Roof boxes - do they improve aerodynamics when towing?

[CarolineSimon]

My OH is querying whether or not having a roof box on when towing would increase economy & aerodynamics.

Can anyone offer any experience on this please?

Thanks

 

[ProfJohnL]

Hi,

Its all down aerodynamics, and that depends on the car, caravan and the make of the roof box and not least on its exact positioning relative to the roof of the car and the leading edge of the caravan.

In essence its a complicated thing to predict. In fairness there have been some people on here who swear it does improve towing and mpg, but probably as many or more who either don't notice any great change or find it degrades mpg.

 

[Lutz]

There's no way of giving definite answer either way. It all depends on the aerodynamics of the car itself, of the roof box and the relationship between the two. On the whole, however, I would not expect much of a difference because, in effect, you are only moving part of the total frontal area of the caravan further forward. However, if the design of the box is favourable, you may get a slight improvementif you are lucky.

 

[Garfield]

We have a Ford S-max that we tow a 21'T/a with and use a roof ski box to go skiing with but not both at the same time. In crosswinds both have about the same effect on the car's stability ie a ~75kg roofbox causes about the same instability in crosswinds as a 1500kg t/a caravan. Roofboxes should only contain low density items that could go in the caravan anyway, so unless you are desperate for space, stay away from them.

 

[mistermac]

I don't know.

What I DO know is....that if I have my roof box on, and my sun-roof on 'tilt' - everything seems to work better....mpg, stability, roof-box stability...

Strange, eh?

Mac

 

[MikeJE]

I found I got more M.P.G. with the roofbox on which seemed rather illogical. Then I realised I only put the roofbox on for long, steady non-stop holiday journeys rather than the usual daily traffic hampered urban cycle. This was driving solo before having a caravan - as suggested above there may be no difference between a 'van with roofbox or without.

 

[RayS]

The aerodynamics of my previous outfit Passtiche 520/4 ML 270cdi were seriously upset by roofbox whatever the fore and aft position. Without the box the outfit towed 'clean' i.e. minimal exterior cleaning required at the end of 1000 or more towing miles. With the box the van was filthy front and back. Tested on several occaisions.

I now have one of the fairly large plastic boxes meant for garden furniture cushions which goes inside the ML when the seats are folded. On site, box comes out and holds neatly all the stuff you seem to gather up for long stays. Appreciate this won't help if you need more than two seats available in the car and/ or the seats don't fold

 

[Kar1]

I'd like to join in and ask....

are there any implications to the rules around towing limits, weights etc when adding a roofbox to carry chairs winbreaks and beach paraphernalia-rather than having it all inside the 'van. I am considering a box so that we can pitch up and not have to empty out a load of stuff before we can have a brew!

 

[Lutz]

No, roof boxes do not affect towloads in any way. It's immaterial whether you've got payload in the boot or on the roof, you just have to make sure that you don't exceed the car's GVW or the maximum load that the manufacturer allows as a roof load.