It will just create more drag & as you say, increase fuel consumption. In order to reduce drag air would have to be directed cleanly past car & over caravan roof but due to the large gap between car/caravan anything on car roof will just create more turbulence & hence more drag.
I beg to differ here. Last year on our trip round Germany and the first time using a roof box I did notice a marginally improved fuel economy. This, of course, could have been something to do with the lower towing speed allowed. However I have not noticed a massive difference and certainly not worse. The only real probllem is what to do with the thing on site! I just leave it on and accept economy solo will not be as good.
i noticed no difference to mpg when towing , less flies plastered to the front of the caravan though.solo with it on , 4 up fully loaded mpg dropped by around 4mpg.
When towing, the drag of the roof box is not as significant as it is solo as you create a vacuum behind it which draws the caravan in, cancelling out much of the drag of the roof box. I would not expect it to reduce fuel consumption though, but increase it less than a roof box would increase the fuel consumption when solo.
A roof box will not work as well as a wind deflector as it has a gap underneath it. Also for a wind deflector to work properly it should be mounted tight to the caravan and also finish flush around the roof and sides of the van. This is rarely if ever the case. If the deflector is not fitted correctly, the aerodynamics will do all manner of strange things. To put this in perspective, look at the aerodynamics of this years McLaren F1 car. With all their experience and money, they have got the aerodynamics badly wrong this year. They cannot blame their problems on the engine either as Jenson Buttons Brawn is using the same engine and he has won 4 races out of 5 this year.
When all is said and done, I am no aerodynamicist, but I do know that you cannot take something pointy and stick it on your roof expecting it to improve the aerodynamics.
I tow an avondale dart 556/6 with a Kia sportage. I recently bought an awning with fibre tech poles annex etc all of which weighs in at 68kgs. So I stuck all this gear in a top box I bought in Spain 5 years ago when my estate car was full of wine and needed to get our luggage, and kids, home. If anything I noticed when towing to Yorkshire that the drive was better with the box on the roof, acceleration was better, feul consumption appeared better, it is hard to tell, the Kia lets you put upto 12 ltrs in tank after the pump at the station cuts out firts time...It definatly seemed to go better with the box on the roof, and will use it in the future, but perhaps with the awning etc in van and the bags in the box. My rear seat is taken up by a GSD (german shepard dog) and the boot by duvets towels etc that I dont leave in van in case of that damp smell......not that my van is damp!
------------- just because I am paranoid it doesnt mean I am not being followed.
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Topic: roof box as spoiler when towing
