1 or 2 roof boxes - what drag, reduction on mpg?

Discussion in 'Motorhome Chat' started by errpaul, Mar 12, 2009.

  1. errpaul

    errpaul Funster

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    I was wondering if anyone can help with the question of how much extra drag a roof box will create, and if this will be increased substantially with 2 boxes?
    We have an OC bed so have a big bubble on the front of the van which is going to reduce aerodynamics greatly anyway.
    The van has a huge roof rack and easily enough space for 2 boxes either side by side or one in front of the other.
    Is it worth using these to save space inside?
    How much will the mpg reduce with 1 or 2 boxes mounted in the different ways as above?
    Thank you.

    :BigGrin::BigGrin:
     
  2. hilldweller

    hilldweller Funster Life Member

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    A few thoughts from an ex-glider pilot.....

    You'll notice a gliders wings are long and thin because the longer air is in contact the more drag, this points to side by side. I'd take a guess that behind the Luton it'll not make any difference at all.

    Having said that, shape is also important so two back to back might be more aerodynamically sound. Confused ? Well that's what you get at this price.

    But, high up is just where you do not want weight.
     
  3. scotjimland

    scotjimland Funster Life Member

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    I'm only guessing, but it's only making a bad shape worse .. 1 - 2 mpg ?

    If it's only to save space inside then I would try to do without.. boxes on the roof are fine for storing light-weight bulky items that you don't use on a daily basis.. but it's like getting an extra room or shed at home ... you'll always fill them up..
     
  4. pappajohn

    pappajohn Funster Life Member

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    much better with a small lockable trailer as it will be in your slipstream anyway plus you wont be using any of the valuable payload.

    as hilldweller said....you dont excess want weight high up on the roof anyway.
     
  5. errpaul

    errpaul Funster

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    Thanks all :thumb:
    The boxes would only be used for bulky lightweight items...or at least that would be the plan!
    We have considered a trailer papajohn, but it was just the extra cost of buying the trailer and having a towbar fitted, which I would imagine is around £500?? A few hundered £s then for the trailer? Does that sound about right?
     
  6. johng

    johng Read Only Funster

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    hi Paul, I have two boxes on roof, streamlined.. I dont think they would make a huge difference to the already bad MPG :)

    I only use them for lightish stuff that isn't needed very often e.g. tent, wetsuit, some spare motorbike gear..

    I fitted a brand new factory issue (ebay - unwanted item takng up space) tow bar to my car, £30 all in and 1 hour :Rofl1: still have some wiring issues to sort but thats ok...

    trailer.. my small box trailer was 40£ but is impossible to reverse! you shouldn't need to pay too much more for a brand new galvanised jobby..

    John
     
  7. barryd

    barryd Read Only Funster

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    Hi. When I got my Kontiki last year the previous owner had a large Fiamma box fitted to the roof. I decided I was never going to use it as Im too heavy and decrepid to climb on the roof and Michelle my wife wasnt too keen either (even though shes fit and light enough unlike me) so I had it taken off. I think it made a difference. It was early days as we hadnt had the van long before I took it off but I think we saved 2-3 mpg. There was certainly a slight difference. The box is now sat at the back of the garage. Is it worth anything by the way as Im never going to use it? Richmond are North Yorks if anybody wants to make me an offer!

    Regards
    Barry
     
  8. Terry

    Terry Funster

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    Hi we put a roof box on ours :Rofl1: it costs us 4 mpg :Eeek:and 10mph off top speed :Rofl1: Before fitting we could feel strong acceleration up to 90 :BigGrin:then slow build up to 100 :Blush: After this dropped to 80 and a slow build up to 90.:Rofl1:
    Our average mpg on 2 / 3 weeks trips was before 31 mpg, after fitting it is 27 mpg--:BigGrin:--May I hasten to add not at the above speeds :thumb: but travelling at 65 / 70 on m.ways then using the van around town/ sight seeing.Aprox mileage 700 m/w (350 each way)then 300 tripping about.I have no doubt that the m/w mpg would be greater but we kept total mileage figures so not to make stupid I get 40/45 mpg claims.As it is used daily when we are away rather than get there and park up I think it reflects a true usage figure. :thumb:IE 31 mpg without and 27 mpg with :Eeek::Rofl1:
    terry
     
  9. johng

    johng Read Only Funster

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    thats quite a difference Barry and Terry :Eek!:
    I think as my van would struggle to reach 60, that it doesnt matter too much :Rofl1:
     
  10. kijana

    kijana Read Only Funster

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    Side-by-side or fore & aft?

    Well, the formula for calculating aerodynamic drag has three main variables: a) the frontal area b) the drag coefficient and c) the speed.

    Clearly fore & aft will reduce the frontal area. It will also improve the drag coefficient: all other things being equal, long & thin makes for lower drag than wide & short. (The reason glider wings are long & narrow is for a different reason: this format increases their aspect ratio, and hence minimises induced drag due to wingtip vortices).

    But the variable that has the greatest effect on drag is velocity. In the formula for calculating drag, the velocity value is squared. So, say, twice the speed gives four times the drag. Even a small increase in speed will have a noticeable increase in drag (& hence fuel consumption).

    But as has been pointed out in this thread, the roofspace you're considering is downstream of the big Luton bulge. The airflow behind that bulge will be turbulent at any sort of usual roadspeed, so the orientation effect of the boxes will be considerably reduced. (Though if you do decide to go side-by-side, you'd do better to fit the boxes as close to the end of the bulge - i.e. as far forward - as you can (this will also help with weight distribution).

    Hope this helps. . .

    Bruce
     
  11. haganap

    haganap Funster Life Member

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    Can only give our experience from our last van.
    Had roof box fitted. killed te mpg and up hill in winds was such a struggle. We took it off and it was so much better probably at a guestermate about 2-5mpg the difference but obviously difficult to get a like for like comparison.

    Have you thought about fitting beeny boxes to the chasis?
     
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