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Old 12-05-2007, 04:36 AM   #1 (permalink)
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Arrow Video: Illustrating the Vast Importance of Aerodynamics (open hatchback experiment)

When my mother moved to her new home in South Regina, she no longer needed her kitchen table set. and thus gave it to me. I managed to stuff the 4 chairs and table in the rear hatch, but not without keeping the hatchback wide open.

Because it was late at night, I managed to be able to cruise at a speed of about 35 miles per hour on the Trans Canada Highway with my precious cargo, all the way home. What amazed me was how much of a blow to my gas mileage occurred, even at that snail-like speed. Because the Kitchen set was maybe only a 100 pounds, the culprit was simple gesture of opening up the hatch that robbed my fuel economy.

Here is a video of just how massive this loss can be when hauling something that requires a hatchback to be open:



Feel free to discuss.

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Old 12-14-2007, 12:37 PM   #2 (permalink)
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I'm not surprised you saw such a big difference with having the hatch open at low speeds.

Based on miscellaneous comments I read all over the web, people often seem to think that aero losses "magically" come into play only above some arbitrary speed.

Even at just 20 mph, 1/3 of the total power (about 1.2 hp) required to move a Metro goes to overcoming aero losses. 1/5 of the power at 15 mph. (Based a RR & aero loss spreadsheet, with all the usual formulas).
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Old 12-14-2007, 12:38 PM   #3 (permalink)
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Hope you don't mind me pasting this here...

--------

Originally from Peakster:

Here's the little test in a nutshell:

Temperature: -4*C
Winds: south @ 11km/h (I was driving west)
Cruise control was used to maintain speeds
Each mpg reading was taken after 2 miles of travel
Route:



Hatch closed:
73.6 mpg @ 35 mph
62.9 mpg @ 45 mph
49.2 mpg @ 55 mph

Hatch open:
62.4 mpg @ 35 mph (15.22% mileage loss)
48.2 mpg @ 45 mph (23.37% mileage loss)
34.4 mpg @ 55 mph (30.08% mileage loss)

Those are huge numbers for something as simple as opening up the rear hatch. What do you think my Cd was when it was open? 0.44? I'm now beginning to believe that small aero mods to these little cars (such as mirror removal, wheel skirts, etc) would help FE in more than just highway travel.
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Old 12-14-2007, 12:40 PM   #4 (permalink)
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Quote:
Originally Posted by Peakster
Those are huge numbers for something as simple as opening up the rear hatch. What do you think my Cd was when it was open? 0.44?
Remember you didn't just change Cd - you dramatically increased A as well. I'm not surprised at the difference. Hunt down my roof rack mini-test to see what that showed.
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Old 12-15-2007, 03:41 AM   #5 (permalink)
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I wonder... I'm getting a wind noise from my drivers side door that indicates that air is leaking into the cabin. Would this hamper mpg by far? I'm guessing it's just the 13 year old rubber seals that I'm trying to fix by adding some lube between it for a better seal, but didn't have really much at all to get the job done. Just wondering, It's difficult to get good Cd in a jeep, but with the roads here it'd be a miracle to make it through with a metro.
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Old 12-15-2007, 10:44 AM   #6 (permalink)
igo
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I might want too add: while nobody drives around with there hatch open. People do drive with ridiculous bike/snowboard/kayak roof racks that kill mpg bad.

Take a look at this test:

http://www.metrompg.com/posts/roof-racks.htm
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Old 12-15-2007, 11:37 AM   #7 (permalink)
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Quote:
Originally Posted by DifferentPointofView View Post
Would this hamper mpg by far?
I doubt it would make any difference, actually.

Quote:
but with the roads here it'd be a miracle to make it through with a metro.
Where's "here"? (You should add your location to your Profile.) So everybody drives 4WD "here"?
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Old 12-30-2007, 08:13 PM   #8 (permalink)
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Fuel Flow Meter

OK, where did you get the Fuel Flow meter?
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Old 12-30-2007, 10:00 PM   #9 (permalink)
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Quote:
Originally Posted by bryan View Post
OK, where did you get the Fuel Flow meter?
Scan Guage



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Old 12-31-2007, 12:38 PM   #10 (permalink)
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I had the same question. Doesn't seem to be available for Honda Accords earlier than 1995.

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