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Discussion Starter #1
If anyone is interested in roof boxes one may wonder what is the penalty in consumption. I did a few tests over last two days (repeated to ensure consistency). All tests were at real GPS speed (not speedo).
I did a loop of 10 miles for easy calculation.

I was gifted 470L roofbox from Halfords - Exodus. Not the most aerodynamic but not too bad either - dimensions: 165x95x41.

Without roofbox @90km/h (56mph): 4.6m/KWh
132336


With roofbox @90km/h (56mph): 4.4m/KWh
132338


Without roofbox @120km/h (75mph): 3.0m/KWh
132339


With roofbox @120km/h (75mph): 2.8m/KWh
132340
 

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Discussion Starter #2
I was honestly surprised that efficiency drop was minimal - especially at high speed. The box makes noticeable noise at 120km/h and I was expecting huge penalty. But figures speak for themselves.

So if small trunk in Kona is a problem for someone, roofbox for holiday trips may be a solution.
 

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It's strange that the drop (0.2 m/kWh) is the same at 56 and 75. I'd have expected it to be 0.3 or 0.4.
Funny stuff aerodynamics. What looks good or bad often isn't.
 

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Well, viewing this objectively and pedantically as practical in a few minutes, the box works out to 0.39m2 against an estimated (by one source) 2.5m2 for the car, a 16% increase in the aerodynamic component of losses assuming the same CD as the car, but I'd bet it's somewhat less. According to one detailed but unvalidated mathematical model of Kona losses, aerodynamic drag represents 46% and 56% of the total losses at 90 and 120 km/h respectively. So, the additional drag might represent an additional (.16 x .46 =) 7% and (.16 x .56 =) 9% more total losses.

The measured differences are 4% and 7% more kWh respectively, where the estimate above says 7% and 9%.
But each measured pair of numbers have a relative potential truncation or rounding error of around 4% and 6%, assuming that a multitude of other practical influences have been avoided while driving. That leaves our measurements indicating a very loose 0-8% and 1-13% against an estimated 7% and 9%, the latter within the margin of that error.

So, it's not unreasonable to suggest that the measurements are not wildly off base, especially if the actual box Cd is better than the car.
 

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Life's too short to wait for a Kona .....
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Well, viewing this objectively and pedantically as practical in a few minutes, the box works out to 0.39m2 against an estimated (by one source) 2.5m2 for the car, a 16% increase in the aerodynamic component of losses assuming the same CD as the car, but I'd bet it's somewhat less. According to one detailed but unvalidated mathematical model of Kona losses, aerodynamic drag represents 46% and 56% of the total losses at 90 and 120 km/h respectively. So, the additional drag might represent an additional (.16 x .46 =) 7% and (.16 x .56 =) 9% more total losses.

The measured differences are 4% and 7% more kWh respectively, where the estimate above says 7% and 9%.
But each measured pair of numbers have a relative potential truncation or rounding error of around 4% and 6%, assuming that a multitude of other practical influences have been avoided while driving. That leaves our measurements indicating a very loose 0-8% and 1-13% against an estimated 7% and 9%, the latter within the margin of that error.

So, it's not unreasonable to suggest that the measurements are not wildly off base, especially if the actual box Cd is better than the car.
I have read the above, imagining the voice of Sheldon Cooper from the Big Bang Theory, and a rapid monologue from Nigel Hawthorne as Sir Humphrey Appleby in Yes Minister...both work :)
Anyway, it seems the answer is “not bad, but try stuffing the boot first before buying a top box”? I find foot wells are often under-used when space becomes a premium.
 

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I have read the above, imagining the voice of Sheldon Cooper from the Big Bang Theory, and a rapid monologue from Nigel Hawthorne as Sir Humphrey Appleby in Yes Minister...both work :)
Anyway, it seems the answer is “not bad, but try stuffing the boot first before buying a top box”? I find foot wells are often under-used when space becomes a premium.
You may have meant your comments tongue in cheek, but to me they read as a somewhat discourteous dumbing down of the discussion from KiwiME and his brief analytical derivation of their meaning.

Why would you do that?

Is it because you can't cope with numerical discussions?
 

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You may have meant your comments tongue in cheek, but to me they read as a somewhat discourteous dumbing down of the discussion from KiwiME and his brief analytical derivation of their meaning.

Why would you do that?

Is it because you can't cope with numerical discussions?
Ah well, can't get humour to work for everybody :) So for Davesul, Yes, I didn't understand the numerical discussions which went straight over my head...I'm thick...I sought to dumb down the discussion to suit my own inadequacies. They were discourteous to KiwiMe for which I humbly apologise.....Happy now Dave?
 

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Now now people. This is a friendly forum, let's try and keep it that way. The trouble with typing words is that you don't get the tone of voice or facial expressions which convey SO much context. That's why emojie's were invented, and I clearly saw a smiley face, which means to me that it was meant as an inoffensive joke. Davesul, have you asked KiwiMe if they were offended by the remark, or are you offended on their behalf?

(Try reading it as David Tennant as the 10th Doctor. That works as well. :) No offence meant, I think your maths is brilliant as I am so rubbish at it.)
 

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This kind of discussion always reminds me of this quote.

"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong."
Richard P. Feynman

And I will always take notice of pages of theory - but more notice of physical experimentation results.
 

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Apologies all, and particularly to KiwiMe for taking this thread off on a tangent. Had I have started by saying "That's very interesting and useful, although I can't help reading the post in the voice of...." it may have sounded a little less "discourteous" and the pathetic attempt at humour might have been recognised. And Davesul, despite the rather pithy reply, I am happy to take it on the chin, as I did read the calculations a number of times and didn't get it so I am clearly not very bright! My comments about my conclusions of storage space preferences stand by the way. Perhaps I should promise to not respond to any thread that mentions drag coefficients, square routes or cubic capacities, and Dave can do likewise where a BBC comedy or Big Bang Theory is mentioned :) Anyway I promise not to add to this thread, so can somebody add some data graphs or technical analysis to get it back on track please. I offer the attached link if it helps


The above link takes you to an interesting discussion of Isosurface of Q-Criterion colored by velocity magnitude values for two different turbulence models on a roof mounted box....which computes realizable k-epsilon, and SAS apparently, but what do I know....although ask me me some stats on Kaley Cuoco and I think I could help.
 

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Cheers all, I'm not at all offended by the Sheldon remark and it's not entirely inaccurate! My dad was an astrophysicist and very much like Sheldon. I'm not remotely as clever but do like to utilise what few analytical skills I've acquired.

So, with that said here I go again ... all I've done in a nutshell is try to apply a quick 'sniff test' to the increase in energy use with the increase in wind resistance. The result is that the reported consumption numbers are plausible within the assumptions made, despite appearing to be somewhat lower than expected.
 

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I have read in other places that a critical feature that affects consumption is the gap between the car's roof and the box. Apparently, it's best either attached to the roof with no gap with no airflow possible through that gap. Or set quite high to allow smooth air to continue to flow over the car and the box to operate as if it was in a wind tunnel on its own. Probably resulting in tiny efficiency differences and such niceties being more academic than of any real benefit.
 

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Discussion Starter #15
I'm not expert but I believe the bars I used are one of the most aerodynamic bars on the market - Yakima Whispbars. Quiet with low drag (flush bars, mounted directly to fix points rather that wrapped around roof rails). There are quite a few other brands that are in the same segment (eg. Thule Wingbar Edge):
132390


Roofbox is not the most aerodynamic (short & wide) and a bit noisy.
Longer and slimmer box would be better but Kona has a rear spoiler which prevents using most of other boxes. It protrudes quite significantly and I just couldn't open the trunk. Also "anchor points" on roof rails are weirdly positioned. There are 3 on each side but the middle one is pretty much useless (to close to front point). So I had to use front + rear points but this means that roof bars are ~95 cm apart - very few roof boxes can accommodate that.

However, after seeing consumption figures I do not feel that further research and cost makes any sense now. If I was loosing 0.5m or more per KWh then I'd look for better box. But 0.2 loss would be rather hard to improve.

I will try to take some pics later today to visualise bars and mounting...
 
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