine the more the MPG loss will be when the bikes are on top. So a small high efficient car will see more loss then a big guzzlier would. The Saabs I had (turbo 93s) would loose 2-4mpg (dependent on whether only the tandem was on top or it and two singles). My current VW 2.5L Golf looses about 4-8mpg in the same situations. Andy.
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Old 01-07-15, 06:12 AM
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Aerodynamic drag is such a big factor that car companies have investigated replacing outside rear-view mirrors with cameras and displays to boost their MPG numbers. The vehicle's shape is carefully optimized and any disturbances to the air flow imposes big drag penalties besides the drag of the object itself..
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Old 01-07-15, 08:30 AM
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Originally Posted by JanMM View Post
With Yakima Landing Pads on the roof (attached to the factory threaded holes in the roof) and matching towers for the crossbars, a roof rack can be attached or removed almost instantly. Quick release mechanism in the towers locks securely to the Landing Pads. Driving around with no roof rack is more efficient than with a roof rack, with or without a fairing.
I used to have an older Yakima SST clamp-on rack that required careful clamp foot alignment to install properly. I placed small patches of electrical tape inside the door openings to mark the proper clip location and that allowed very fast installation so there was no incentive to leave the rack in place when it wasn't carrying bikes.

That said, the roof rack did cause a noticeable drop in gas mileage, a lot of noise and the ever-present danger of driving under a low clearance and forgetting the bikes were up there. I've now gone to a hitch rack and that has far less influence on gas consumption, is much quieter and has no overhead clearance problems. Before someone points out the danger of a rear-end collision damaging the bikes, the driver who hits you is liable. Forgetting the overhead clearance puts the damage cost on you.
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Old 01-07-15, 08:50 AM
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Consumer Reports tested this a few years back. Their findings agree with everyone else: the MPG cost of even a bare roof rack is real and can be significant.
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Old 01-07-15, 09:11 AM
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It's Less , if you drive slower. say 50 vs 65mph.. Moved out of Cal with a bunch of stuff on my Roof ..

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Old 01-07-15, 09:22 AM
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Originally Posted by fietsbob View Post
It's Less , if you drive slower.
Yes, and the penalty is zero if the car is stationary.
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Old 01-07-15, 09:24 AM
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Originally Posted by HillRider View Post
Before someone points out the danger of a rear-end collision damaging the bikes, the driver who hits you is liable. Forgetting the overhead clearance puts the damage cost on you.
This has always been my number 1 concern regarding rooftop racks; getting forgetful can cost you damage to the bike(s), car and overhang in one go.
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Old 01-07-15, 09:28 AM
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Originally Posted by cajunpedaler View Post
I drive a Honda Odyssey. I have roof racks with a nice Yakima rack for 2 bikes. On one of our long road trips, we hauled the bikes up top (had bikes inside too) and it seemed our gas MPG's suffered. Has anyone else ever notice a decrease in MPG hauling bikes on top?
Thanks.
Perry
Is this a real question?
What would you expect.
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Old 01-07-15, 09:35 AM
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Originally Posted by dsbrantjr View Post
Aerodynamic drag is such a big factor that car companies have investigated replacing outside rear-view mirrors with cameras and displays to boost their MPG numbers. The vehicle's shape is carefully optimized and any disturbances to the air flow imposes big drag penalties besides the drag of the object itself..
Warning thread drift.

Does anyone else see the irony in having rear view cameras and object detectors and then the car ads have the fine print along the bottom of the TV image, something like, " do not rely on camera image or indicator when backing up"? I saw this disclaimer for the first time a few days ago. Andy
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Old 01-07-15, 09:40 AM
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Originally Posted by Andrew R Stewart View Post
Does anyone else see the irony in having rear view cameras and object detectors and then the car ads have the fine print along the bottom of the TV image, something like, " do not rely on camera image or indicator when backing up"? I saw this disclaimer for the first time a few days ago. Andy
Lawyer-speak. Actually, I treat the rear view camera the way I do a rear view mirror. If you see something it is certainly there. If you see nothing, look again as you may have missed it.
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Old 01-07-15, 10:02 AM
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Thanks for all the replies..I guess my query should have been something along the lines of "I wonder how *much* the roof racks affect mileage?" I do not have a math or complex physics brain, so simplistic enough, I just thought a rack that is made of essentially pipes and bars and attaching a lightweight narrow profile bike would affect MPGs some, but I didn't know how much. I guess it's just like when you are on the bike riding it, wind is a variable, but constant factor...
Thanks all, even the poster who asked if this is a "real question"...
Happy New Year...let's all ride our bikes.
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Old 01-07-15, 10:11 AM
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Originally Posted by cajunpedaler View Post
Thanks for all the replies..I guess my query should have been something along the lines of "I wonder how *much* the roof racks affect mileage?" I do not have a math or complex physics brain, so simplistic enough, I just thought a rack that is made of essentially pipes and bars and attaching a lightweight narrow profile bike would affect MPGs some, but I didn't know how much. I guess it's just like when you are on the bike riding it, wind is a variable, but constant factor...
Thanks all, even the poster who asked if this is a "real question"...
Happy New Year...let's all ride our bikes.
Yes better LOL Not sure how much but it is less that's for sure even with out bikes on it.
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Old 01-07-15, 10:24 AM
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Originally Posted by HillRider View Post
Yes, and the penalty is zero if the car is stationary.
But then your getting 0 MPG, so the penalty would be as bad as it can get.
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